How does this C++ code return 0,0,3

Question

Here is the code :

#include<iostream>
using namespace std;
int a, b;
int f(int c)
{
    int n = 1;
    for (int i = 0; i < c, i++;)
    {
        int a = n + i;                              
    }                                     
    return a;
}
void main()
{
    int i = 3;
    int b = f(i);          
    cout << a << b << i;
}

The output is 0,0,3

I understand the a = 0 because it is a global variable and defaults to 0, and i = 3, but I can't figure out how or why b = 0

Name your variables better and you will find out. You have two declarations of `a`, so your call to `f()` returns the global one. — Tas, Mar 20 '18 at 02:50
`-Wall` reveals the bug as well: `weird.cpp:9:13: warning: unused variable ‘a’ [-Wunused-variable]` — Stephen Newell, Mar 20 '18 at 02:51
you have two 'a' one global and one local to that 'for-loop' at the bottom of 'f' it returns the global 'a' that is 0 — nullqube, Mar 20 '18 at 02:51
Your loop in `f` has a comma, shouldn't that be a semicolon? — Robert, Mar 20 '18 at 02:52

score 2 · Accepted Answer · answered Mar 20 '18 at 02:51

2

In your program, f() always returns 0. The a inside the for loop is a different a from the global variable (it "shadows" it). Therefore, local variable b in main() (which shadows global b) is initialized to 0 by the call to f().

answered Mar 20 '18 at 02:51

Carl Norum

219,201
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422
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score 1 · Answer 2 · answered Mar 20 '18 at 02:57

1

Because in for loop of function f, a is a local varaible. Its scople is effective only in for loop. So return value is global a; So return 0;

answered Mar 20 '18 at 02:57

PZY

131
1
16

Max Vollmer · Answer 3 · 2018-03-20T04:30:04.937

While others have already pointed out that you write to a local a in your loop, but return the global a, it's also worth nothing that even if you fix that, your loop will actually never enter it's body, because i < c, i++ evaluates to 0, which will be interpreted as false. (See What does the comma operator , do? .)

You need to fix your entire loop to this:

for (int i = 0; i < c; i++)
{
    a = n + i;                              
}

How does this C++ code return 0,0,3

3 Answers3