C :

#include<stdio.h>
#include<string.h>

void main(){
	char a[81];
	int i;
	gets(a);
	
	for(i=0;i<strlen(a);i++){
		if(a[i]>='A'&&a[i]<='Z'){
			a[i]=a[i]+32;
		}else if(a[i]>='a'&&a[i]<='z'){
			a[i]=a[i]-32;
		}
	}
	printf("%s",a);
}

C++ :

#include<bits/stdc++.h>
using namespace std;

int main(){
	char s[1000];
	gets(s);
	int i;
	for(i=0;i<strlen(s);i++){
		if(s[i]>='a'&&s[i]<='z'){
			s[i]=s[i]-('a'-'A');
		}else if(s[i]>='A'&&s[i]<='Z'){
			s[i]=s[i]+('a'-'A');
		}
		
	}
	cout<<s;
}

Java :

import java.util.Scanner;
public class Main{
	public static void main(String[] args) {
		Scanner scanner=new Scanner(System.in);
		String scannerStr=scanner.nextLine();
		char[] charArray=scannerStr.toCharArray();
		for(int i=0;i<charArray.length;i++){
			int k=charArray[i];
			if(k>=65&&k<=90){
				charArray[i]=(char)((int)charArray[i]+32);
			}
			if(k<=122&&k>=97){
				charArray[i]=(char)((int)charArray[i]-32);
			}
			System.out.print(charArray[i]);
		}
	}
}

Python :

s=input()
s1=""
for i in s:
    if i>="A" and i<="Z":
        s1=s1+i.lower()
    elif i>="a" and i<="z":
        s1=s1+i.upper()
    else:
        s1=s1+i
print(s1)