{"topic":{"id":12717,"title":"OP 遇到了灵异事件","slug":"topic","categoryId":4,"createdAt":"2024-05-26 17:45:09.444178","bumpedAt":"2024-05-27 12:09:27.125875","views":496,"postsCount":27,"highestPostNumber":34,"likeCount":7,"closed":false,"pinned":false,"pinnedGlobally":false,"pinnedAt":null,"pinnedUntil":null,"isBanner":false,"excerpt":"如题。起因是这个： https://xjtu.app/t/topic/12644/17 OP 发现在修改不相关的代码之后，竟然导致在该行之前，有变量的值多次运行结果不同。 2.22507e-308 == DBL_MIN，是 OP 在前面为该变量赋的初始值，但在正常的运行中，该变量应该被更新为 3.33333：这是 OP 用 GDB 单步执行检查的结果。在第 66 行处，subsidy 的值被赋给 max_m。这件事的灵异之处就是，这…","deletedAt":null,"isAnonymous":false,"category":{"id":4,"name":"谈笑风生","slug":"general","color":"25AAE2"},"author":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif"},"tags":[]},"posts":[{"id":34384,"postNumber":1,"raw":"如题。起因是这个：\n\nhttps://xjtu.app/t/topic/12644/17\n\nOP 发现在修改不相关的代码之后，竟然导致在该行之前，有变量的值多次运行结果不同。\n\n2.22507e-308 == DBL_MIN，是 OP 在前面为该变量赋的初始值，但在正常的运行中，该变量应该被更新为 3.33333：\n\n![image|690x264](/uploads/image/25/19/2519f90312d006e8bbcdfe1f42ac17225802400bdda8780a20ab6b9def35692a.avif)\n\n这是 OP 用 GDB 单步执行检查的结果。在第 66 行处，`subsidy` 的值被赋给 `max_m`。\n\n![image|690x431](/uploads/image/1c/59/1c5999654ee39547b424c11040bff70dd77bc738163ff6425b2a11d7e18c940e.avif)\n\n这件事的灵异之处就是，这个赋值好像在某些次执行的时候会失败。比如如下的运行结果（这几次编译之间 OP 没有修改原文件内容，只是换用了不同的优化等级。这有可能是使用了未定义操作导致的。但是！**其中有两次 `-O2` 优化后执行，但两次结果竟然不同**）：\n\n![image|623x500](/uploads/image/55/49/55494e5ccbf72b7e8d0cbb593af2b7e7e95139e4d266e17cb0b9782c4d553ecc.avif)\n\n于是 OP 决定抓个现行，用 GDB debug 一个会产生错误结果（输出 1）的编译结果。\n\n![image|690x417](/uploads/image/bf/38/bf38ae9e74d92568360e43ecd9ca478eee87151799b2f2390bba581336e7435d.avif)\n\n一路追踪下去，发现最重要的 `max_m` 竟然成功被更新。\n\n![image|690x431](/uploads/image/76/4d/764dd88b60fb05f3d9ce66b96242c296cea51d908612b3e67d3b22bc2bb75586.avif)\n\nOP 觉得可能是循环结束之后的哪里出了问题，然后继续执行下去，结果发现：竟然输出了正确的结果。\n\n![image|690x431](/uploads/image/12/6b/126b10ee16a4489c5f92d0e8aec1cace45e999cd8c1f3513fe0ae33d1cec8154.avif)\n\nOP 觉得难以置信，在本次 debug 之后紧接着又运行了一遍，发现还是输出正确结果。在写这句话的时候又运行了一遍，发现又错了：\n\n![image|690x290](/uploads/image/0d/0e/0d0ef6f97758f3c5e01e0bba097d2eae45aaad0d6783f7e9f7bc932ebc8861c3.avif)","cooked":"

如题。起因是这个：

\n\n

OP 发现在修改不相关的代码之后，竟然导致在该行之前，有变量的值多次运行结果不同。

2.22507e-308 == DBL_MIN，是 OP 在前面为该变量赋的初始值，但在正常的运行中，该变量应该被更新为 3.33333：

$\"image\"$

这是 OP 用 GDB 单步执行检查的结果。在第 66 行处，subsidy 的值被赋给 max_m。

$\"image\"$

这件事的灵异之处就是，这个赋值好像在某些次执行的时候会失败。比如如下的运行结果（这几次编译之间 OP 没有修改原文件内容，只是换用了不同的优化等级。这有可能是使用了未定义操作导致的。但是！其中有两次 -O2 优化后执行，但两次结果竟然不同）：

