Main.cc: In function 'int main()': Main.cc:7:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<s.size();i++) ^ Main.cc:10:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<s.size();i++){ ^ Main.cc:11:54: error: expected ';' before '}' token if(a[s[i]-'a']==1) {cout<<s[i]<<endl;return 0} ^