Compilare: user.cpp: In function ‘int calc(int, int)’: user.cpp:22:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0;i<strlen(S[l][c]);) ^ user.cpp:25:70: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while ((S[l][c][i]<'0' || S[l][c][i]>'9') && i<strlen(S[l][c]))i++; ^ user.cpp:26:72: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while ((S[l][c][i]>='0' && S[l][c][i]<='9') && i<strlen(S[l][c])) ^ user.cpp:37:70: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while ((S[l][c][i]<'0' || S[l][c][i]>'9') && i<strlen(S[l][c]))i++; ^ user.cpp:39:1: warning: control reaches end of non-void function [-Wreturn-type] } ^