c++ - 如何获得 if 表达式的哪一部分为真？

Question

假设我有如下代码：

if(condition1 || condition2 || condition 3 || condition4)
{
// this inner part will be executed if one of the conditions is true.
// Now I want to know by which condition this part is executed.
}

Answer 1

我相信有更好的方法可以做到这一点，这里有一个：

int i = 0;
auto check = [&i](bool b)->bool
{
    if (!b) ++i;
    return b;
};

if (check(false) || // 0
    check(false) || // 1
    check(true)  || // 2
    check(false))   // 3
{
    std::cout << i; // prints 2
}

Answer 2

|| 是短路评估，所以你可以有这样的代码：

if(condition1 || condition2 || condition 3 || condition4)
{
    if (condition1 ) 
    {
            //it must be condition1 which make the overall result true
    }
    else if (condition2)
    {
            //it must be condition2 which make the overall result true
    }
    else if (condition3)
    {
            //it must be condition3 which make the overall result true
    }
    else
    {
            //it must be condition4 which make the overall result true
    }

    // this inner part will executed if one of the condition true. Now I want to know by which condition this part is executed.
}
else
{

}

Answer 3

如果条件相互独立，则需要单独检查，或者，如果它们属于一个变量，则可以使用 switch 语句

bool c1;
bool c2
if ( c1 || c2 )
{
    // these need to be checked separately
}


int i; // i should be checked for multiple conditions. Here switch is most appropriate
switch (i)
{
    case 0: // stuff
            break;
    case 1: // other stuff
            break;
    default: // default stuff if none of the conditions above is true
}

Answer 4

没有 aswitch你只能使用orandif语句：

if(condition1 || condition2 || condition 3 || condition4) {
  // this inner part will executed if one of the condition true. 
  //Now I want to know by which condition this part is executed.
  if ( condition1 || condition2 ) { 
    if ( condition1 ) 
       printf("Loop caused by 1");
    else 
       printf("Loop caused by 2");
  else 
    if ( condition3) 
       printf("Loop caused by 3");
    else
       printf("Loop caused by 4");
}

我不确定这是你见过的最有效的方法，但它会确定四种情况中的哪一种导致进入if ...区块。

Answer 5

如果您出于编程原因需要知道，即根据哪个条件为真运行不同的代码，您可以执行以下操作

if (condition1)
{
    ...
}
else if (condition2)
{
    ...
}
else if (condition3)
{
    ...
}
else if (condition4)
{
    ...
}
else
{
    ...
}

如果您只是出于调试原因想知道，只需打印输出即可。

Answer 6

逗号运算符呢？通过使用该逻辑运算符遵循短路评估方法，以下工作正常：

 int w = 0; /* w <= 0 will mean "no one is true" */
 if ( (w++, cond1) || (w++, cond2) || ... || (w++, condN) )
   printf("The first condition that was true has number: %d.\n", w);