chapter-09.md

Loop statements

In creating a computer program, there are bound to be code that needs to be executed repeatedly. Loop statement is a code statement that executes again and again until (or while) a certain condition is met.

Let's say you want to write a program that "prints number 1 to 10 into the console". You can do it by using console.log 10 times:

console.log(1);
console.log(2);
console.log(3);
console.log(4);
console.log(5);

// and so on..

It works, but there is a better way to write this kind of repetitive code. With the loop statement for, you can write it like this:

for ( x = 1; x < 11; x++) {
  console.log(x);
}

There you go! A for statement is followed by a parentheses (()) which contains 3 expressions:

• Initialization expression, where you declare a variable to be used as the source of the loop condition
• Condition expression, where the variable in initialization will be evaluated for a specific condition
• Arithmetic expression, where the variable value is either incremented or decremented by the end of each loop

After these expressions, the curly brackets ({}) will be used to mark the code that will be executed as long as the condition expression is true. You can identify which expression is which by paying attention to the semicolon (;) which ends the statement.

for ( [initialization]; [condition]; [arithmetic expression]) {
  // begin execution ..
}

The arithmetic expression can be an increment (++) or a decrement (--) or a shorthand arithmetic operators like += and -=. It is run once each time the execution of the code inside the curly brackets end:

// for statement with decrement expression
for ( x = 10; x >= 1; x--) {
  console.log(x);
}

// divider just for easier reading in the console
console.log('===============');

// for statement with shorthand arithmetic expression
for ( x = 1; x < 20; x+=3) {
  console.log(x);
}

The while loop

The while statement is an alternative loop statement that can be used to execute repetitive task, and the syntax goes like this:

let x = 1;
while (x < 11) {
  console.log(x);
  x++;
}

A while statement is followed by a parentheses (()) for the condition, and while the condition is true, it will run the code inside the curly brackets ({}).

Please note that inside the while statement block there is an increment operation executed

x++

This is so that the while condition evaluation can return false and stop the while statement from executing forever.

The do..while loop

Sometimes, you want to execute a code at least once regardless if the while condition evaluates to true or false. A Do..While loop statement ensures that a code will be executed once before checking on the conditional:

let i = 2;
do {
  console.log(i);
  i++;
} while ( i < 1);

The above example will execute the code block inside do statement once, even though the condition is already false because the variable i is 2, which is already greater than 1.

Code exercise

Write a small piece of program that prints all even numbers from 1 to 50

Then write another program to print a half triangle using the asterisk (*). Here is the result:

*
**
***
****
*****

Then print a reversed half triangle next:

*****
****
***
**
*

Hint: for both cases, the number of the loop is 5.