Minor Problem

Hi,

I have just been rounding off learning the basics of working with C++ at the console level by coding a very simple game. Part of this entails opting to play the game again.

Basically, i if the user enters "y" then i need to call the start function again otherwise the program can terminate normally.

Code:

int main() {
 start();

 cout << "Play Again? (y/n)... ";
 
 cin.ignore();
 cin.getline(again,2);
 cout << "|" << again << "|" << endl; //debug: again is empty?
 if (strcmp(again,"y") == 0) start();

 system("pause");

 return 0;
}

I also have (before the above):

Code:

char again[2];

Before strcmp i also tried: if (again == "y") {

The problem seems to be before that though, with getline as the again variable is empty.

How should i be doing this?

Thank you

You can't use cout on a c-string.
I'm not sure what the best way to output a c-string is (you could write the function to loop through and print each char, but it may have been done better/before elsewhere*).

The problem goes away if you use the string class though. Also simplifies the comparisons and input.

* quick c-string output, probably not perfect:

Code:

void strOut(char start[], ostream& out = cout) {
     while(*start != '\0')
     {
       out << *start;
       start++;
     }
};

Much cleaner. String has been implemented so you can use the stream insertions with it (cout << and cin>>). You can also directly compare them, no messing around with strcmp.

Code:

#include <iostream>
#include <string>

using namespace std;

void start() {
     cout << "Start stub" << endl;     
};


int main() {
    string againChoice; // holds user input choice
    string repeat("y"); // value to flag a repeat
    
 start();

 cout << "Play Again? (y/n)... ";
 
 cin >> againChoice;
 cout << "|" << againChoice << "|" << endl; //debug: should look good
 if (againChoice == repeat) 
    start();

 system("pause");

 return 0;
}

Thank you =)

I wasn't aware you could set a variable as type string, i had thought you could only use an array with char arr[n] in c++.

Originally Posted by AH_Tze

You can't use cout on a c-string.

...

The problem goes away if you use the string class though. Also simplifies the comparisons and input.

Of course you can. (But just because you can doesn't mean you should. C++ gives us the string class. I agree that it's waaaay better --- usually.)

Code:

#include <iostream>

using std::cout;
using std::endl;
using std::cin;

int main()
{
  char again[2];

  cout << "Play Again? (y/n)... ";
 
  cin.getline(again,2);

  cout << "Here's again |" << again << "|" << endl;

  if (strcmp(again,"y") == 0){
    cout << "strcmp() evaluated to zero" <<endl;
  }
  else {
    cout << "strcmp() did not give zero" << endl;
  }

  return 0;
}

Regards,

Dave

Code:

char again[2];
Before strcmp i also tried: if (again == "y")

char literals are denoted with single quotes:

if(again=='y')

while string literals are denoted with double quotes:

"some text"

A string literal can be used to initialize a char array:

char again[] = "y";

However, that is a two step process. If I've got this right, what happens is that when the compiler sees a string literal, it first tacks on a '\0' to the end of the string literal:

y\0

and stores it in memory somewhere. Then, the address is assigned to the variable 'again'.
So, in actuality, a char array name is a pointer to a string in memory(see below as to why that is difficult to prove). Therefore, when you do this:

if (again == "y")

you are comparing an address stored in the pointer 'again' to "y". But, an address looks something like this: 006BFDF4, so they will never be equal.

To compare c-strings, you have to use strcmp() in <cstring>. strcmp() returns 0 if the cstrings are equal:

Code:

char again[] = "y";

if(strcmp(again, "y")==0)
{
	cout<<"they are the same"<<endl;
}
else
{
	cout<<"not the same"<<endl;
}

Originally Posted by AH_Tze

You can't use cout on a c-string.

In fact, cout<< is specifically tailored to handle c-strings. c-string variables are pointers to the strings, and normally when you cout<< a pointer variable, you get an address:

Code:

int num = 10;
int* p = #
cout<<p<<endl; //006BFDF4

However, with c-strings the <<operator is programmed to automatically convert the address to the value at that address, and display the string:

Code:

char text[] = "some text";
cout<<text<<endl;  //some text

That is why it is hard to know that a char array name is actually a pointer.

Where you may have gotten confused is that there is a distinction between an "array-of-chars" and a c-string. The terms char arrays and c-strings are sometimes used interchangeably, but then you have to have a term to identify another entity, which I'm calling an "array-of-chars". An "array-of-chars" is different from a c-string/char array. The defining aspect of a c-string/char array is that the array ends with a '\0' character. That '\0' character enables the <cstring> functions and cout<< to know when the end of the array has been reached, and to stop. With an array-of-chars, there is no '\0' character at the end of the array, so the <cstring> functions and cout<< will run past the end of the array and go out of bounds, spewing out all kinds of junk characters. Here is an example:

char my_array[] = {'a', 'b', 'c'};
cout<<my_array<<endl; //abc#@$

If you try that, it will display abc, but then you will also get a bunch of junk after that, as the <<operator runs past the end of the array. There is no '\0' char to signal the end of the array, so the <<operator keeps going.

To ouput an array-of-chars, you have to use a for-loop and output each char one at a time. You also need to know the size of the array, so you can tell the for-loop when to stop, and prevent it from going out of bounds:

Code:

char my_array[] = {'a', 'b', 'c'};

for(int i=0; i<3; i++)
{
	cout<<my_array[i];
}
cout<<endl;

Presumably, the <<operator is programmed to ouput a c-string in much the same way, it just has to check for the '\0' char so it doesn't go out of bounds:

Code:

char my_array[] = "abc";

int i = 0;
while(my_array[i] != '\0')
{
	cout<<my_array[i];
	i++;
}

Similarly, the <cstring> functions are all pretty simple functions that just loop through a char array until they come across a '\0', and then they end.