Write a C program to read a sentence and replace lowercase characters with uppercase and vice versa.

Sure, here is a simple C program that reads a sentence and replaces lowercase characters with uppercase and vice versa.

#include <stdio.h>

void convertCase(char *str) {
    int i = 0;
    while(str[i] != '\0') {
        if(str[i] >= 'a' && str[i] <= 'z') {
            str[i] = str[i] - 32;
        }
        else if(str[i] >= 'A' && str[i] <= 'Z') {
            str[i] = str[i] + 32;
        }
        i++;
    }
}

int main() {
    char str[100];
    printf("Enter a sentence: ");
    fgets(str, sizeof(str), stdin);
    convertCase(str);
    printf("After converting case: %s", str);
    return 0;
}

This program works as follows:

1. It includes the standard input/output library.
2. It defines a function convertCase that takes a string as input and converts all lowercase characters to uppercase and all uppercase characters to lowercase.
3. In the main function, it declares a character array str of size 100 to store the input sentence.
4. It prompts the user to enter a sentence and reads the input using fgets.
5. It calls the convertCase function to convert the case of the characters in the sentence.
6. It prints the sentence after converting the case.
7. It returns 0 to indicate successful execution of the program.