Language: C

include <stdio.h>

	char aft[1000];
	int n, j;

	void change(char aft[1000]){
		int a;
		for (a = 0;aft[a] != NULL;a++) {
			switch (aft[a]) {
			case'A':aft[a] = 'D';break;
			case'B':aft[a] = 'E';break;
			case'C':aft[a] = 'F';break;
			case'D':aft[a] = 'G';break;
			case'E':aft[a] = 'H';break;
			case'F':aft[a] = 'I';break;
			case'G':aft[a] = 'J';break;
			case'H':aft[a] = 'K';break;
			case'I':aft[a] = 'L';break;
			case'J':aft[a] = 'M';break;
			case'K':aft[a] = 'N';break;
			case'L':aft[a] = 'O';break;
			case'M':aft[a] = 'P';break;
			case'N':aft[a] = 'Q';break;
			case'O':aft[a] = 'R';break;
			case'P':aft[a] = 'S';break;
			case'Q':aft[a] = 'T';break;
			case'R':aft[a] = 'U';break;
			case'S':aft[a] = 'V';break;
			case'T':aft[a] = 'W';break;
			case'U':aft[a] = 'X';break;
			case'V':aft[a] = 'Y';break;
			case'W':aft[a] = 'Z';break;
			case'X':aft[a] = 'A';break;
			case'Y':aft[a] = 'B';break;
			case'Z':aft[a] = 'C';break;
			}
		}
		int main() 
		{
	
			scanf("%s", &aft);
			printf("%s\n",change(aft));
			return 0;
		}

Compiler output:
main.c:1:9: error: expected '=', ',', ';', 'asm' or '__attribute__' before '<' token
    1 | include <stdio.h>
      |         ^
main.c: In function 'change':
main.c:8:38: error: 'NULL' undeclared (first use in this function)
    8 |                 for (a = 0;aft[a] != NULL;a++) {
      |                                      ^~~~
main.c:1:1: note: 'NULL' is defined in header '<stddef.h>'; this is probably fixable by adding '#include <stddef.h>'
  +++ |+#include <stddef.h>
    1 | include <stdio.h>
main.c:8:38: note: each undeclared identifier is reported only once for each function it appears in
    8 |                 for (a = 0;aft[a] != NULL;a++) {
      |                                      ^~~~
main.c: In function 'main':
main.c:41:25: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
   41 |                         scanf("%s", &aft);
      |                         ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | include <stdio.h>
main.c:41:25: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
   41 |                         scanf("%s", &aft);
      |                         ^~~~~
main.c:41:25: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:42:25: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   42 |                         printf("%s\n",change(aft));
      |                         ^~~~~~
main.c:42:25: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:42:25: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:42:25: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:42:39: error: invalid use of void expression
   42 |                         printf("%s\n",change(aft));
      |                                       ^~~~~~~~~~~
main.c: In function 'change':
main.c:44:17: error: expected declaration or statement at end of input
   44 |                 }
      |                 ^