$\"image\"$

于是 OP 决定抓个现行，用 GDB debug 一个会产生错误结果（输出 1）的编译结果。

$\"image\"$

一路追踪下去，发现最重要的 max_m 竟然成功被更新。

$\"image\"$

OP 觉得可能是循环结束之后的哪里出了问题，然后继续执行下去，结果发现：竟然输出了正确的结果。

$\"image\"$

OP 觉得难以置信，在本次 debug 之后紧接着又运行了一遍，发现还是输出正确结果。在写这句话的时候又运行了一遍，发现又错了：

$\"image\"$

\n","createdAt":"2024-05-26 17:45:09.569316","updatedAt":"2024-05-26 17:45:09.569316","replyToPostNumber":null,"replyCount":0,"likeCount":0,"wordCount":388,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34385,"postNumber":2,"raw":"这个变量定义在这个地方：\n\n![image|690x290](/uploads/image/49/fb/49fb147f9e7f617d6d2da1208da2203188acff7e703ed464ce11baedc5e598fb.avif)\n\n以下是 source 和测试用例：\n\n```cpp\n#include \n#include \n#include \n\nusing namespace std;\n\ntypedef long long ll;\n\nconst string no_solution = \"NO SOLUTION\";\n\nvoid solve() {\n int expected;\n cin >> expected;\n int sales_at_expected;\n\n double min_M = DBL_MAX;\n double max_m = DBL_MIN;\n\n map input;\n\n // w = sales * subsidy + c\n // c = sales * (price - cost)\n int cost, sales_at_cost;\n cin >> cost >> sales_at_cost;\n input[cost] = sales_at_cost;\n \n int p, s;\n int max_p;\n int max_p_s;\n while (cin >> p >> s && p != -1) {\n max_p = max(p, max_p);\n\n if (p == expected) {\n sales_at_expected = s;\n }\n\n input[p] = s;\n // cout << p << \" \" << s << endl;\n }\n\n // calculate unknown\n max_p_s = input[max_p];\n // cout << max_p << endl;\n // cout << max_p_s << endl;\n int decrement;\n cin >> decrement;\n while ((max_p_s -= decrement) >= 0) {\n // cout << max_p + 1 << \" \" << max_p_s << endl;\n input[++max_p] = max_p_s;\n }\n\n sales_at_expected = input[expected];\n\n // calculate intersections\n ll c_expected = sales_at_expected * (expected - cost);\n for (const auto &pair : input) {\n auto &pi = pair.first;\n auto &si = pair.second;\n ll ci = (pi - cost) * si;\n if (si - sales_at_expected == 0) continue;\n double subsidy = double(c_expected - ci) / double(si - sales_at_expected);\n \n // cout << \"subsidy \" << subsidy << endl;\n\n if (pi < expected && subsidy < min_M) min_M = subsidy;\n else if (pi > expected && subsidy > max_m) max_m = subsidy;\n }\n\n // find result\n // cout << max_m << \" \" << min_M << endl;\n // ll ans_min = ll(max_m + 1.0);\n // ll ans_max = ll(min_M);\n // cout << ans_min << \" \" << ans_max << endl; \n // if (ans_min > ans_max) cout << no_solution << endl;\n // else {\n // // find minimum absolute value\n // if (ans_min <= 0 && ans_max >= 0) cout << 0 << endl;\n // else if (ans_min >= 0) cout << ans_min << endl;\n // else cout << ans_max << endl;\n // }\n\n ll ans = ll(max_m + 1.0);\n\n if (ans > min_M) cout << no_solution << endl;\n else cout << ans << endl;\n}\n\nint main() {\n ios::sync_with_stdio(false);\n\n solve();\n\n return 0;\n}\n```\n\n```text\n31\n28 130\n30 120\n31 110\n-1 -1\n15\n```","cooked":"

这个变量定义在这个地方：

$\"image\"$

以下是 source 和测试用例：

#include <iostream>\n#include <cfloat>\n#include <map>\n\nusing namespace std;\n\ntypedef long long ll;\n\nconst string no_solution = \"NO SOLUTION\";\n\nvoid solve() {\n    int expected;\n    cin >> expected;\n    int sales_at_expected;\n\n    double min_M = DBL_MAX;\n    double max_m = DBL_MIN;\n\n    map<int, int> input;\n\n    // w = sales * subsidy + c\n    // c = sales * (price - cost)\n    int cost, sales_at_cost;\n    cin >> cost >> sales_at_cost;\n    input[cost] = sales_at_cost;\n    \n    int p, s;\n    int max_p;\n    int max_p_s;\n    while (cin >> p >> s && p != -1) {\n        max_p = max(p, max_p);\n\n        if (p == expected) {\n            sales_at_expected = s;\n        }\n\n        input[p] = s;\n        // cout << p << \" \" << s << endl;\n    }\n\n    // calculate unknown\n    max_p_s = input[max_p];\n    // cout << max_p << endl;\n    // cout << max_p_s << endl;\n    int decrement;\n    cin >> decrement;\n    while ((max_p_s -= decrement) >= 0) {\n        // cout << max_p + 1 << \" \" << max_p_s << endl;\n        input[++max_p] = max_p_s;\n    }\n\n    sales_at_expected = input[expected];\n\n    // calculate intersections\n    ll c_expected = sales_at_expected * (expected - cost);\n    for (const auto &pair : input) {\n        auto &pi = pair.first;\n        auto &si = pair.second;\n        ll ci = (pi - cost) * si;\n        if (si - sales_at_expected == 0) continue;\n        double subsidy = double(c_expected - ci) / double(si - sales_at_expected);\n        \n        // cout << \"subsidy \" << subsidy << endl;\n\n        if (pi < expected && subsidy < min_M) min_M = subsidy;\n        else if (pi > expected && subsidy > max_m) max_m = subsidy;\n    }\n\n    // find result\n    // cout << max_m << \" \" << min_M << endl;\n    // ll ans_min = ll(max_m + 1.0);\n    // ll ans_max = ll(min_M);\n    // cout << ans_min << \" \" << ans_max << endl; \n    // if (ans_min > ans_max) cout << no_solution << endl;\n    // else {\n    //     // find minimum absolute value\n    //     if (ans_min <= 0 && ans_max >= 0) cout << 0 << endl;\n    //     else if (ans_min >= 0) cout << ans_min << endl;\n    //     else cout << ans_max << endl;\n    // }\n\n    ll ans = ll(max_m + 1.0);\n\n    if (ans > min_M) cout << no_solution << endl;\n    else cout << ans << endl;\n}\n\nint main() {\n    ios::sync_with_stdio(false);\n\n    solve();\n\n    return 0;\n}\n

31\n28 130\n30 120\n31 110\n-1  -1\n15\n

\n","createdAt":"2024-05-26 17:47:10.359438","updatedAt":"2024-05-26 17:47:10.359438","replyToPostNumber":null,"replyCount":1,"likeCount":0,"wordCount":404,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34386,"postNumber":3,"raw":"错误答案 1 是 `max_m = DBL_MIN` 导致的，而正确答案 4 是 `max_m` 被成功更新为 3.33333 得出的。\n\n本 source code 并不是原题目的正确解答，只能通过 2/4 个测试，但是这是因为 OP 读题读漏了一点，在修改的过程中发现本问题。","cooked":"

