Getting the warning: control reaches end of non-void function [-Wreturn-type] in the while

Can anybody tell me how I should fix this error? I cannot print the numbers of 10 to 19 individually.

# include <iostream>
using namespace std;

int while_loop(int a)
{
    a = 10;
    while (a < 20)
    {
        a   ;
        return a;
    }
}

int main()
{
    int a, result;
    result = while_loop(a);
    cout << "The result of while_loop is : " << result << endl;
    return 0;
}

CodePudding user response：

The compiler is not perfect in this case:

int while_loop(int a)
{
    a = 10;
    while (a < 20)
    {
        a   ;
        return a;
    }
}

Since you set a initially to 10, this loop will always terminate (after exactly one iteration). However, in the general case, a while loop will run multiple times or never, and that means that you need to add a return statement after the loop.

The code you have written does not use the loop, as it always returns 11 due to the unconditional return a; inside your loop. What you want is probably:

    while (a < 20)
    {
        a  ;
    }
    return a;

Also, your main function passes an uninitialized value to your loop function:

    int a, result; // change that to int a = 10
    result = while_loop(a);

and then you can remove the a = 10 from inside the function.

CodePudding user response：

Mistake 1

When you wrote:

 int a, result;
 result = while_loop(a);//UNDEFINED BEHAVIOR

The variables a and result have garbage value since they are of built in type(in local scope) and you haven't initialized them. And using/accessing that garbage value leads to undefined behavior. So while_loop(a) leads to undefined behavior.

To solve this you should initialize a to 10 as shown below.

int a =10, result;//INITIALIZED a to 10
result = while_loop(a);

Mistake 2

Inside the while_loop function if the condition while (a < 20) is not met then the control flow will not encounter any return statement and so you get the mentioned warning.

To solve this you can move the return a; outside while loop as shown below:

int while_loop(int a)
{
    //a = 10;//NO NEED FOR THIS
    while (a < 20)
    {
        a   ;
        
    }
    return a;//return moved outside while
}

Corrected Program

So the modified program looks like:

# include <iostream>
using namespace std;

int while_loop(int a)
{
    //a = 10;NO NEED FOR THIS
    while (a < 20)
    {
        a   ;
        
    }
    return a;//return MOVED OUTSIDE while
}

int main()
{
    int a = 10, result;//INITIALIZE a to 10
    result = while_loop(a);
    cout << "The result of while_loop is : " << result << endl;
    return 0;
}