Write a C program to read a sentence and replace lowercase characters with uppercase and vice versa without using fgets function and without using the pointer

Sure, here is a simple C program that reads a sentence and replaces lowercase characters with uppercase and vice versa. This program uses the getchar() function to read the characters one by one and the putchar() function to print the characters. It doesn't use the fgets function or pointers.

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

int main() {
    char ch;

    printf("Enter a sentence: ");

    while ((ch = getchar()) != '\n') {
        if (islower(ch)) {
            putchar(toupper(ch));
        } else if (isupper(ch)) {
            putchar(tolower(ch));
        } else {
            putchar(ch);
        }
    }

    return 0;
}

Here's how it works:

1. The program first includes the necessary header files: stdio.h for input/output functions and ctype.h for character handling functions.

2. The main function starts. It declares a character variable ch to hold the characters read from the input.

3. The program then prints a prompt asking the user to enter a sentence.

4. The program enters a while loop that continues until the user hits the Enter key (which sends a newline character '\n'). Inside the loop, the program reads a character from the input using the getchar() function and assigns it to ch.

5. The program then checks if the character is a lowercase letter using the islower() function. If it is, the program converts the character to uppercase using the toupper() function and prints it using the putchar() function.

6. If the character is not a lowercase letter, the program checks if it's an uppercase letter using the isupper() function. If it is, the program converts the character to lowercase using the tolower() function and prints it.

7. If the character is neither a lowercase nor an uppercase letter (for example, it's a digit or a punctuation mark), the program simply prints it as it is.

8. The main function returns 0 to indicate successful execution of the program