错误答案 1 是 max_m = DBL_MIN 导致的，而正确答案 4 是 max_m 被成功更新为 3.33333 得出的。

本 source code 并不是原题目的正确解答，只能通过 2/4 个测试，但是这是因为 OP 读题读漏了一点，在修改的过程中发现本问题。

\n","createdAt":"2024-05-26 17:50:32.994576","updatedAt":"2024-05-26 17:50:32.994576","replyToPostNumber":null,"replyCount":0,"likeCount":0,"wordCount":84,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34388,"postNumber":5,"raw":"OP 的测试环境是 Fedora 40 + zsh shell + g++ 14.1.1：\n\n![image|601x499](/uploads/image/9e/3e/9e3e9ffb6bf1f29a2b3db1edac15371cfbbc1bb1398968145f446153550cea45.avif)\n\n又拿 clang 编译了一遍：\n\n![image|690x210](/uploads/image/72/ee/72ee5e23fba83737b020acda312c0e1a10648f9ff99d6bc7ab394cfc8adfdf20.avif)\n\n\n将 source 发给朋友在 Ubuntu 上测试得到了同样的结果：\n\n![image|586x232](/uploads/image/1d/89/1d89a30ee6c6c5aae93d918f8b5f5cae5e357e5fb4c7682179daa9e6e7233cc1.avif)","cooked":"

OP 的测试环境是 Fedora 40 + zsh shell + g++ 14.1.1：

$\"image\"$

又拿 clang 编译了一遍：

$\"image\"$

将 source 发给朋友在 Ubuntu 上测试得到了同样的结果：

$\"image\"$

\n","createdAt":"2024-05-26 18:01:04.207578","updatedAt":"2024-05-26 18:09:45.843021","replyToPostNumber":null,"replyCount":0,"likeCount":0,"wordCount":47,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":2,"publicVersion":2,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34389,"postNumber":6,"raw":"OP 拿 g++ 又编译了一次，本来想多尝试几次 GDB 抓个现行，但现在又复现不出来错误结果了。\n\n![image|520x500](/uploads/image/04/ff/04ffed932f54436855d098b95fddb480221673bc621b62c4c7797a665345d9d9.avif)\n\n\nOP 感到十分困惑，希望能得到解答","cooked":"

OP 拿 g++ 又编译了一次，本来想多尝试几次 GDB 抓个现行，但现在又复现不出来错误结果了。

$\"image\"$

OP 感到十分困惑，希望能得到解答

\n","createdAt":"2024-05-26 18:03:38.159849","updatedAt":"2024-05-26 18:10:34.080923","replyToPostNumber":null,"replyCount":0,"likeCount":0,"wordCount":55,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":2,"publicVersion":2,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34392,"postNumber":7,"raw":"看完代码了，暂时先放一个疯四在这里 :exploding_head:","cooked":"

看完代码了，暂时先放一个疯四在这里 🤯

\n","createdAt":"2024-05-26 18:21:40.485367","updatedAt":"2024-05-26 18:21:40.485367","replyToPostNumber":null,"replyCount":0,"likeCount":1,"wordCount":18,"liked":false,"bookmarked":false,"polls":[],"reactions":[{"reaction":"hugging_face_exploding_head","count":1,"users":[{"id":4648,"username":"nonsuch","name":null}],"hasReacted":false},{"reaction":"❤️","count":1,"users":[{"id":4648,"username":"nonsuch","name":null}],"hasReacted":false}],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":601,"username":"m0rsun","name":null,"avatarUrl":"/uploads/image/37/7f/377f4ae853850691ab959270f01d52509ee07caf808041ddb0ab22488a0e9f85.avif","admin":false,"moderator":true,"signatureRaw":null,"signatureCooked":null}},{"id":34395,"postNumber":10,"raw":"OP 又一次发现：在命令行执行会出错的 build，用 GDB 执行一定会得到正确答案\n\n[grid mode=\"tile\"]\n![image|690x377](/uploads/image/bb/fd/bbfdff3694bff1ed72df677da3e0af8b1429fad5dc6af620e8f6072f72b151d6.avif)\n\n![image|690x377](/uploads/image/0a/8b/0a8bfd74376821496e8d34b71273369298444a2838afb4f71cda4bb2d011caa5.avif)\n[/grid]","cooked":"

OP 又一次发现：在命令行执行会出错的 build，用 GDB 执行一定会得到正确答案

$\"image\"$

\n","createdAt":"2024-05-26 18:23:24.692228","updatedAt":"2024-05-26 18:23:24.692228","replyToPostNumber":null,"replyCount":0,"likeCount":0,"wordCount":36,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34397,"postNumber":12,"raw":"什么，李大夫能帮忙诊断一下吗","cooked":"

什么，李大夫能帮忙诊断一下吗

\n","createdAt":"2024-05-26 18:24:43.056409","updatedAt":"2024-05-26 18:24:43.056409","replyToPostNumber":11,"replyCount":1,"likeCount":0,"wordCount":14,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34398,"postNumber":13,"raw":"我算法很差来着。已经上床了，明天也许有空。正好最近打算重拾算法了","cooked":"

我算法很差来着。已经上床了，明天也许有空。正好最近打算重拾算法了

\n","createdAt":"2024-05-26 18:25:34.097501","updatedAt":"2024-05-26 18:25:34.097501","replyToPostNumber":12,"replyCount":1,"likeCount":0,"wordCount":32,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":1628,"username":"Lorange","name":"主治医师李大华","avatarUrl":"/uploads/image/f8/ec/f8ecbeb887c4a1311f4f0bf05928476c7b3f632d15bb2b24e782baf2ba6f0d71.avif","admin":false,"moderator":true,"signatureRaw":null,"signatureCooked":null}},{"id":34400,"postNumber":15,"raw":"这不是算法的问题啊 :anxious: 不知道是系统还是编译器的问题","cooked":"

这不是算法的问题啊 😰 不知道是系统还是编译器的问题

\n","createdAt":"2024-05-26 18:28:58.775306","updatedAt":"2024-05-26 18:28:58.775306","replyToPostNumber":13,"replyCount":1,"likeCount":0,"wordCount":24,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34401,"postNumber":16,"raw":"我可以用 ubuntu 帮你测测看，或者 google 一下。不过可能帮不上什么忙，毕竟我连 gdb 都没用过😭","cooked":"

