Iterating through char* and comparing each char to another char

Question

I'm not well versed in c and I'm having issues with

1. iterating through a char* character by character
2. correctly comparing the individual character with another character

given a string like "abcda", I want to count the number of "a"s and return the count

    #include <stdio.h>
    #include <unistd.h>
    #include <string.h>

    int main(int argc, char** argv){
        char* string_arg;
        int counter = 0;
        if(argc == 2){
            for(string_arg = argv[1]; *string_arg != '\0'; string_arg++){
                printf(string_arg);
                printf("\n");
                /*given abcda, this prints
                abcda                    a
                bcda                     b
                cda        but i want    c
                da                       d 
                a                        a */

                if(strcmp(string_arg, "a") == 0){ //syntax + logical error
                    counter++;
                }
            }
         printf(counter);
         }
         else{
             printf("error");
         }
         return(0);
    }

I'm also not supposed to use strlen()

How do I compare one character at a time, correctly?

Answer 1

if (strcmp(string_arg, "a") == 0) { 
     counter++;
}

The call to strcmp is not appropriate in your case, as it compares strings. With this statement you compare a string starting at the element pointed to by string_arg with the string "a", not the character constant 'a'. Note that "a" is equal to 'a'+ '\0'.

Instead, You need to compare *string_arg with 'a':

if (*string_array == 'a') { 
     counter++;
}

puts(string_arg); prints a string. That is not what you want. You want to print only a single character. Use printf("%c", *string_arg); instead to print a character.

Note that something like printf(string_arg); is dangerous. Always use a format specifier: printf("%c", *string_arg);. Reasony why is explained under the following link:

Why is printf with a single argument (without conversion specifiers) deprecated?

---

This shall be what you want:

#include <stdio.h>
#include <unistd.h>
#include <string.h>

int main (int argc, char** argv) {

    char* string_arg;
    int counter = 0;

    if (argc == 2){

        for (string_arg = argv[1]; *string_arg != '\0'; string_arg++) {

            printf("%c", *string_arg);
            printf("\n");

            if (*string_arg == 'a') { 
                counter++;
            }
        }

        printf("%d times character 'a' encountered.", counter);
     }
     else {
        printf("Error: No second argument at the program invocation!");
     }

     return 0;
}