Question

我对编码非常陌生，并且遇到了一个带有if-else程序的奇怪障碍。我的代码中有很多注释来解释发生了什么，所以我只是发布代码，但基本问题是当我尝试简化嵌套语句时，我只执行一个条件（覆盖所有其他条件））。例如，如果我想在“x = 4”时发生某些事情，那么只有那个条件被执行 - 即使它是假的！但是，如果我将其声明为“x＆lt; 5＆amp;＆amp; x＆gt; 3”，那么程序运行正常（适用于真假条件的执行），但这看起来很混乱 - 特别是因为我想要多个某些响应输入。

我搜索了网站但找不到这个具体问题，“类似问题”似乎也不适用。那么任何想法是怎么回事？有什么东西可以使程序只执行一个if-else语句并忽略所有其他语句，即使该语句是假的吗？

代码：问题在于结尾附近的两个elseif语句（用注释标记）。

#include <iostream>

using namespace std;

int main()
{//This a program using loop functions to have the user
 //guess variable "guess" (which we'll make "5"), until the user gets it
 //right, and provides hints to help the user out
    cout<<"Can you guess the number I'm thinking? It's between 1 and 10.\n";
    //Prompts the user to input a guess
    int guess;
    //Declares the variable that the user will input
    cin>>guess;
    //Before the loop begins, the user input a guess
    cout<<"\n";
    //This gives us a line break after the answer is submitted
    while (guess != 5){//Guessing incorrectly starts the loop
                       //The program is going to keep asking the user to guess
                       //until the user guesses "5"
        if (guess < 1 ){//The program will warn the user if they're out of range, here it's too low
                cout<<"No, that's too low! Guess a number between 1 and 10.\n";
                cin>>guess; //Allow the user to guess again
                cout<<"\n"; //Add a line break after the input
    }
            else //Now, give responses for other conditions
                if(guess > 10){//Warn the user if they guess too high
                cout<<"Too high! Guess a number between 1 and 10.\n";
                cin>>guess;
                cout<<"\n";
            }
            else //Tell the user if they're getting close.
                 //Not sure why I can't simply say if "guess = 4,6"
                 //Doing so causes only this response to execute, ignoring
                 //the others, except the above ">" and "<" statements
                 //which is why I'm continung to use </> statements here
                if(guess > 5 && guess < 7 || guess < 5 && guess > 3){
                cout<<"VERY close! Keep trying.\n";
                cin>>guess;
                cout<<"\n";
            }
            else //Tell the user if they're sort of close
                 //Again, not sure why I can't just say "guess=3,7"
                if(guess > 2 && guess < 4 || guess > 6 && guess < 8){
                cout<<"Close, but no cigar. Try again.\n";
                cin>>guess;
                cout<<"\n";
            }
            else {//For all other responses, we output this
                cout<<"Guess again.\n";
                cin>>guess;
                cout<<endl; //We need to end the loop here, as these are all the conditions
                //This kept creating a line break. My assumption is that
                //by virtue of ending the loop, a line break was created
            }
    }
    if(guess = 5){//Outside the loop, we need to specify the correct answer
                  //because the loop is only working while the answer in incorrect
        cout<<"Good job. "<<guess<<" is right!\n";
    }
    cin.get();//This keeps the program from closing after completion
}
//Done

Answer 1

有一点是：

 if(guess = 5){

应该是

 if(guess == 5){

您的意思是logical comparison而不是assignment。

另一点是根据C++ Operator Precedence，&&的优先级高于||。您应该在逻辑条件中使用()来清楚地表达您的逻辑：

 if((guess > 2 && guess < 4) || (guess > 6 && guess < 8))

Answer 2

谈论整数时，

(guess > 5 && guess < 7 || guess < 5 && guess > 3)

相当于

(guess == 6 || guess == 4)

因此，至少。重组如下：

while (guess != 5) {
    if (guess < 1 ) {
    }
    else if (guess > 10)
    }
    else if (guess == 6 || guess == 4) {
    }
    else if (guess == 3 || guess == 7) {
    }
    else { // executes for 1, 2, 5 (then leaves loop), 8, 9, 10
    }
}

你的意思是......

if (guess == 5) // equivalence check

不

if (guess = 5) // assignment and "if" becomes true (always)

Answer 3

在C ++中，问if(x = whatever)在语法上不正确。但是，它没有按照你的想法做到。

区别在于：

//is the value of x equal to 4?
if(x == 4) 

//does the assignment of 4 to the variable x return true? (meaning it worked)`
if(x = 4)

所以正在发生的事情是你认为错误的事实。

嵌套的if-else语句只执行一个条件（C ++）

3 个答案: