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C String Programs

Counting vowels is a common task in text processing. This tutorial guides you through writing a C program that counts vowels in a string.

What Are Vowels?

Vowels are the speech sounds represented by the letters "A," "E," "I," "O," and "U". In the English alphabet, these five letters are crucial for word construction and pronunciation. Recognizing vowels in strings is necessary for many text-processing tasks.

Basic Vowel Count Program in C

Here is the simple C program that counts the number of vowels in a given string:

#include <stdio.h>

int main() {
    char str[100];  // String array declaration with a fixed size
    int count = 0;  // Variable to keep track of vowel count

    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin); // fgets is used to read a string from the user

    for(int i = 0; str[i] != '\0'; i++) { // Iterate over each character
        // Using a if statement to check for vowels, both lowercase and uppercase
        if(str[i]=='a' || str[i]=='e' || str[i]=='i' || 
           str[i]=='o' || str[i]=='u' || str[i]=='A' || 
           str[i]=='E' || str[i]=='I' || str[i]=='O' || str[i]=='U') {
            count++; // Increment count for each vowel

    printf("Number of vowels in the string: %d\n", count); // Output the result

    return 0;

Program Output:

Enter a string: Hello W3schools
Number of vowels in the string: 4

Here in the above example, the vowels are "e" and "o" in "Hello" and "o" and "o" in "w3schools".


  1. The program begins by declaring a char array named str to hold the user's input and an integer count for the number of vowels.
  2. It prompts the user to enter a string and reads the input using fgets to prevent buffer overflow.
  3. A for loop iterates through each character in the string until it hits the null terminator ('\0'), which signifies the end of the string.
  4. An if statement checks for vowels. If one is found, the count is incremented.
  5. After the loop finishes, the program prints out the number of vowels found.

Improved Vowel Count Program in C

While the basic program works well, it can be refined to improve its functionality and make it more efficient:

#include <stdio.h>
#include <ctype.h>  // Include ctype.h for character type functions
#include <string.h> // Include string.h for string functions

#define MAX_STR_LEN 100 // Define a constant for the maximum string length

// Function to count vowels in a given string
int countVowels(const char *str) {
    int count = 0; // Vowel counter
    size_t length = strlen(str); // Calculate the string length once to optimize the loop

    for (size_t i = 0; i < length; i++) { // Use size_t for index, which is the proper type for sizes
        char lower = tolower((unsigned char)str[i]); // Cast to unsigned char to avoid undefined behavior
        // Check for vowels with a switch statement
        switch (lower) {
            case 'a': case 'e': case 'i': case 'o': case 'u':

    return count; // Return the total count of vowels

int main() {
    char str[MAX_STR_LEN]; // Allocate buffer for string input

    printf("Enter a string: "); // Ask for user input
    // Check for input errors and null termination
    if (!fgets(str, MAX_STR_LEN, stdin)) {
        printf("Error reading string.\n"); // Error message
        return 1;

    str[strcspn(str, "\n")] = 0; // Remove trailing newline character, if any

    // Call countVowels and display the result
    printf("Number of vowels: %d\n", countVowels(str));

    return 0;

Enhancements Made:

  1. Function Usage: Separated the logic into a countVowels function for better code organization and reusability.
  2. Performance Optimization: The strlen function is called once before the loop instead of in each iteration.
  3. Type Safety: The tolower function is used with an (unsigned char) cast to avoid undefined behavior with negative char values.
  4. String Handling: Added code to remove the newline character from the input string.
  5. Error Handling: Included a check to ensure fgets successfully reads


You just learned how to count the vowels in a string in a C program. This program can handle any text length and is a fundamental skill for text analysis in C. Try modifying the code to add new features or to count vowels in a particular way.

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