Main.cc: In function 'std::string zh(std::string)': Main.cc:6:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<a.size();i++){ ^ Main.cc:10:60: error: lvalue required as left operand of assignment if(a[i]=='a'||a[i]=='e'||a[i]=='i'||a[i]=='o'||a[i]='u'){ ^