我可以用 ubuntu 帮你测测看，或者 google 一下。不过可能帮不上什么忙，毕竟我连 gdb 都没用过😭

\n","createdAt":"2024-05-26 18:34:51.039784","updatedAt":"2024-05-26 18:36:16.563662","replyToPostNumber":15,"replyCount":0,"likeCount":0,"wordCount":38,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":1628,"username":"Lorange","name":"主治医师李大华","avatarUrl":"/uploads/image/f8/ec/f8ecbeb887c4a1311f4f0bf05928476c7b3f632d15bb2b24e782baf2ba6f0d71.avif","admin":false,"moderator":true,"signatureRaw":null,"signatureCooked":null}},{"id":34402,"postNumber":17,"raw":"OP 觉得这真得可能是一个优化问题。因为：\n\n- gdb 运行一定会得到正确结果\n- 不指定优化等级 `g++ -Wall -g -o p1023 p1023.cpp` 的时候（默认使用 `-O0`）也一定会得到正确结果\n- 但是当指定了 `-O2` 或 `-O3` 后就会时不时出错\n- 又试了一下指定 `-O1` 发现每次都一定会得到错误结果\n\n不对，但朋友在 Ubuntu 上用 `g++ filename.cpp -o filename.o` 编译的，一样会跑飞","cooked":"

OP 觉得这真得可能是一个优化问题。因为：

gdb 运行一定会得到正确结果
不指定优化等级 g++ -Wall -g -o p1023 p1023.cpp 的时候（默认使用 -O0）也一定会得到正确结果
但是当指定了 -O2 或 -O3 后就会时不时出错
又试了一下指定 -O1 发现每次都一定会得到错误结果

不对，但朋友在 Ubuntu 上用 g++ filename.cpp -o filename.o 编译的，一样会跑飞

\n","createdAt":"2024-05-26 18:37:04.892451","updatedAt":"2024-05-26 18:38:46.000888","replyToPostNumber":null,"replyCount":0,"likeCount":0,"wordCount":129,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34404,"postNumber":19,"raw":"-O2 优化过后的汇编是真看不懂吧，，","cooked":"

-O2 优化过后的汇编是真看不懂吧，，

\n","createdAt":"2024-05-26 18:39:30.639108","updatedAt":"2024-05-26 18:39:30.639108","replyToPostNumber":18,"replyCount":0,"likeCount":0,"wordCount":15,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34406,"postNumber":21,"raw":"我看什么","cooked":"

我看什么

\n","createdAt":"2024-05-26 18:40:37.637191","updatedAt":"2024-05-26 18:40:37.637191","replyToPostNumber":20,"replyCount":0,"likeCount":0,"wordCount":4,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34408,"postNumber":22,"raw":"@aibot 这里可能存在的 undefined behavior 是什么？如果你能找到导致 OP 结果错误的 undefined behavior，我就会给你$10 小费","cooked":"

@aibot 这里可能存在的 undefined behavior 是什么？如果你能找到导致 OP 结果错误的 undefined behavior，我就会给你$10 小费

\n","createdAt":"2024-05-26 18:42:34.259504","updatedAt":"2024-05-26 18:42:34.259504","replyToPostNumber":2,"replyCount":1,"likeCount":1,"wordCount":38,"liked":false,"bookmarked":false,"polls":[],"reactions":[{"reaction":"🥳️","count":1,"users":[{"id":4648,"username":"nonsuch","name":null}],"hasReacted":false},{"reaction":"❤️","count":1,"users":[{"id":4648,"username":"nonsuch","name":null}],"hasReacted":false}],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":601,"username":"m0rsun","name":null,"avatarUrl":"/uploads/image/37/7f/377f4ae853850691ab959270f01d52509ee07caf808041ddb0ab22488a0e9f85.avif","admin":false,"moderator":true,"signatureRaw":null,"signatureCooked":null}},{"id":34409,"postNumber":23,"raw":"在代码中，可能存在未定义行为的情况如下：\n\n1. **未初始化变量**：变量 `max_p` 在第一次使用前未初始化。在下面这行代码中：\n ```cpp\n max_p = max(p, max_p);\n ```\n `max_p` 在第一次迭代时未被赋值，这将导致未定义行为。这可能会导致不同的运行结果，尤其是在开启不同的优化等级时。\n\n2. **超出边界操作**：在进行 `map` 操作时，可能存在访问未定义的行为。例如：\n ```cpp\n max_p_s = input[max_p];\n ```\n 要确保每次访问 `max_p`和`max_p_s`时它们在`map` 中有有效值。\n\n3. **浮点操作中的未定义行为**：在计算 `subsidy` 时，可能会发生浮点未定义行为：\n ```cpp\n double subsidy = double(c_expected - ci) / double(si - sales_at_expected);\n ```\n 当`si == sales_at_expected`时，会导致除以 0 的情况。虽然前面有 `continue` 来避免这种情况，但需要确保在所有情况下都避免可能的浮点精度问题。\n\n此外，根据论坛中的讨论，未初始化变量的使用问题非常常见，并且会随着编译优化等级的变化而导致不同的行为。因此，建议首先解决变量 `max_p`未初始化的问题。这可以通过给`max_p` 赋初始值来解决，例如`int max_p = 0;`。\n\n希望这能帮助找到代码中的未定义行为。如果还有其他问题，请告诉我！","cooked":"

在代码中，可能存在未定义行为的情况如下：

\n
未初始化变量：变量 max_p 在第一次使用前未初始化。在下面这行代码中：
\n
```
max_p = max(p, max_p);\n
```
\n
max_p 在第一次迭代时未被赋值，这将导致未定义行为。这可能会导致不同的运行结果，尤其是在开启不同的优化等级时。
\n
\n
超出边界操作：在进行 map 操作时，可能存在访问未定义的行为。例如：
\n
```
max_p_s = input[max_p];\n
```
\n
要确保每次访问 max_p和max_p_s时它们在map 中有有效值。
\n
\n
浮点操作中的未定义行为：在计算 subsidy 时，可能会发生浮点未定义行为：
\n
```
double subsidy = double(c_expected - ci) / double(si - sales_at_expected);\n
```
\n
当si == sales_at_expected时，会导致除以 0 的情况。虽然前面有 continue 来避免这种情况，但需要确保在所有情况下都避免可能的浮点精度问题。
\n

