CODEWITHSUNDEEP
ccommand-line-arguments
sopatd
sopatd

Reputation: 27

How to check if last command line argument is a valid digit - C programming

I am trying to write a program which will check a text file, but also uses a number which is the last argument passed. I want to do some error checking to make sure that the last argument is a digit.

Upvotes: 0

Views: 522

Answers (1)

Miguel Carvalho
Miguel Carvalho

Reputation: 416

ORIGINAL

I do not know what is the digit() function, but whatever this function is, it will never check if the string argv[argc-1] is a digit successfully. That is because the atoi() function will return zero if argv[argc-1] is not a numeric string as an error, but will also return zero if argv[argc-1] is "0". To verify if argv[argc-1] is one and only one digit, you need to:

  1. Check the length of the string (argv[argc-1][1] == '\0');
  2. Check if the first character is a digit character (isdigit( argv[argc-1][0] )).

Here is the final code.

char *a = argv[argc-1];
if ( (a[1] != '\0') || !isdigit(a[0]) ) {
    printf("No valid digit entered\n");
    exit(1);
}

EDIT #1

My original answer has two problems, as some users mentioned below.

In first place, I assumed that strings in argv cannot be empty. However, if a program is called like ./program "", argv[1] is going to be an empty string.

For demonstration purpose, here is a program that prints in stdout all argv strings and their lengths.

/* program.c */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main (int argc, char **argv) {
    for (int i = 0; i < argc; i++)
        printf("strlen(\"%s\") = %zu\n", argv[i], strlen(argv[i]));
    return EXIT_SUCCESS;
}

Here is the output, confirming argv[1] is an empty string.

$ gcc -Wall -std=c99 -o program program.c
$ ./program ""
strlen("./program") = 9
strlen("") = 0
$

The second problem is related to the usage of isdigit() function. According to the manual (man isdigit on terminal), the argument passed to isdigit() must me an unsigned char.

int isdigit(int c);

These functions check whether c, which must have the value of an unsigned char or EOF, falls into a certain character class according to the specified locale.

Here is the final solution without the problems mentioned in the comments below.

char *a = argv[argc-1];
if ( !isdigit((unsigned char) a[0]) || (a[1] != '\0') ) {
    printf("No valid digit entered\n");
    exit(1);
}

Upvotes: 2

Related Questions