C++ 试图从 main 调用函数

Question

I'm trying to write a function that will find the reverse of a number, so if 1234 is input, the reverse is 4321.

#include <iostream>
using namespace std;

int reverse(int);

int main()
{
    cout << "This is the special difference calculator. Please enter 
positive integers: " << endl;

reverse();

}

int reverse(int num)
{
    int num, remainder, reversenum;
    cin >> num;
    while (num != 0)
    {
        remainder = num % 10;
        reversenum = reversenum * 10 + remainder;
    }
    return reversenum;
}

I also tried making a variable in main and setting it equal to reverse(int) but it showed it was wrong. I honestly have no idea what I'm doing so any help would be greatly appreciated!

Answer 1

You still do not understand how functions work. Your logic was wrong (as far I was concerned) I coded this for 2 digit numbers. I left you the math equations for 3, 4, and 5 digit numbers. Use nested loops or input manipulation to choose the correct equation. Read the following stuff to learn more about functions. Stack Overflow is not for homework help, next time do more reading and try more in your code before asking for help. Read: http://www.cplusplus.com/doc/tutorial/functions/

#include <iostream>
using namespace std;

int rev_function (int num)
{
  int r;
   //For 2 digit numbers 
  r =  (10)*(num % 10) + (num - (num % 10)) / 10 ;

//For 3 digit numbers 
//f(x) = (100)(x % 10) + ((x - (x % 10)) % 100) + (x - (x % 100)) / 100 

//For 4 digit numbers 
//f(x) = (1000)(x % 10) + ((x - (x % 10)) % 100) * 10 + ((x - (x % 100)) % 1000) / 10 + (x - (x % 1000)) / 1000 

//For 5 digit numbers 
//f(x) = (10000)(x % 10) + ((x - (x % 10)) % 100) * 100 + ((x - (x % 100)) % 1000) + ((x - (x % 1000)) % 10000) / 100 + (x - (x % 10000)) / 10000
  return r;
}

int main ()
{
  int z;
  int num;
  cout << "\nThis is the special difference calculator.";
    cout << "\n Please enter positive integers: ";
    cin >> num;
  z = rev_function (num);
  cout << "The result is " << z;
}

So the output would be:

This is the special difference calculator.
 Please enter positive integers: 12
The result is 21 

Answer 2

There are a few logic errors in your code. Try the following code ( please note the comments ):

#include <iostream>
using namespace std;

int reverse();

int main()
{
    cout << "This is the special difference calculator. Please enter  positive integers: " << endl;

    reverse();

}

int reverse()
{
    int num, remainder, reversenum=0;//reversenum should be initialized to zero
    cin >> num;
    while (num != 0)
    {
        remainder = num % 10;
        num = num/10; // Add this line so that while statement works correctly
        reversenum = reversenum * 10 + remainder;
    }
    return reversenum;
}