此外，根据论坛中的讨论，未初始化变量的使用问题非常常见，并且会随着编译优化等级的变化而导致不同的行为。因此，建议首先解决变量 max_p未初始化的问题。这可以通过给max_p 赋初始值来解决，例如int max_p = 0;。

希望这能帮助找到代码中的未定义行为。如果还有其他问题，请告诉我！

\n","createdAt":"2024-05-26 18:42:47.508591","updatedAt":"2024-05-26 18:42:47.508591","replyToPostNumber":22,"replyCount":1,"likeCount":0,"wordCount":393,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":-4,"username":"aibot_old","name":"小AI同學","avatarUrl":"/uploads/image/02/1b/021bb0b0848388a911ae795dde7c8e5b3f1962316b2d88853208172cd0e89de2.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34411,"postNumber":24,"raw":"```cpp\nif (si - sales_at_expected < 1e-5) continue;\n```\n\n修改了防除 0 措施，现在检测一个 epsilon 范围，但与本问题无关","cooked":"

if (si - sales_at_expected < 1e-5) continue;\n

修改了防除 0 措施，现在检测一个 epsilon 范围，但与本问题无关

\n","createdAt":"2024-05-26 18:45:06.672094","updatedAt":"2024-05-26 18:45:06.672094","replyToPostNumber":23,"replyCount":1,"likeCount":0,"wordCount":35,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34412,"postNumber":25,"raw":"好的，那我们再仔细查看和考虑代码中的其他潜在未定义行为。既然已经排除了浮点除零的问题，让我们着重分析剩余部分。\n\n### 变量初始化问题\n\n关键问题依然可能出在未初始化变量上。让我们再具体分析：\n\n1. `max_p` 变量：在第一次使用前未初始化，可能会导致不确定的值及行为。\n\n2. `sales_at_expected` 变量：虽然它在`input[p] = s;`循环内被条件性赋值，但使用该值的先后顺序仍需确认，以避免未初始化的风险。\n\n### 安全检查与修改\n\n下面是改进代码以确保没有未定义行为的建议：\n\n1. 初始化 `max_p`:\n ```cpp\n int max_p = INT_MIN; // 设置为一个较小值，确保在循环中能正确更新\n ```\n 这样确保在循环中 `max_p` 能正确获取最大值。\n\n2. 在`if (p == expected)`判断后增加检查:\n ```cpp\n if (p == expected) {\n sales_at_expected = s;\n }\n ```\n 确保在相关代码段前 `sales_at_expected` 已被正确赋值。\n\n根据这些建议，我们可以改写代码如下：\n\n```cpp\n#include \n#include \n#include \n#include // 引入 INT_MIN\n\nusing namespace std;\n\ntypedef long long ll;\n\nconst string no_solution = \"NO SOLUTION\";\n\nvoid solve() {\n int expected;\n cin >> expected;\n int sales_at_expected = 0; // 初始化为 0\n\n double min_M = DBL_MAX;\n double max_m = DBL_MIN;\n\n map input;\n\n // w = sales * subsidy + c\n // c = sales * (price - cost)\n int cost, sales_at_cost;\n cin >> cost >> sales_at_cost;\n input[cost] = sales_at_cost;\n \n int p, s;\n int max_p = INT_MIN; // 初始化为一个最小值\n int max_p_s;\n while (cin >> p >> s && p != -1) {\n max_p = max(p, max_p);\n\n if (p == expected) {\n sales_at_expected = s;\n }\n\n input[p] = s;\n }\n\n // calculate unknown\n max_p_s = input[max_p];\n int decrement;\n cin >> decrement;\n while ((max_p_s -= decrement) >= 0) {\n input[++max_p] = max_p_s;\n }\n\n sales_at_expected = input[expected];\n\n // calculate intersections\n ll c_expected = sales_at_expected * (expected - cost);\n for (const auto &pair : input) {\n auto &pi = pair.first;\n auto &si = pair.second;\n ll ci = (pi - cost) * si;\n if (si - sales_at_expected < 1e-5) continue; // 已修正的除零检查\n double subsidy = double(c_expected - ci) / double(si - sales_at_expected);\n\n if (pi < expected && subsidy < min_M) min_M = subsidy;\n else if (pi > expected && subsidy > max_m) max_m = subsidy;\n }\n\n // find result\n ll ans = ll(max_m + 1.0);\n\n if (ans > min_M) cout << no_solution << endl;\n else cout << ans << endl;\n}\n\nint main() {\n ios::sync_with_stdio(false);\n\n solve();\n\n return 0;\n}\n```\n\n通过初始化关键变量 `max_p`和`sales_at_expected` 以防止未定义行为，可以提高代码稳定性。请测试修改后的代码，看看是否解决了 OP 遇到的问题。","cooked":"

好的，那我们再仔细查看和考虑代码中的其他潜在未定义行为。既然已经排除了浮点除零的问题，让我们着重分析剩余部分。

变量初始化问题

关键问题依然可能出在未初始化变量上。让我们再具体分析：

\n
max_p 变量：在第一次使用前未初始化，可能会导致不确定的值及行为。
\n
\n
sales_at_expected 变量：虽然它在input[p] = s;循环内被条件性赋值，但使用该值的先后顺序仍需确认，以避免未初始化的风险。
\n

安全检查与修改

下面是改进代码以确保没有未定义行为的建议：

初始化 max_p:

int max_p = INT_MIN; // 设置为一个较小值，确保在循环中能正确更新\n

这样确保在循环中 max_p 能正确获取最大值。

\n
在if (p == expected)判断后增加检查:
\n
```
if (p == expected) {\n    sales_at_expected = s;\n}\n
```
\n
确保在相关代码段前 sales_at_expected 已被正确赋值。
\n

