{"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":501,"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}}],"hasMore":true,"lastReadPostNumber":1,"hasUrlPostNumber":false}