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Difference between while(1) and while(0) in C language

  • Difficulty Level : Basic
  • Last Updated : 30 Oct, 2020

Prerequisite: while loop in C/C++
In most computer programming languages, a while loop is a control flow statement that allows code to be executed repeatedly based on a given boolean condition. The boolean condition is either true or false

while(1)

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It is an infinite loop which will run till a break statement is issued explicitly. Interestingly not while(1) but any integer which is non-zero will give a similar effect as while(1). Therefore, while(1), while(2) or while(-255), all will give infinite loop only.  

while(1) or while(any non-zero integer)
{ 
    // loop runs infinitely
}

A simple usage of while(1) can be in the Client-Server program. In the program, the server runs in an infinite while loop to receive the packets sent from the clients. 
But practically, it is not advisable to use while(1) in real-world because it increases the CPU usage and also blocks the code i.e one cannot come out from the while(1) until the program is closed manually. while(1) can be used at a place where condition needs to be true always. 



C




// C program to illustrate while(1)
#include <stdio.h>
int main()
{
    int i = 0;
    while (1) {
        printf("%d\n", ++i);
        if (i == 5)
            break; // Used to come
                   // out of loop
    }
    return 0;
}

C++




#include <iostream>
using namespace std;
 
int main()
{
    int i = 0;
    while (1) {
        cout << ++i << "\n";
        if (i == 5)
            // Used to come
            // out of loop
            break;
    }
    return 0;
}
Output
1
2
3
4
5

while(0)
It is opposite of while(1). It means condition will always be false and thus code in while will never get executed. 

while(0)
{ 
    // loop does not run
}

C




// C program to illustrate while(0)
#include<stdio.h>
int main()
{
  int i = 0, flag=0;
  while ( 0 )
  {
    // This line will never get executed
    printf( "%d\n", ++i ); 
    flag++;
    if (i == 5)
      break;
  }
  if (flag==0)
     printf ("Didn't execute the loop!");
 return 0;
}

C++




#include <iostream>
using namespace std;
 
int main() {
  int i = 0, flag=0;
  while ( 0 )
  {
    // This line will never get executed
    cout << ++i << "\n"
    flag++;
    if (i == 5)
      break;
  }
  if (flag==0)
     cout << "Didn't execute the loop!";
 return 0;
}
Output
Didn't execute the loop!

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