根据这些建议，我们可以改写代码如下：

#include <iostream>\n#include <cfloat>\n#include <map>\n#include <climits> // 引入 INT_MIN\n\nusing namespace std;\n\ntypedef long long ll;\n\nconst string no_solution = \"NO SOLUTION\";\n\nvoid solve() {\n    int expected;\n    cin >> expected;\n    int sales_at_expected = 0; // 初始化为 0\n\n    double min_M = DBL_MAX;\n    double max_m = DBL_MIN;\n\n    map<int, int> input;\n\n    // w = sales * subsidy + c\n    // c = sales * (price - cost)\n    int cost, sales_at_cost;\n    cin >> cost >> sales_at_cost;\n    input[cost] = sales_at_cost;\n    \n    int p, s;\n    int max_p = INT_MIN; // 初始化为一个最小值\n    int max_p_s;\n    while (cin >> p >> s && p != -1) {\n        max_p = max(p, max_p);\n\n        if (p == expected) {\n            sales_at_expected = s;\n        }\n\n        input[p] = s;\n    }\n\n    // calculate unknown\n    max_p_s = input[max_p];\n    int decrement;\n    cin >> decrement;\n    while ((max_p_s -= decrement) >= 0) {\n        input[++max_p] = max_p_s;\n    }\n\n    sales_at_expected = input[expected];\n\n    // calculate intersections\n    ll c_expected = sales_at_expected * (expected - cost);\n    for (const auto &pair : input) {\n        auto &pi = pair.first;\n        auto &si = pair.second;\n        ll ci = (pi - cost) * si;\n        if (si - sales_at_expected < 1e-5) continue; // 已修正的除零检查\n        double subsidy = double(c_expected - ci) / double(si - sales_at_expected);\n\n        if (pi < expected && subsidy < min_M) min_M = subsidy;\n        else if (pi > expected && subsidy > max_m) max_m = subsidy;\n    }\n\n    // find result\n    ll ans = ll(max_m + 1.0);\n\n    if (ans > min_M) cout << no_solution << endl;\n    else cout << ans << endl;\n}\n\nint main() {\n    ios::sync_with_stdio(false);\n\n    solve();\n\n    return 0;\n}\n

通过初始化关键变量 max_p和sales_at_expected 以防止未定义行为，可以提高代码稳定性。请测试修改后的代码，看看是否解决了 OP 遇到的问题。

\n","createdAt":"2024-05-26 18:45:28.555687","updatedAt":"2024-05-26 18:45:28.555687","replyToPostNumber":24,"replyCount":1,"likeCount":0,"wordCount":635,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":-4,"username":"aibot_old","name":"小AI同學","avatarUrl":"/uploads/image/02/1b/021bb0b0848388a911ae795dde7c8e5b3f1962316b2d88853208172cd0e89de2.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34413,"postNumber":26,"raw":"建议阅读：https://www.ralfj.de/blog/2019/07/14/uninit.html","cooked":"

建议阅读："What The Hardware Does" is not What Your Program Does: Uninitialized Memory

\n","createdAt":"2024-05-26 18:46:46.538768","updatedAt":"2024-05-26 18:46:46.538768","replyToPostNumber":null,"replyCount":1,"likeCount":1,"wordCount":5,"liked":false,"bookmarked":false,"polls":[],"reactions":[{"reaction":"❤️","count":1,"users":[{"id":4648,"username":"nonsuch","name":null}],"hasReacted":false}],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":1277,"username":"ucw","name":null,"avatarUrl":"/uploads/image/03/ea/03eac2176be50969b549f432e72df28bd719033042da4c6c41c3fde705e8f13a.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34414,"postNumber":27,"raw":"所有变量都已初始化，但没用","cooked":"

所有变量都已初始化，但没用

\n","createdAt":"2024-05-26 18:47:46.663999","updatedAt":"2024-05-26 18:47:46.663999","replyToPostNumber":25,"replyCount":1,"likeCount":0,"wordCount":13,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34415,"postNumber":28,"raw":"既然所有变量都已初始化，并且排除了浮点除零的问题，那我们需要进一步细化调试过程和代码结构，来检查是否存在其他潜在的未定义行为，尤其是在不同优化等级下的表现不同。\n\n### 分析与调试思路\n\n1. **检查循环和赋值**：确保所有循环和条件赋值都正确执行，尤其在高优化级别下。\n2. **调试日志**：在关键位置添加调试日志，输出变量值和执行路径，帮助确定在哪个环节出现异常。\n3. **优化等级影响**：不同优化等级-O0, -O1, -O2, -O3 会导致编译器进行不同的代码优化路径，需要确认在每种优化等级下的行为。\n\n### 调试日志\n\n通过在代码关键位置增加调试信息，帮助分析变量状态和执行流程。示例如下：\n\n```cpp\n#include \n#include \n#include \n#include // 引入 INT_MIN\n\nusing namespace std;\n\ntypedef long long ll;\n\nconst string no_solution = \"NO SOLUTION\";\n\nvoid solve() {\n int expected;\n cin >> expected;\n int sales_at_expected = 0; // 初始化为 0\n\n double min_M = DBL_MAX;\n double max_m = DBL_MIN;\n\n map input;\n\n // w = sales * subsidy + c\n // c = sales * (price - cost)\n int cost, sales_at_cost;\n cin >> cost >> sales_at_cost;\n input[cost] = sales_at_cost;\n \n int p, s;\n int max_p = INT_MIN; // 初始化为一个最小值\n int max_p_s;\n while (cin >> p >> s && p != -1) {\n max_p = max(p, max_p);\n\n if (p == expected) {\n sales_at_expected = s;\n }\n\n input[p] = s;\n }\n\n // 打印调试信息\n cerr << \"Initial input map: \";\n for (const auto &pair : input) {\n cerr << \" (\" << pair.first << \",\" << pair.second << \")\";\n }\n cerr << endl;\n\n // calculate unknown\n max_p_s = input[max_p];\n int decrement;\n cin >> decrement;\n while ((max_p_s -= decrement) >= 0) {\n input[++max_p] = max_p_s;\n }\n\n sales_at_expected = input[expected];\n\n // 打印调试信息\n cerr << \"Updated input map: \";\n for (const auto &pair : input) {\n cerr << \" (\" << pair.first << \",\" << pair.second << \")\";\n }\n cerr << endl;\n\n // calculate intersections\n ll c_expected = sales_at_expected * (expected - cost);\n for (const auto &pair : input) {\n auto &pi = pair.first;\n auto &si = pair.second;\n ll ci = (pi - cost) * si;\n if (si - sales_at_expected < 1e-5) continue; // 已修正的除零检查\n double subsidy = double(c_expected - ci) / double(si - sales_at_expected);\n\n cerr << \"pi = \" << pi << \", si = \" << si << \", subsidy = \" << subsidy << endl;\n\n if (pi < expected && subsidy < min_M) min_M = subsidy;\n else if (pi > expected && subsidy > max_m) max_m = subsidy;\n }\n\n // find result\n ll ans = ll(max_m + 1.0);\n\n cerr << \"max_m = \" << max_m << \", min_M = \" << min_M << \", ans = \" << ans << endl;\n\n if (ans > min_M) cout << no_solution << endl;\n else cout << ans << endl;\n}\n\nint main() {\n ios::sync_with_stdio(false);\n\n solve();\n\n return 0;\n}\n```\n\n通过在关键位置打印调试信息，可以帮助追踪变量值和代码执行路径，尤其在不同的优化等级下。具体调试思路如下：\n\n1. **编译和运行**：分别在不同优化等级 `-O0`, `-O1`, `-O2`, `-O3` 下编译和运行代码。\n2. **检查调试输出**：对比不同优化等级下输出的调试日志，分析是否存在变量值异常或执行路径不一致的地方。\n\n请尝试使用上述调试日志分析不同运行结果，以确定代码在不同优化等级下为何会表现不同。这将有助于进一步确定未定义行为的根源。","cooked":"

