Language: C

/* ex 5_4
   potter0723sw */

#include <stdio.h>
#include <string.h>
#define SIZE = 11

int main(void){
    int i,pre,counter = 0,ans = 0,;
    char str1[SIZE],sub = "A", *ret;
    scanf("%s", str1);
    while(counter != 3){
        pre=0;
        while((ret = strstr(str+pre,sub)) != NULL){
            i = 0
            while(*ret + i == 'A' || *ret + i == 'T' || *ret + i == 'c' || *ret + i == 'G'){
                i++;
            }
            if(i > ans){
                ans = i;
            }
            pre+=strlen(str1)-strlen(ret)+strlen(sub);
        }
        switch(counter){
            case 0:
                sub = "T";
                break;
            case 1:
                sub = "C";
                break;
            case 2:
                sub = "G";
                break;
        }
        counter++
    }
    printf("%d\n", ans);
    return 0;
}

Compiler output:
main.c: In function 'main':
main.c:9:35: error: expected identifier or '(' before ';' token
    9 |     int i,pre,counter = 0,ans = 0,;
      |                                   ^
main.c:6:14: error: expected expression before '=' token
    6 | #define SIZE = 11
      |              ^
main.c:10:15: note: in expansion of macro 'SIZE'
   10 |     char str1[SIZE],sub = "A", *ret;
      |               ^~~~
main.c:10:27: error: initialization of 'char' from 'char *' makes integer from pointer without a cast [-Wint-conversion]
   10 |     char str1[SIZE],sub = "A", *ret;
      |                           ^~~
main.c:14:29: error: 'str' undeclared (first use in this function); did you mean 'str1'?
   14 |         while((ret = strstr(str+pre,sub)) != NULL){
      |                             ^~~
      |                             str1
main.c:14:29: note: each undeclared identifier is reported only once for each function it appears in
main.c:14:37: error: passing argument 2 of 'strstr' makes pointer from integer without a cast [-Wint-conversion]
   14 |         while((ret = strstr(str+pre,sub)) != NULL){
      |                                     ^~~
      |                                     |
      |                                     char
In file included from main.c:5:
/usr/include/string.h:350:58: note: expected 'const char *' but argument is of type 'char'
  350 | extern char *strstr (const char *__haystack, const char *__needle)
      |                                              ~~~~~~~~~~~~^~~~~~~~
main.c:15:18: error: expected ';' before 'while'
   15 |             i = 0
      |                  ^
      |                  ;
   16 |             while(*ret + i == 'A' || *ret + i == 'T' || *ret + i == 'c' || *ret + i == 'G'){
      |             ~~~~~ 
main.c:26:21: error: assignment to 'char' from 'char *' makes integer from pointer without a cast [-Wint-conversion]
   26 |                 sub = "T";
      |                     ^
main.c:29:21: error: assignment to 'char' from 'char *' makes integer from pointer without a cast [-Wint-conversion]
   29 |                 sub = "C";
      |                     ^
main.c:32:21: error: assignment to 'char' from 'char *' makes integer from pointer without a cast [-Wint-conversion]
   32 |                 sub = "G";
      |                     ^
main.c:35:18: error: expected ';' before '}' token
   35 |         counter++
      |                  ^
      |                  ;
   36 |     }
      |     ~