Compilare: main.cpp: In function 'int main()': main.cpp:20:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < strlen(s); i++){ ~~^~~~~~~~~~~ main.cpp:23:55: warning: NULL used in arithmetic [-Wpointer-arith] if(s[i] == 'x' && (isdigit(s[i-1]) == NULL || i == 0) ){ ^~~~ main.cpp:39:44: warning: NULL used in arithmetic [-Wpointer-arith] while(isdigit(s[i]) != NULL && i < strlen(s)){ ^~~~ main.cpp:39:54: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while(isdigit(s[i]) != NULL && i < strlen(s)){ ~~^~~~~~~~~~~ main.cpp:44:27: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses] if(s[i] == 'x') ^ main.cpp:47:27: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses] if(s[i] != 'x') ^ main.cpp:53:27: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses] if(s[i] == 'x') ^ main.cpp:56:27: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses] if(s[i] != 'x') ^