既然所有变量都已初始化，并且排除了浮点除零的问题，那我们需要进一步细化调试过程和代码结构，来检查是否存在其他潜在的未定义行为，尤其是在不同优化等级下的表现不同。

分析与调试思路

检查循环和赋值：确保所有循环和条件赋值都正确执行，尤其在高优化级别下。
调试日志：在关键位置添加调试日志，输出变量值和执行路径，帮助确定在哪个环节出现异常。
优化等级影响：不同优化等级-O0, -O1, -O2, -O3 会导致编译器进行不同的代码优化路径，需要确认在每种优化等级下的行为。

调试日志

通过在代码关键位置增加调试信息，帮助分析变量状态和执行流程。示例如下：

#include <iostream>\n#include <cfloat>\n#include <map>\n#include <climits> // 引入 INT_MIN\n\nusing namespace std;\n\ntypedef long long ll;\n\nconst string no_solution = \"NO SOLUTION\";\n\nvoid solve() {\n    int expected;\n    cin >> expected;\n    int sales_at_expected = 0; // 初始化为 0\n\n    double min_M = DBL_MAX;\n    double max_m = DBL_MIN;\n\n    map<int, int> input;\n\n    // w = sales * subsidy + c\n    // c = sales * (price - cost)\n    int cost, sales_at_cost;\n    cin >> cost >> sales_at_cost;\n    input[cost] = sales_at_cost;\n    \n    int p, s;\n    int max_p = INT_MIN; // 初始化为一个最小值\n    int max_p_s;\n    while (cin >> p >> s && p != -1) {\n        max_p = max(p, max_p);\n\n        if (p == expected) {\n            sales_at_expected = s;\n        }\n\n        input[p] = s;\n    }\n\n    // 打印调试信息\n    cerr << \"Initial input map: \";\n    for (const auto &pair : input) {\n        cerr << \" (\" << pair.first << \",\" << pair.second << \")\";\n    }\n    cerr << endl;\n\n    // calculate unknown\n    max_p_s = input[max_p];\n    int decrement;\n    cin >> decrement;\n    while ((max_p_s -= decrement) >= 0) {\n        input[++max_p] = max_p_s;\n    }\n\n    sales_at_expected = input[expected];\n\n    // 打印调试信息\n    cerr << \"Updated input map: \";\n    for (const auto &pair : input) {\n        cerr << \" (\" << pair.first << \",\" << pair.second << \")\";\n    }\n    cerr << endl;\n\n    // calculate intersections\n    ll c_expected = sales_at_expected * (expected - cost);\n    for (const auto &pair : input) {\n        auto &pi = pair.first;\n        auto &si = pair.second;\n        ll ci = (pi - cost) * si;\n        if (si - sales_at_expected < 1e-5) continue; // 已修正的除零检查\n        double subsidy = double(c_expected - ci) / double(si - sales_at_expected);\n\n        cerr << \"pi = \" << pi << \", si = \" << si << \", subsidy = \" << subsidy << endl;\n\n        if (pi < expected && subsidy < min_M) min_M = subsidy;\n        else if (pi > expected && subsidy > max_m) max_m = subsidy;\n    }\n\n    // find result\n    ll ans = ll(max_m + 1.0);\n\n    cerr << \"max_m = \" << max_m << \", min_M = \" << min_M << \", ans = \" << ans << endl;\n\n    if (ans > min_M) cout << no_solution << endl;\n    else cout << ans << endl;\n}\n\nint main() {\n    ios::sync_with_stdio(false);\n\n    solve();\n\n    return 0;\n}\n

通过在关键位置打印调试信息，可以帮助追踪变量值和代码执行路径，尤其在不同的优化等级下。具体调试思路如下：

编译和运行：分别在不同优化等级 -O0, -O1, -O2, -O3 下编译和运行代码。
检查调试输出：对比不同优化等级下输出的调试日志，分析是否存在变量值异常或执行路径不一致的地方。

请尝试使用上述调试日志分析不同运行结果，以确定代码在不同优化等级下为何会表现不同。这将有助于进一步确定未定义行为的根源。

\n","createdAt":"2024-05-26 18:48:01.98508","updatedAt":"2024-05-26 18:48:01.98508","replyToPostNumber":27,"replyCount":0,"likeCount":0,"wordCount":843,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":-4,"username":"aibot_old","name":"小AI同學","avatarUrl":"/uploads/image/02/1b/021bb0b0848388a911ae795dde7c8e5b3f1962316b2d88853208172cd0e89de2.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34417,"postNumber":29,"raw":"应该是这个问题，我再看看","cooked":"

