c++ - 我可以知道为什么这段代码没有给出任何输出吗？

Question

请帮助我解决该代码在特定行上无限运行的查询。
它没有给出任何输出，因为在代码末尾，我编写了代码以打印 vector 。即使在我为 vector “result”手动分配了任何值之后，它也没有给出任何输出。为什么会这样呢？

#include<bits/stdc++.h>
using namespace std;

bool authorize(int strValue, int value, int M) 
{
   long int newValue = (strValue - (value * 131) % M);
   if (newValue >= 48 && newValue <= 57)
      return true;
   if (newValue > 65 && newValue <= 90)
      return true;
   if (newValue >= 97 && newValue <= 122)
      return true;
   return false;
}

int hashingfunct(string str, int M) 
{
   long int P, F, sum = 0;
   int len = str.length();
   for (int i = 0; i < len; i++)
   {
      P = pow(131, len - i - 1);
      F = (int)str[i];
      sum += (F * P) % M;
   }
   sum = sum % M;
   return sum;
}

int main()
{
   int n = 5;
   string str1, str2;
   vector<vector<string> > events;
   for (int i = 0; i < n; i++) {
      cin >> str1 >> str2;
      vector<string > temp;
      temp.push_back(str1);
      temp.push_back(str2);
      events.push_back(temp);
   }
   for (int i = 0; i < n; i++) {
      cout << events[i][0] << events[i][1];
   }
   /*
   INPUT FORMAT:
   setpassword 1
   setpassword 2
   setpassword 3
   authorize 49
   authorize 50
   */
   vector<int> result;
   int j = 0;
   long int m = pow(10, 9);
   long int M = m + 7;
   long int value, strValue;
   for (int i = 0; i < events.size(); i++)
   {
      strValue = stoi(events[i][1]);
      if (events[i][0] == "setPassword") {
         value = hashingfunct(events[i][1], M);
      }
      else if (strValue == value)
         result[j++] = 1;
      else if (authorize(strValue, value, M))
         result[j++] = 1;
      else
         result[j++] = 0;
   }

   for (int i = 0; i < result.size(); i++) {
      cout << result[i];
   }
}

Answer 1

您的程序具有完整的未定义行为。
让我们开始第一个问题。在下面的检查代码中

long int value, strValue;  // not initialised
for (int i = 0; i < events.size(); i++)
{
   // ... 
   // here it should have been "setpassword" (i.e. all are small letters)
   if (events[i][0] == "setPassword")
   {
       // if the check fails the `value` never get initialised!
       value = hashingfunct(events[i][1], M);
   }
   // If the `value` not been initialised, check happens with any garbage value here!
   else if (strValue == value) 

   // ...other code
}

您正在检查字符串是否是 "setPassword"而不是 "setpassword"(即在 events vector 中看到，所有字符串都是小写字母)。
如果这是错误的，则 value将永远不会初始化，这意味着将其初始化为 holds any garbage value，因此执行此检查 else if (strValue == value)可以 cause any behaviour to your program (aka Undefined Behaviour)。
其次， vector<int> result;在开始时为空。因此，稍后通过 std::vector::operator[]访问元素

result[j++] = 1;
// ...
result[j++] = 1;
// ...
result[j++] = 0;

触发 access out of bounds (UB) 。在那里，您只需要 result.emplace_back(/*value*/);或 result.push_back(/*value*/);，而不需要冗余变量 j。
简而言之，您需要

#include <iostream>
#include <vector>
#include <string>

// ..other functions
int main()
{
   std::vector<std::vector<std::string> > events { 
      {"setpassword", "1"}, // can be also user input, like in your example
      {"setpassword", "2"},
      {"setpassword", "3"},
      {"authorize", "49" },
      {"authorize", "50" }
   };

   std::vector<int> result;
   const long int M = pow(10, 9) + 7;
   long int value{ 0 }, strValue{ 0 }; // default initialization
   for (const std::vector<std::string> row: events) // better use range-based loop
   {
      strValue = std::stoi(row[1]);
      if (row[0] == "setpassword") {
         value = hashingfunct(row[1], M);

         if (strValue == value)
            result.emplace_back(1);
         else if (authorize(strValue, value, M))
            result.emplace_back(1);
      }
      else
         result.emplace_back(0);
   }
}

作为旁注，

请do not use using namespacestd;

Why should I not #include <bits/stdc++.h>?