如何獲得if表達式的哪一部分爲真？

Question

假設我有這樣的代碼：

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

我敢肯定有更好的方法來做到這一點，這裏有一個：

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

如果條件是相互獨立的，你需要單獨檢查他們，或者，如果他們屬於一個變量，你可以使用switch語句

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 3

如果沒有switch只能使用or和if聲明：

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"); 
} 

我不確定這是否是您見過的最有效率的事情，但它會識別導致進入if ...區塊的四個條件中的哪一個。

Answer 4

如果您需要了解程序的原因，即運行取決於哪個條件爲真不同的代碼，你可以做這樣的事情

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

如果你只是想知道調試的原因，只是做了打印。

Answer 5

||是短路評估，所以你可以有這樣的代碼：

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 6

怎麼樣的逗號運算符？ 通過使用邏輯運算符遵循短路評價方法，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);