Converting user input to an array of characters, and filtering letters from other characters?

Question

#include "stdafx.h"
#include "stdlib.h"
#include <ctype.h>

int num = 0;
int i = 0;
int ch = 0;

int letter_index_in_alphabet(int ch) {

        if (isalpha(ch) == true) {
            char temp_str[2] = { ch };
            num = strtol(temp_str, NULL, 36) - 9;
            printf("%d is a letter, with %d as its location in the alphabet!", ch, num);
        }
        else {
            return -1;
        }

}

int main()
{
    char input_str[10];
    printf("Please enter a series of up to 10 letters and numbers: \n");

     fgets(input_str, 10, stdin);

    for (i == 0; i <= 10; i++) {
        ch = input_str[i];
        letter_index_in_alphabet(ch);

    }

    return 0;
}

Hello everyone, this is my first post on SOF! The goal of this program is to read characters from the standard input to EOF. For each character, report if it is a letter. If it is a letter, print out its respective index in the alphabet ('a' or 'A' = 1, 'b' or 'B' = 2..etc). I have been searching some other posts on stackoverflow and this has helped me get this far(using fgets and strtol functions). I have no visible syntax errors when I run this code, but after I enter a string of characters (ex: 567gh3fr) the program crashes.

Basically, I am trying to use 'fgets' to bring each character entered into a string with the appropriate index. Once I have that string, I check each index for a letter and if it is, I print the number assigned to that letter of the alphabet.

Any help or insight into why this isn't working as intended is greatly appreciated, Thanks!

Answer 1

You have a few problems.

First, char input_str[10] is only big enough for the user to enter 9 characters, not 10, because you need to allow one character for the null byte that ends a string.

Second, your loop goes too far. For a string with 10 characters, indexes go up to 9, not 10. It also should stop when it gets to the null byte, since the user might not have entered all 9 characters.

To get the position in the alphabet, you can simply subtract the value of A or a from the value of the character. Use tolower() or toupper() to convert the character to the case that you're going to use. Your method works, but it's overly complicated and confusing.

letter_index_in_alphabet() is declared to return int. But when the character is a letter, it doesn't execute a return statement. I'm not sure why it's supposed to return something, since you never use the return value, but I've changed it to return the position (maybe the caller should be the one that prints the message, so the function just does the calculation).

In the for loop, it should be i = 0 to perform an assignment, not i == 0 which is comparison.

You also shouldn't use global variables so much. And system header files should have <> around them, not "".

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

int letter_index_in_alphabet(int ch) {

    if (isalpha(ch)) {
        int num = tolower(ch) - 'a' + 1;
        printf("%d is a letter, with %d as its location in the alphabet!\n", ch, num);
        return num;
    } else {
        return -1;
    }
}

int main()
{
    char input_str[10];
    printf("Please enter a series of up to 9 letters and numbers: \n");

    fgets(input_str, sizeof(input_str), stdin);

    for (int i = 0; input_str[i]; i++) {
        letter_index_in_alphabet(input_str[i]);
    }

    return 0;
}