C Exercises: Find the first capital letter in a string

Last update on August 07 2025 12:50:42 (UTC/GMT +8 hours)

20. Find First Capital Letter Recursion Variants

Write a program in C to find the first capital letter in a string using recursion.

Pictorial Presentation:

C Exercises: Find the first capital letter in a string

Sample Solution:

#include <stdio.h>
#include <string.h>
#include <ctype.h> 
 
char checkCapital(char *);
 
int main()
{
    char str1[20], singLet;
	
    printf("\n\n Recursion : Find the first capital letter in a string :\n");
	printf("------------------------------------------------------------\n"); 	
 
    printf(" Input a string to including one or more capital letters : ");
    scanf("%s", str1);
    singLet = checkCapital(str1);
    if (singLet == 0)
    {
        printf(" There is no capital letter in the string :  %s.\n", str1);
    }
    else
    {
        printf(" The first capital letter appears in the string %s is %c.\n\n", str1, singLet);    }
        return 0;
    }
    char checkCapital(char *str2)
    {
        static int i = 0;
        if (i < strlen(str2))
        {
            if (isupper(str2[i]))
            {
                return str2[i];
            }
            else
            {
                i = i + 1;
                return checkCapital(str2);
            }
        }
        else return 0;
    }

Sample Output:

Recursion : Find the first capital letter in a string :                                                      
------------------------------------------------------------                                                  
 Input a string to including one or more capital letters : testString                                         
 The first capital letter appears in the string testString is S.

Explanation:

char checkCapital(char *str2)
    {
        static int i = 0;
        if (i < strlen(str2))
        {
            if (isupper(str2[i]))
            {
                return str2[i];
            }
            else
            {
                i = i + 1;
                return checkCapital(str2);
            }
        }
        else return 0;

The function checkCapital() takes a pointer to a string ‘str2’ as input and returns the first capital letter in the string, if there is any.

The function uses recursion to iterate through the characters in the string.
It starts by initializing a static variable i to zero.
Then, it checks if the current character at index i in the string is a capital letter using the isupper() function from the ctype.h library.
If it is a capital letter, the function returns that letter.
Otherwise, it increments i by one and calls itself recursively to check the next character in the string.

If the function reaches the end of the string without finding a capital letter, it returns zero.

Time complexity and space complexity:

The time complexity of this function is O(n), where n is the length of the input string, since it needs to iterate through all the characters in the string.

The space complexity is O(1), since it only uses a constant amount of additional memory regardless of the size of the input.

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to find the last capital letter in a string using recursion.
Write a C program to return the index of the first uppercase letter in a string recursively.
Write a C program to check recursively if a string contains any capital letters and print a boolean result.
Write a C program to extract all capital letters from a string using recursion and display them.

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