c++ - 在 If 语句中分配变量 [C++]

Question

如果一个变量等于另一个值，我正在尝试分配一个变量。这是我的问题的简化版本:

#include <iostream>
using namespace std;

int main()
{
   int a = 5;
   int b;

   cout << "A is this value (before if):  " << a << endl;
   cout << "B is this value (before if):  " << b << endl;


   if (a==5)
   {
       cout << "A equals five" << endl;
       int b = 6;
       cout << "B is this value (if stmt):  " << b << endl;
   }

cout << "B is this value (outside):  " << b << endl;

   return 0;

}

输出如下:

A is this value (before if):  5
B is this value (before if):  0
A equals five
B is this value (if stmt):  6
B is this value (outside):  0

为什么变量“b”在离开 if 语句后不保持分配为 6？有没有更好的方法来分配它？在我的实际代码中，我将五个变量与 a 进行比较。

Answer 1

您在 if block 中声明了一个新变量。用赋值替换变量声明。

此外，您应该初始化原始的b 变量。打印它的值而不初始化它会导致未定义的行为。

#include <iostream>
using namespace std;

int main()
{
   int a = 5;
   int b = 0;

   cout << "A is this value (before if):  " << a << endl;
   cout << "B is this value (before if):  " << b << endl;

   if (a==5)
   {
       cout << "A equals five" << endl;
       b = 6;
       cout << "B is this value (if stmt):  " << b << endl;
   }

   cout << "B is this value (outside):  " << b << endl;
   return 0;
}