C Program to convert lowercase character to uppercase & vice versa

For a better understanding, we always recommend you to learn the basic topics of C programming listed below:

How to convert lower case to upper case in C?

In this c program, we have to read a sentence and replace lowercase characters with uppercase and vice-versa. It means we have to change the uppercase letters into lowercase and lowercase characters to uppercase.

What are 'islower' and 'isupper' functions for?

For checking whether a character is uppercase or lowercase we can use the built-in function of c islower() and isupper(). The 'islower()' function will return true if the given character is in lowercase. The "isupper()" function will return true if the given character is in uppercase.

What are 'toupper' and 'tolower' functions for?

To convert a character to uppercase we can use "toupper()" and to convert a character to lowercase we can use "tolower()".For using all functions we have to include the header file "ctype.h".

The logic behind this c program is to read the sentence from the user and find out the length of the sentence by just traversing it. Put the length of the sentence into the variable count. After that display the user-given sentence. Then take each character of the string and check it is uppercase or lowercase by using the function "isupper()" and "islower()".

If the character is in uppercase it will change to lowercase by calling the function "tolower()". If the character is in lowercase it will change to uppercase by using "toupper()". After the conversion, it will display the character by using the function putchar(ch). It will repeat the conversion and display the character until it reaches the end of the sentence.


STEP 1: Include the Header files to use the built-in functions in the C program.

STEP 2: Include the Header file "ctype.h" to perform the character testing like "isupper()","islower()".

STEP 3: Declare the Character Array Sentence[100] .

STEP 4: Accept the Sentence from the user to the Array sentence.

STEP 5: Find out the Length of the Sentence by just traversing through it by using a for loop

STEP 6: At the end of the Sentence add a Null character.

STEP 7: Display the Given Sentence.

STEP 8: By using a for loop and check each Character of the String is 'uppercase' or 'lowercase'. If the Character is Uppercase then change it into Lowercase. If the Character is in Lowercase change it into Uppercase by using the 'tolower()','toupper()' function.

STEP 9: After the conversion, it displays each character of the string by using the function putchar(ch).

C Source Code

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

void main()
  char sentence[100];
  int count, ch, i;
  printf("Enter a sentence\n");
  for (i = 0;
    (sentence[i] = getchar()) != '\n'; i++) /* enters the user sentence to change the case*/ {
  sentence[i] = '\0'; /* add the null character at the end of sentence */
  count = i; /*shows the number of chars accepted in a sentence*/
  printf("The given sentence is   : %s", sentence);
  printf("\nCase changed sentence is: ");
  for (i = 0; i < count; i++) {
    ch = islower(sentence[i]) ? toupper(sentence[i]) : tolower(sentence[i]); /* check each letter of sentence using islower function if it is lowercase use toupper function to change it to uppercase else use tolower function to change uppercase to lowercase */
} /*End of main()*/


Enter a sentence
World is Beautiful

The given sentence is: World is Beautiful
Case changed sentence is: wORLD IS bEAUTIFUL