C Program to Check if String is Alphabetic

Use a loop to check each character with isalpha() from ctype.h and if all are alphabetic, the string is alphabetic; otherwise, it is not.

Examples

InputHelloWorld

OutputThe string is alphabetic.

InputHello123

OutputThe string is not alphabetic.

Input

OutputThe string is alphabetic.

How to Think About It

To check if a string is alphabetic, look at each character one by one. If every character is a letter (A-Z or a-z), then the string is alphabetic. If you find any character that is not a letter, stop and say the string is not alphabetic.

Algorithm

1

Get the input string from the user.

2

Start from the first character and check if it is alphabetic.

3

If any character is not alphabetic, stop and return 'not alphabetic'.

4

If all characters are alphabetic, return 'alphabetic'.

Code

c

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

int main() {
    char str[100];
    int i = 0, isAlpha = 1;

    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    while (str[i] != '\0' && str[i] != '\n') {
        if (!isalpha((unsigned char)str[i])) {
            isAlpha = 0;
            break;
        }
        i++;
    }

    if (isAlpha)
        printf("The string is alphabetic.\n");
    else
        printf("The string is not alphabetic.\n");

    return 0;
}

Output

Enter a string: HelloWorld
The string is alphabetic.

Dry Run

Let's trace the input 'Hello123' through the code

1

Input string

str = 'Hello123\n', i = 0, isAlpha = 1

2

Check characters one by one

Check 'H' -> alphabetic, i=1 Check 'e' -> alphabetic, i=2 Check 'l' -> alphabetic, i=3 Check 'l' -> alphabetic, i=4 Check 'o' -> alphabetic, i=5 Check '1' -> not alphabetic, set isAlpha=0, break loop

3

Result

isAlpha = 0, print 'The string is not alphabetic.'

Index	Character	isalpha()	isAlpha
0	H	true	1
1	e	true	1
2	l	true	1
3	l	true	1
4	o	true	1
5	1	false	0

Why This Works

Step 1: Using isalpha()

The function isalpha() checks if a character is a letter from A-Z or a-z.

Step 2: Loop through string

We check each character until we find a non-letter or reach the end.

Step 3: Decide result

If all characters are letters, the string is alphabetic; otherwise, it is not.

Alternative Approaches

Manual ASCII check

c

#include <stdio.h>

int main() {
    char str[100];
    int i = 0, isAlpha = 1;

    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    while (str[i] != '\0' && str[i] != '\n') {
        if (!((str[i] >= 'A' && str[i] <= 'Z') || (str[i] >= 'a' && str[i] <= 'z'))) {
            isAlpha = 0;
            break;
        }
        i++;
    }

    if (isAlpha)
        printf("The string is alphabetic.\n");
    else
        printf("The string is not alphabetic.\n");

    return 0;
}

This method avoids using <code>ctype.h</code> but requires manual range checks.

Using recursion

c

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

int isAlphabetic(const char *str) {
    if (*str == '\0' || *str == '\n') return 1;
    if (!isalpha((unsigned char)*str)) return 0;
    return isAlphabetic(str + 1);
}

int main() {
    char str[100];
    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    if (isAlphabetic(str))
        printf("The string is alphabetic.\n");
    else
        printf("The string is not alphabetic.\n");

    return 0;
}

This recursive approach is elegant but uses more stack space for long strings.

Complexity: O(n) time, O(1) space

Time Complexity

The program checks each character once, so time grows linearly with string length.

Space Complexity

Only a few variables are used, so space is constant regardless of input size.

Which Approach is Fastest?

The loop with isalpha() is fast and simple; manual ASCII checks are similar but less readable; recursion uses more memory and is slower for long strings.

Approach	Time	Space	Best For
Using isalpha()	O(n)	O(1)	Readability and standard use
Manual ASCII check	O(n)	O(1)	Avoiding library functions
Recursion	O(n)	O(n)	Elegant code but uses more memory