Compilare: user.cpp: In function ‘void calc(int, int)’: user.cpp:12:44: warning: NULL used in arithmetic [-Wpointer-arith] else {val=e[start]-'0'; if(e[start+1]==NULL) b=b+val*egal;} ^ user.cpp:14:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=start+1;i<=strlen(e)-1 && e[i-1]!='=' && e[i]!=NULL;i++) ^ user.cpp:14:58: warning: NULL used in arithmetic [-Wpointer-arith] for(i=start+1;i<=strlen(e)-1 && e[i-1]!='=' && e[i]!=NULL;i++) ^ user.cpp:17:20: warning: NULL used in arithmetic [-Wpointer-arith] if(e[i+1]==NULL) b=b+val*egal*s;} ^