foo.cc: In function ‘int main()’:
foo.cc:10:26: warning: right operand of comma operator has no effect [-Wunused-value]
   10 |                 if(b=4,6,9,11)
      |                          ^
foo.cc:10:28: warning: right operand of comma operator has no effect [-Wunused-value]
   10 |                 if(b=4,6,9,11)
      |                            ^~
foo.cc:13:15: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
   13 |           if(b=2)
      |              ~^~
foo.cc:19:21: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
   19 |                 if(b=2)
      |                    ~^~
foo.cc:23:18: warning: right operand of comma operator has no effect [-Wunused-value]
   23 |         if(b=4,6,9,11)
      |                  ^
foo.cc:23:20: warning: right operand of comma operator has no effect [-Wunused-value]
   23 |         if(b=4,6,9,11)
      |                    ^~
foo.cc:18:7: warning: suggest explicit braces to avoid ambiguous ‘else’ [-Wdangling-else]
   18 |     if(a%400==r)
      |       ^
foo.cc:18:5: warning: ‘r’ is used uninitialized [-Wuninitialized]
   18 |     if(a%400==r)
      |     ^~
foo.cc:6:14: note: ‘r’ was declared here
    6 |    int a,b,c,r;
      |              ^