Language: C

int main() {
    int H, W, i, n, m, x, y;
    for(; ;){
        scanf("%d%d", &H, &W);
        if(H == 0 && W == 0){break;}
        for(n=1; n <= H;n++){
            y=n;
            for(x=1; x <= 3000; x++){
                y=y%2;
            }
            if(y == 0){
                for(m=1;m <= W;m++){
                    y=m;
                    for(x=1;x <= 3000;x++){
                        y=y%2;
                    }
                    if(y == 0){
                        printf("#");
                    }else{
                        printf(".");
                    }
                }
                printf("\n");
            }else{
                for(m=1;m <= W;m++){
                    y=m;
                    for(x=1;x <= 3000;x++){
                        y=y%2;
                    }
                    if(y == 0){
                        printf(".");
                    }else{
                        printf("#");
                    }
                }
                printf("\n");
            }
        }
        printf("\n");
    }
    printf("\n");
}return 0;

Compiler output:
main.c: In function 'main':
main.c:4:9: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    4 |         scanf("%d%d", &H, &W);
      |         ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | int main() {
main.c:4:9: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    4 |         scanf("%d%d", &H, &W);
      |         ^~~~~
main.c:4:9: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:18:25: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   18 |                         printf("#");
      |                         ^~~~~~
main.c:18:25: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:18:25: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:18:25: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:20:25: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   20 |                         printf(".");
      |                         ^~~~~~
main.c:20:25: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:23:17: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   23 |                 printf("\n");
      |                 ^~~~~~
main.c:23:17: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:31:25: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   31 |                         printf(".");
      |                         ^~~~~~
main.c:31:25: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:33:25: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   33 |                         printf("#");
      |                         ^~~~~~
main.c:33:25: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:36:17: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   36 |                 printf("\n");
      |                 ^~~~~~
main.c:36:17: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:39:9: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   39 |         printf("\n");
      |         ^~~~~~
main.c:39:9: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:41:5: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   41 |     printf("\n");
      |     ^~~~~~
main.c:41:5: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c: At top level:
main.c:42:2: error: expected identifier or '(' before 'return'
   42 | }return 0;
      |  ^~~~~~