foo.c: In function 'main':
foo.c:8:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s);
  ^~~~
In file included from /usr/include/stdio.h:862:0,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:232:1: note: declared here
 gets (char *__str)
 ^~~~
foo.c:11:6: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
   if(s[i]='5')
      ^
foo.c:17:3: error: expected ';' before 'for'
   for(i=j;s[i]!=0;i++)
   ^~~
foo.c:7:24: warning: variable 'sum' set but not used [-Wunused-but-set-variable]
  int a[100000]={0},i,j,sum,cnt=0;
                        ^~~
foo.c:7:6: warning: variable 'a' set but not used [-Wunused-but-set-variable]
  int a[100000]={0},i,j,sum,cnt=0;
      ^
foo.c:8:2: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
  gets(s);
  ^~~~~~~

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