我刚刚学习了 C++ 函数；我可以在函数返回值上使用 if 语句吗？

I just learned about C++ functions; can I use if statements on function return values?

提问by Sagistic

What I am confused on is about the isNumPalindrome() function. It returns a boolean value of either true or false. How am I suppose to use that so I can display if it's a palindrome or not. For ex. if (isNumPalindrome == true) cout << "Your number is a palindrome"; else cout << "your number is not a palindrome.";

我对 isNumPalindrome() 函数感到困惑。它返回一个 true 或 false 的布尔值。我该如何使用它，以便我可以显示它是否是回文。例如。if (isNumPalindrome == true) cout << "Your number is a palindrome"; else cout << "your number is not a palindrome.";

#include "stdafx.h"


int _tmain(int argc, _TCHAR* argv[])
{
 return 0;
}

#include <iostream>
#include <cmath>

using namespace std;

int askNumber();
bool isNumPalindrome();

int num, pwr;

int main()
{
 askNumber();

 return 0;
}

bool isNumPalindrome()
{
 int pwr = 0;

 if (num < 10)
  return true;
 else
 {
  while (num / static_cast<int>(pow(10.0, pwr)) >=10)
   pwr++;
  while (num >=10)
  {
   int tenTopwr = static_cast<int>(pow(10.0, pwr));

   if ((num / tenTopwr) != (num% 10))
    return false;
   else
   {
    num = num % tenTopwr;
    num = num / 10;
    pwr = pwr-2;
   }
  }

  return true;
 }
}

int askNumber()
{
 cout << "Enter an integer in order to determine if it is a palindrome: " ; 
 cin >> num;
 cout << endl;

if(isNumPalindrome(num))
{
cout << "It is a palindrome." ;
cout << endl;
}
else
{
cout << "It is not a palindrome." ;
cout << endl;
}
 return num;
}

回答by miked

The return value for a function can be used just like a variable of the same type.

函数的返回值可以像相同类型的变量一样使用。

Your main program should look something like this:

你的主程序应该是这样的：

int main()
{
  int num=askNumber();
  bool isPal=isNumPalindrome(num);
  if (isPal)
  {
    //do something
  }
  else
  {
    //do something else
  }

  return 0;
}

or you could be even more succinct:

或者你可以更简洁：

int main()
{
  if (isNumPalindrome(askNumber()))
  {
    //do something
  }
  else
  {
    //do something else
  }

  return 0;
}

What you don't want to do is use those global variables you defined. In more complicated programs that will be a recipe for disaster.

您不想做的是使用您定义的那些全局变量。在更复杂的程序中，这将是灾难的秘诀。

Edit: you'll want to make sure you edit your isNumPalindrome function to accept the number it's working with:

编辑：您需要确保编辑 isNumPalindrome 函数以接受它正在使用的数字：

bool isNumPalindrom(int num)
{
   ...
}

回答by Paulo Santos

Yes, you can do such thing.

是的，你可以做这样的事情。

Actually you could do just...

其实你可以只...

if (isNumPalindrome()) { ... }

回答by Rachel

if(isNumPalindrome())
{
    cout << "Your number is a palindrome";
}
else
{
    cout << "Your number is not a palindrome";
}

回答by Chris H

when a function returns a type you can think of that function as being replaced by the return value and type. so for you:

当函数返回一个类型时，您可以认为该函数被返回值和类型替换。所以对你来说：

isNumPalindrome() -> {true/false}

isNumPalindrome() -> {true/false}

so you can write for example:

所以你可以写例如：

if(isPalindrome())
  cout<<"it is!"<<endl;
else
  cout<<"it is not :("<<endl;

回答by outis

First, you shouldn't use globalsfor numand pwr; you should pass them as arguments to functions:

首先，你不应该对numand使用全局变量pwr；您应该将它们作为参数传递给函数：

bool isNumPalindrome(int num);
...
num = askNumber();
isNumPalindrome(num);

Second, there's no need to compare a boolean value with true(or false); simply use the boolean value.

其次，不需要将布尔值与true(或false)进行比较；只需使用布尔值。

It's hard to tell what exact syntax you're trying to use in your example "if" statement, but one thing you can't do is have an "if" statement in an expression. In C++, there are expressions and statements. Expressions have values; statements don't.

很难说出您在示例“if”语句中尝试使用的确切语法，但您不能做的一件事是在表达式中包含“if”语句。在 C++ 中，有表达式和语句。表达式有值；陈述没有。

// valid
if (isNumPalindrome(num)) {
    std::cout << '"' << num << "\" is a palindrome." << std::endl;
} else {
    std::cout  << '"' << num << "\" is not a palindrome." << std::endl;
}

// invalid
std::cout << '"' << num << (if (isNumPalindrome(num)) {
    "\" is a palindrome.";
} else {
    "\" is not a palindrome.";
}) << std::endl;

// valid, but not recommended
std::cout << '"' << num << "\" is " << (isNumPalindrome(num) ? "" : "not ") << "a palindrome." << std::endl;

For the ternary operator (?:), read "To ternary or not to ternary?"

对于三元运算符 (?:)，请阅读“要三元还是不要三元？”

回答by Corwin

As many people have stated, the return value of a function essentially becomes the function's value.

正如很多人所说，函数的返回值本质上就是函数的值。

Here's an example of a ternary operation for printing the result.

这是用于打印结果的三元运算的示例。

cout << "The number " << (isNumPalindrome()) ? "is a palindrome" : "is NOT a palindrome";

This is a bit weird looking for a lot of beginners, but it shows how ternary operators can be used to print conditional responses.

对于很多初学者来说，这有点奇怪，但它展示了如何使用三元运算符来打印条件响应。

回答by Dominic.wig

yet another solution ;-)

另一个解决方案;-)

#include <iostream>
#include <sstream>
#include <algorithm>

bool is_palindrome( const int num )
{
    std::ostringstream os;
    os << num;
    const std::string& numStr = os.str();
    std::string reverseNumStr = numStr;
    std::reverse( reverseNumStr.begin(), reverseNumStr.end() );
    const bool result = ( numStr == reverseNumStr );
    return result;
}

int main()
{
    int num = 0;
    std::cout << "Enter an integer in order to determine if it is a palindrome: ";  
    std::cin >> num; 

    std::string inset;
    if( !is_palindrome( num ) )
    { 
        inset = "not ";
    } 
    std::cout << "It is " << inset << "a palindrome." << std::endl; 
}

回答by sinek

In a single sentence:

一句话概括：

"As the condition of the ifstatement you can use any expressionwhose result, once when expression is evaluated, can be implicitlyconverted to 'bool'."

“作为if语句的条件，您可以使用其结果的任何表达式，一旦表达式被评估，就可以隐式转换为 ' bool'。”

回答by liewl

You could call isNumPalindrome() inside askNumber(), and have the returned value from isNumPalindrome() be used in a conditional test. It's better to pass num as an argument to isNumPalindrome though: isNumPalindrome(int num)

您可以在 askNumber() 中调用 isNumPalindrome()，并将 isNumPalindrome() 的返回值用于条件测试。最好将 num 作为参数传递给 isNumPalindrome： isNumPalindrome(int num)

int askNumber()
{
 cout << "Enter an integer in order to determine if it is a palindrome: " ; 
 cin >> num;
 if(isNumPalindrome(num)){
   cout << "it is a palindrome";
 }
 cout << endl;

 return num;
}

then main can call only askNumber()

那么 main 只能调用 askNumber()