global and local variable

Question

#include<>
int calc(int,int,char);
void main()
{
    int a,b;
    char c;
    printf("enter 2 nos");
    scanf("%d%d",&a,&b);
    printf("enter op");
    scanf("%s",&c);
    printf("the ans is %d\n",calc(a,b,c));
}
int calc(int a,int b,char c)
{
    int ans;
    switch(c)
    {
        case'+':ans=a+b;break;
        case'-':ans=a-b;break;
    }
    return ans;
}

why does this program give the output as b...it works when i give a, b and c as global variables...what change should i do if i want them as local variables...using functions

See [What should `main()` return in C and C++](http://stackoverflow.com/questions/204476/) for why `void main()` is generally not a good idea. — Jonathan Leffler, Feb 28 '16 at 08:14

score 2 · Answer 1 · answered Feb 28 '16 at 07:03

2

scanf("%s",&c); causes undefined behavior. You are storing at least two characters ['+', '\0'] and you have only allocated space for one.

You might consider scanf(" %c", &c);. Note that I intentionally added a space in the format string to eat any whitespace that the user might include.

answered Feb 28 '16 at 07:03

Bill Lynch

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but it works when i declare a and b out of the main..i mean as global variables – Aishwarya Shah Feb 28 '16 at 07:12
3

It's still undefined behavior if it's a global variable. A possible result from undefined behavior is that your code appears to work. – Bill Lynch Feb 28 '16 at 07:16
i want it to work when i declare a,b and c within main..how do i do that? – Aishwarya Shah Feb 28 '16 at 07:18
@AishwaryaShah by following the advice in this answer – M.M Feb 28 '16 at 08:02

Jabberwocky · Accepted Answer · 2016-02-28T07:55:40.593

char c;
...
scanf("%s",&c);

Is plain wrong, even if c is a global variable.

Your scanf will take input from the keyboard and store the string you type at the address given as second parameter (in your case the address of c). But c has only space for a single character, and your string inputted with scan will take at least 2 characters (the one you type and th terminating zero).

In your case you get undefined behaviour, which means that your program may

appear to work
crash each time you run it
appear to work with a certain string you type, and crash with another
or any other odd behaviour

You need this:

char c[10];
...
scanf("%9s",c);
...
printf("the ans is %d\n",calc(a,b,c[0]));

which lets allows you to input a string of length 9 (9 characters + terminating zero = 10).

global and local variable

2 Answers2