How does one correctly format command line arguments? - c

Question

I am trying to construct a simple programme to acquaint myself with command line functionality, I seem to have formatted something incorrectly, but I find it very difficult to understand precisely what the resulting error message means. My intenion is to create a programme which checks that all the characters in the command line are digits. Here is my code:
#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
int main(int argc, char *argv[])
{
for (int i = 0; i < strlen(argv); i++)
{
if (!isalnum(argv[i]))
{
printf("Please provide letters or numbers only");
}
else
{
printf("Success!");
}
}
}
The error message I receive when I attempt to compile is: c:9:32: error: incompatible pointer types passing 'char **' to parameter of type 'const char *'; dereference with * [-Werror,-Wincompatible-pointer-types].

Answer 1

argv is an array (pointer) of pointers to strings.
isalnum is checking a character, not a whole string, so you will need two loops: to check each strings and to check each characters in the strings.
This code won't get the warning:
#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
int main(int argc, char *argv[])
{
for (int c = 1; c < argc; c++) // loop for checking each strings
{
for (int i = 0; i < strlen(argv[c]); i++) // loop for checking each characters in the strings
{
if (!isalnum(argv[c][i]))
{
printf("Please provide letters or numbers only");
}
else
{
printf("Success!");
}
}
}
}
But you may want this code with implovements:
Print the message only once instead of for each characters.
Use size_t for looping until the length of the string.
Call strlen() once before the loop instead of calling in each iteration.
Use puts instead of printf to print newline character at the end of output.
Add return 0; to clarify that the code returns 0.
Remove unnecessary #includes, including non-standard one.
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(int argc, char *argv[])
{
int all_ok = 1;
for (int c = 1; c < argc; c++)
{
size_t len = strlen(argv[c]);
for (size_t i = 0; i < len; i++)
{
all_ok = all_ok && isalnum(argv[c][i]);
}
}
if (!all_ok)
{
puts("Please provide letters or numbers only");
}
else
{
puts("Success!");
}
return 0;
}