C. invalid operands to binary * (have ‘char *’ and ‘int’)

Question

Hy! I learn #define in C. My program about the absolute value. I have 2 files: ft_abs.h:

#ifndef FT_ABS_H
# define FT_ABS_H

# define ABS(Value) ((Value < 0) ? (Value * (-1)) : (Value))

#endif

and main.c:

#include "ft_abs.h"
#include <stdio.h>
#include <string.h>

int     main(int argc, char **argv)
{
    printf("%s", ABS(argv[1]));
    return (0);
}

compile

gcc -o abs ft_abs.h main.c

and...

In file included from main.c:2:0: main.c: In function ‘main’: ft_abs.h:16:41: error: invalid operands to binary * (have ‘char *’ and ‘int’) # define ABS(Value) ((Value < 0) ? (Value * (-1)) : (Value)) main.c:17:18: note: in expansion of macro ‘ABS’ printf("%s", ABS(argv[1]));

When I change (Value * (-1)) to (-Value) it is not work too (wrong type argument to unary minus). If I change (-Value) to (Value) all ok, but it is nonsense. How should I change the code to make it work (using define of course)?

What result do you expect by taking the absolute value of a c-string? That doesn't make any sense. — tkausl, Oct 01 '18 at 00:01
This is not relevant to your problem, but I'd replace `(Value * (-1))` by `(-Value)`. — Keith Thompson, Oct 01 '18 at 00:07
@KeithThompson Both lack the parentheses around the argument, which are important in macros. — Osiris, Oct 01 '18 at 00:18

Osiris · Accepted Answer · 2018-10-01T00:13:27.427

2

argv[1] is of type char *. It does not make any sense to calculate the absolute value. You probably want to transform the argument to an int type and then calculate the absolute value.

printf("%d", ABS(strtol(argv[1], NULL, 10)));

In addition you should always put parentheses around your arguments when writing macros.

#define ABS(Value) (((Value) < 0) ? ((Value) * (-1)) : (Value))

edited Oct 01 '18 at 00:13

answered Oct 01 '18 at 00:04

Osiris

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@PeteLensky `atoi` has no error handling as descriped by [this answer](https://stackoverflow.com/a/22866001/9017315). `strtol` is probably the better option. – Osiris Oct 01 '18 at 00:15
These facts will be useful. – Pete Lensky Oct 01 '18 at 00:19

C. invalid operands to binary * (have ‘char *’ and ‘int’)

1 Answers1