Main.cc: In function 'int main()': Main.cc:7:35: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<a.size()-b.size();i++){ ^ Main.cc:9:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int j=i,k=0;k<b.size();j++,k++){ ^ Main.cc:17:13: error: return-statement with no value, in function returning 'int' [-fpermissive] return; ^