/in/foo.cc: In function 'int main()':
/in/foo.cc:19:30: warning: operation on 'k' may be undefined [-Wsequence-point]
   b[k++] = abs(a[k] - a[k-1]);
                              ^
/in/foo.cc:19:30: warning: operation on 'k' may be undefined [-Wsequence-point]