应该是这个问题，我再看看

\n","createdAt":"2024-05-26 18:55:04.834548","updatedAt":"2024-05-26 18:55:04.834548","replyToPostNumber":26,"replyCount":0,"likeCount":0,"wordCount":12,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34448,"postNumber":30,"raw":"不对啊，你的 max_p 在第一次塞入 max 前没有赋值？这是一个 UB(应该)，会导致你最后 input[max_p] 的时候取到不在 map 内的 key\n\n所以 aibot，你第一句话就发现了吗？好厉害，可惜没有小费能给你 :wink:","cooked":"

不对啊，你的 max_p 在第一次塞入 max 前没有赋值？这是一个 UB(应该)，会导致你最后 input[max_p] 的时候取到不在 map 内的 key

所以 aibot，你第一句话就发现了吗？好厉害，可惜没有小费能给你 😉

\n","createdAt":"2024-05-27 03:18:13.594288","updatedAt":"2024-05-27 04:01:26.608446","replyToPostNumber":null,"replyCount":2,"likeCount":1,"wordCount":74,"liked":false,"bookmarked":false,"polls":[],"reactions":[{"reaction":"❤️","count":1,"users":[{"id":4648,"username":"nonsuch","name":null}],"hasReacted":false}],"wiki":false,"version":2,"publicVersion":2,"deletedAt":null,"isAnonymous":false,"user":{"id":601,"username":"m0rsun","name":null,"avatarUrl":"/uploads/image/37/7f/377f4ae853850691ab959270f01d52509ee07caf808041ddb0ab22488a0e9f85.avif","admin":false,"moderator":true,"signatureRaw":null,"signatureCooked":null}},{"id":34450,"postNumber":31,"raw":"c--的 undefined behavior 真 tm sb","cooked":"

c--的 undefined behavior 真 tm sb

\n","createdAt":"2024-05-27 03:52:42.120047","updatedAt":"2024-05-27 03:52:42.120047","replyToPostNumber":30,"replyCount":0,"likeCount":1,"wordCount":7,"liked":false,"bookmarked":false,"polls":[],"reactions":[{"reaction":"❤️","count":1,"users":[{"id":4648,"username":"nonsuch","name":null}],"hasReacted":false}],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4668,"username":"IO","name":null,"avatarUrl":"/uploads/image/86/f2/86f267c0aaa73835a999b7cc3a7e7a701605574eda9a2c2a56f5ddbbba64d1a9.avif","admin":true,"moderator":false,"signatureRaw":"","signatureCooked":null}},{"id":34461,"postNumber":32,"raw":"所以编译器优化之后，对于未定义变量会随便取一个值，然后同一次运行只内每次取的也能不一样么","cooked":"

所以编译器优化之后，对于未定义变量会随便取一个值，然后同一次运行只内每次取的也能不一样么

\n","createdAt":"2024-05-27 09:01:18.137986","updatedAt":"2024-05-27 09:01:18.137986","replyToPostNumber":30,"replyCount":2,"likeCount":0,"wordCount":44,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":4648,"username":"nonsuch","name":null,"avatarUrl":"/uploads/image/e0/1a/e01a94ce578d11e72d80c156cd492926e5060c3cac94c7134e40f07ffe4c3e76.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34465,"postNumber":33,"raw":"根据楼上的 link，确实是这样的\n不对不对，是内存，刚没注意到 :grinning_face_with_sweat:","cooked":"

根据楼上的 link，确实是这样的
\n不对不对，是内存，刚没注意到 😅

\n","createdAt":"2024-05-27 10:09:44.619184","updatedAt":"2024-05-27 12:30:28.007601","replyToPostNumber":32,"replyCount":0,"likeCount":0,"wordCount":27,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":2,"publicVersion":2,"deletedAt":null,"isAnonymous":false,"user":{"id":601,"username":"m0rsun","name":null,"avatarUrl":"/uploads/image/37/7f/377f4ae853850691ab959270f01d52509ee07caf808041ddb0ab22488a0e9f85.avif","admin":false,"moderator":true,"signatureRaw":null,"signatureCooked":null}},{"id":34468,"postNumber":34,"raw":"准确地说，如果内存是未初始化的，编译器可以在每次引用这部分内存时假设它是任意值（一般是对于求值来说更“方便”的值，以便进行优化），并且不需要保持前后任意次对同一块未初始化内存区域的这种假设的一致性。\n\n但是这可能还并不能完全解释清楚程序的结果在 1 和 4 之间反复横跳的全部原因，个人建议使用 `-fsanitize` 编译选项，指定适当的 Sanitizer，以便更为严格地捕获异常行为。","cooked":"

准确地说，如果内存是未初始化的，编译器可以在每次引用这部分内存时假设它是任意值（一般是对于求值来说更“方便”的值，以便进行优化），并且不需要保持前后任意次对同一块未初始化内存区域的这种假设的一致性。

但是这可能还并不能完全解释清楚程序的结果在 1 和 4 之间反复横跳的全部原因，个人建议使用 -fsanitize 编译选项，指定适当的 Sanitizer，以便更为严格地捕获异常行为。

\n","createdAt":"2024-05-27 12:09:27.125875","updatedAt":"2024-05-27 12:12:27.74075","replyToPostNumber":32,"replyCount":0,"likeCount":2,"wordCount":166,"liked":false,"bookmarked":false,"polls":[],"reactions":[{"reaction":"🥰️","count":1,"users":[{"id":4648,"username":"nonsuch","name":null}],"hasReacted":false},{"reaction":"❤️","count":2,"users":[{"id":4113,"username":"filehelper","name":"文件传输助手"},{"id":4648,"username":"nonsuch","name":null}],"hasReacted":false}],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":1277,"username":"ucw","name":null,"avatarUrl":"/uploads/image/03/ea/03eac2176be50969b549f432e72df28bd719033042da4c6c41c3fde705e8f13a.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}}],"hasMore":false,"lastReadPostNumber":26,"hasUrlPostNumber":true}