Why is scanf always giving me zero for my first input?

Question

I'm a Java programmer trying to learn C for a class and man, I can't wrap my head around this. There's no reason why this shouldn't work and yet it doesn't. I'm trying to write a simple calculator app, and no matter how I write it, the first number I input (variable a) ends up being 0, but the 2nd one is fine. With 5 + 6 as input, the output is 6. What am I missing?

#include <stdio.h>

int main()
{
    long int a, b, c;

    char op;

    c = 0;

    printf("Enter the expression: ");
    scanf("%ld %s %ld", &a, &op, &b);

    switch(op){
        case('+'): c = a+b; break;
        case('-'): c = a-b; break;
        case('*'): c = a*b; break;
        case('/'): c = a/b; break;

        default: break;
    }

    printf("\n%ld", c);

    return 0;

}

`%c` is for a char, not `%s`. `scanf` also has a return value you should be checking to make sure it read the number of items you expect. — Retired Ninja, Sep 07 '18 at 03:53
thanks. fixed it, but I still get the same output. Every scanf tutorial I looked up online tells me this is the proper way to set it up — Nihilish, Sep 07 '18 at 04:08
Just got it to work. For some reason, when I remove the spaces between %ld %c and %ld and adjust my input accordingly (5+6), it works. — Nihilish, Sep 07 '18 at 04:13
Please provide an example of the *input*/a session as a copy paste from the console where you typed the input and the corresponding output was shown — Antti Haapala -- Слава Україні, Sep 07 '18 at 05:46

score 1 · Answer 1 · edited Sep 07 '18 at 04:22

The scanf function returns the value of the macro EOF if an input failure occurs before the first conversion (if any) has completed. Otherwise, the function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

So always check if scanf is successful and if the number of items are correctly assigned.

And it is good to know about the conversion specifiers (what follows after % character) and the length modifiers and their meanings in C.

Check out C Committee Draft (N1570) sections 7.21.6.2 The fscanf function and 7.21.6.4 The scanf function and you will get a good idea of how to use scanf.

score 0 · Accepted Answer · answered Sep 07 '18 at 04:44

0

I am not sure if you are giving 5<space>+<space>6 or 5+6(without spaces) as input. If you are doing the first, then try doing the second one, i.e, number1+number2 without spaces between the characters. And remove the spaces in the scanf("%ld %s %ld") too.

Hope this helps.

answered Sep 07 '18 at 04:44

Harshith Rai

3,018
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Indeed, removing the spaces in the input and the scanf declaration did the trick. But why? – Nihilish Sep 07 '18 at 15:31
I think this link might be helpful @Nihilish. https://stackoverflow.com/questions/47903523/white-space-in-format-string-of-scanf – Harshith Rai Sep 10 '18 at 05:31

Why is scanf always giving me zero for my first input?

2 Answers2