View Compilation Error

0_0_28599008_16279.cpp: In function 'void check(int)':
0_0_28599008_16279.cpp:21:38: error: 'len' was not declared in this scope
  for(int i = 1;a - i > -1 && a + i < len;i++){
                                      ^
0_0_28599008_16279.cpp: In function 'int main()':
0_0_28599008_16279.cpp:35:16: error: 'check1' was not declared in this scope
    check1(i,i-1);
                ^
0_0_28599008_16279.cpp:37:12: error: 'check2' was not declared in this scope
    check2(i);
            ^