How does this code work?

[talktime104]

#include <iostream>
#include <cmath>
using namespace std;
int main() {
int  n;   // Number to test for prime-ness
int  i;   // Loop counter
int  is_prime = true;   // Boolean flag...
// Assume true for now.
// Get a number from the keyboard. 
cout << "Enter a number and press ENTER: ";
cin >> n;
// Test for prime by checking for divisibility
//  by all whole numbers from 2 to sqrt(n).
i = 2;
while (i <= sqrt(n)) {   // While i is <= sqrt(n),
if (n % i == 0)        //  If i divides n,
is_prime = false;  //    n is not prime.
i++;                   //  Add 1 to i.
}
// Print results
if (is_prime)
cout << "Number is prime." << endl;
else
cout << "Number is not prime." << endl;
system("PAUSE");
return 0;
}

Its a full working program that checks for a prime number, the code works well, but i could not understand what does this means.
if (is_prime)
cout << "Number is prime." << endl;
else
cout << "Number is not prime." << endl;

what is if(is_prime) mean.

kindly help.

[vlad from moscow]

This statement is equivalent to

if ( is_prime == true )

By the way the code contains a bug because 1 is not a prime number.:)

[JockX]

In if (condition) statement condition can evaluate to only two states - that is true or false.
In C++ false means 0. If it is not zero, then it is true. Since is_prime is an integer, it can hold values such as 1, -1, 3, 888, -200. Any of these values means true. Only 0 means false.

Notation
if(condition) is equivalent to
if(condition == true) or in yet other words
if(condition != false) or
if(condition != 0)

[talktime104]

thanks for the prompt reply

why doesn't it mean if(is_prime==false) coz there is no specification.

of course this program need some improvement

[Andrei15193]

In C++, the condition for if, while, for is evaluated to "false" if it's 0 (zero) and evaluated to "true" otherwise.

A code like

if (1){
    // some code
}

will always execute the if code block because 1 evaluates to true.

This can be useful because

int n = 1;
if (n){
    // some code
}

has the same effect.

Lower case true (not TRUE, that's something else) and false are boolean literals. The false literal has like no choice but to evaluate to 0 (as a number) since that's C standard. On the other hand the true literal can evaluate to any non-zero number, however it usually evaluates to 1.

In other words, if you do int n = true; n will most likely be 1 and if you do int n = false; n will be be most likely 0.

If a variable is wanted to take only two values (true or false) it is best to use the bool data type since it's specially designed for that and it makes the code more readable.

More about C++ types: http://www.cplusplus.com/doc/tutorial/variables/

[talktime104]

i really thanks all of you, very happy to be here among u ppl, i'm a newbie in c++.
i will of course post issues, which for u ppl be very simple but for me its challenging.

[JockX]

why doesn't it mean if(is_prime==false) coz there is no specification?

In the language it is assumed that you always check if what is in the brackets is true.
The logical value of the statement is_prime==false as a whole is true when is_prime == 0 and false otherwise.