=CS50 PSET 2 CAESAR= How do I convert the key to digit? It gives me this error if I use atoi()-CodePudding

I'm having a problem with atoi() in the only_digits function. I asked on discord and they said that I am passing a char type arg to atoi() which doesn't work since atoi() only takes string or char * (array of char) as arguments. I don't get it. I'm confused with the difference of string and char. Aren't I passing argv[1] (which is a string) to only_digits? Which means inputKey is a string as well? So what do they mean that I am passing a char type arg to atoi()? How exactly do I make atoi() work? I'm stuck with this problem for 2 days now.

// Encrypts text using Caesar's cipher
// ci = (pi   k) % 26

#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

bool only_digits (string inputKey);
char rotate (char plaintext[], int key);

int main(int argc, string argv[])
{
    string p;

    // Make sure program was run with just one command-line argument

    if (argc != 2)
    {
        printf("Enter exactly one input\n");
        return 1;
    }

    // Make sure every character in argv[1] is a digit (DOESN'T WORK)

    /*else if (only_digits(argv[1]))
    {
        printf("Usage: ./caesar key\n");
        return 1;
    }*/

    else
    {
        p = get_string("plaintext: ");
    }

    // Convert argv[1] from a string to an int

    int k = atoi(argv[1]);
    char c[strlen(p)   1];

    // Convert ASCII range down to a value from 0 to 25



    // For each character in the plaintext: (DOESN'T WORK)

    for (int i = 0, n = strlen(p); i <= n; i  )
    {
        // Rotate the character if it's a letter // ci = (pi   k) % 26

        if (isalpha(p[i]))
        {
            if (isupper(p[i]))
            {
                c[i] = ((p[i])   k) % 26;
            }
            else if (islower(p[i]))
            {
                c[i] = ((p[i])   k) % 26;
            }
        }
    }

    printf("ciphertext: %s\n", c);
}

// Function to encrypt plaintext

/*char rotate (char plaintext[], int key)
{
    char c[strlen(plaintext)   1];

    return c;
}*/

// Function to check if key is a digit (DOESN'T WORK)

bool only_digits (string inputKey)
{
    int flag = 0;

    for (int i = 0, n = strlen(inputKey); i < n; i  )
    {
        // Converts string input to int

        int response = atoi(inputKey[i]);

        // Check if it is a digit

        if (!(isdigit(response)))
        {
            flag  ;
        }
    }

    if (flag != 0)
    {
        return false;
    }
    else
    {
        return true;
    }
}

CodePudding user response：

Strings in C are defined as a sequence of nonzero bytes (characters), followed by a null-terminating byte:

char str[] = "hello"; /* in memory: { 'h', 'e', 'l', 'l', 'o', '\0' } */

atoi (ASCII to integer) expects a proper null-terminated string (char *). You can not pass it a single char.

int number = atoi("1672387");

isdigit expects a single character.

int is_true = isdigit('5');

In your program inputKey[i] is a single character. You can test it directly with isdigit, there is no need to convert it to an integer representation beforehand.

You can also simply return early if you encounter a non-digit character.

bool only_digits(string inputKey) {
    for (size_t i = 0, length = strlen(inputKey); i < length; i  )
        if (!isdigit(inputKey[i]))
            return false;

    return true;
}

Note: size_t is the return type of strlen, and the most appropriate type for indexing memory.