In programming, a while loop is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition.
So for example, if you execute the following piece of code:
while(1==1)
{
System.out.println("Hello World");
}
The console would just print out Hello World an infinite amount of times because 1 is always equal to 1. For the while loop to stop running forever, the condition has to become false.
Let's take a look at another example. If we execute the following piece of code:
int i =0;
while(i<10)
{
i+=2;
System.out.println(i);
}
The result would be:
2
4
6
8
10
As we can see the loop runs 5 times. Before the loop starts running, i is equal to 0. But when we start running the loop for the first time, the variable i increases by 2, because of the i+=2; line. Therefore the variable i becomes 2, and thus it prints 2. The reason why it prints an arithmetic pattern with an incrementation of 2 is that, everytime the loop runs, the value of i increases by 2(i+=2;). Before the loop runs for the fifth time, i holds the value of 8. But when we run the loop for the 5th time, we add 2 to i, and thus the value of i becomes 10. But the loop doesn't run for the 6th time because the condition for the loop is i<10. Remember that after we ran the loop for the 5th time, the value of i was 10. And 10 is not less than 10. Therefore the loop doesn't run anymore because 10<10 evaluates to false.
We can also create an air conditioner program by using while loops:
public class AirConditioner
{
public static void main(String[]args)
{
System.out.println("Welcome to the air conditioner program");
double rate = .5; //degrees per minute
int time =0; //in minutes
double currentTemp = 79;
double requiredTemp = 71;
if(requiredTemp>currentTemp)
{
while(currentTemp<requiredTemp)//Heating the room because the required temperature is higher than the current temperature
{
currentTemp+=rate;
time++;
}
System.out.println("It took " + time + " minutes to heat the room to " + requiredTemp + " degrees Fahrenheit.");
}
else if(requiredTemp<currentTemp)//Cooling the room because the required temperature is lower than the current temperature
{
while(currentTemp>requiredTemp)
{
currentTemp-=rate;
time++;
}
System.out.println("It took " + time + " minutes to cool the room to " + requiredTemp + " degrees Fahrenheit.");
}
else //Does nothing because this condition is run only if the required temperature and the current temperature are the same
{
System.out.println("Required temperature already met.");
}
}
}
In this program, we are either heating or cooling the room based on the currentTemp and the requiredTemp variable. If the requiredTemp variable is greater than the currentTemp, we heat the room by incrementing the currentTemp by rate. The other way around, if the requiredTemp variable is less than the currentTemp, we cool the room by decrementing the currentTemp by rate. We calculate the time by increasing 1 to the time variable, every time the loop runs. The loop stops running when the currentTemp variable is equal to the requiredTemp variable. After that, we just print out how much time it took to cool/heat the room the required temperature.
If you would like to learn about while loops from a video, please click here.