Main.cc: In function 'int main()': Main.cc:14:30: error: 'strlen' was not declared in this scope int i=0,lena=strlen(a),lenahalf=lena/2; ^ Main.cc:16:32: warning: operation on 'i' may be undefined [-Wsequence-point] temp(a[i++],a[i++]); ^ Main.cc:14:17: warning: unused variable 'lena' [-Wunused-variable] int i=0,lena=strlen(a),lenahalf=lena/2; ^