Can a for loop increment/decrement by more than one?

Question

Are there other ways to increment a for loop in Javascript besides i++ and ++i? For example, I want to increment by 3 instead of one.

for (var i = 0; i < myVar.length; i+3) {
   //every three
}

Trudy

Cant you just substitute i = i + 3 for the third argument? Or is that only in Java? – Villeinage

Yes that's fine i++ and ++i are like i+=1 if used in the for loop declaration so i+=3 would work. – Immesh

A for loop doesn't increment anything. Your code used in the for statement does. It's entirely up to you how/if/where/when you want to modify i or any other variable for that matter. – Canteen

That's not a for loop, it's an infinite loop. You mean i+=3. – Clouse

@user1689607 I should have said 'can a for loop be incremented'. Sorry for the poor grammar. – Trudy

@ninjagecko, only if myVar.length is infinite, right? And thanks, =+ was what I was looking for. – Trudy

@brentonstrine: No, my point wasn't the grammer, but rather that your code can modify i in just the same way as you would in any other part of your code. – Canteen

@user1689607 Oh, gotcha. Thanks, I wasn't even thinking of putting it anywhere but the third expression. – Trudy

I get that this is a simple question with a simple solution, but what is the reason for the downvotes? I'm pretty sure it isn't a duplicate. – Trudy

Answer 1

Use the += assignment operator:

for (var i = 0; i < myVar.length; i += 3) {

Technically, you can place any expression you'd like in the final expression of the for loop, but it is typically used to update the counter variable.

For more information about each step of the for loop, check out the MDN article.

Lotz

Thanks for the details--I was pretty sure there was a way to put more advanced expressions into the third slot--I had just forgotten that it needs to define the variable, so obviously i+3 doesn't work. – Trudy

What is the cause of the endless loop when we do i+3? – Fetid

@SanjayShr i+3 does not change the value of i – Vague

@AndrewWhitaker Actually, the MDN article you linked to has absolutely no example of incrementing other than the i++ and i-- simpletons as of today, which is a shame, since I thought only W3Schools stick to the essential for the simple minded. Hence the absolutely valid question here on SO. – Micamicaela

Answer 2

A for loop:

for(INIT; TEST; ADVANCE) {
    BODY
}

Means the following:

INIT;
while (true) {
    if (!TEST)
        break;
    BODY;
    ADVANCE;
}

You can write almost any expression for INIT, TEST, ADVANCE, and BODY.

Do note that the ++ operators and variants are operators with side-effects (one should try to avoid them if you are not using them like i+=1 and the like):

• ++i means i+=1; return i
• i++ means oldI=i; i+=1; return oldI

Example:

> i=0
> [i++, i, ++i, i, i--, i, --i, i]
[0, 1, 2, 2, 2, 1, 0, 0]

Clouse

Answer 3

for (var i = 0; i < 10; i = i + 2) {
    // code here
}​

Illomened

or for (var i = 0; i < 10; i+=2), because i=i+2 is equal to i+=2 – Ductile

Answer 4

Andrew Whitaker's answer is true, but you can use any expression for any part.
Just remember the second (middle) expression should evaluate so it can be compared to a boolean true or false.

When I use a for loop, I think of it as

for (var i = 0; i < 10; ++i) {
    /* expression */
}

as being

var i = 0;
while( i < 10 ) {
    /* expression */
    ++i;
}

Astrophysics

Answer 5

for (var i = 0; i < myVar.length; i+=3) {
   //every three
}

additional

Operator   Example    Same As
  ++       X ++        x = x + 1
  --       X --        x = x - 1
  +=       x += y      x = x + y
  -=       x -= y      x = x - y
  *=       x *= y      x = x * y
  /=       x /= y      x = x / y
  %=       x %= y      x = x % y

Dreadful

Answer 6

You certainly can. Others have pointed out correctly that you need to do i += 3. You can't do what you have posted because all you are doing here is adding i + 3 but never assigning the result back to i. i++ is just a shorthand for i = i + 1, similarly i +=3 is a shorthand for i = i + 3.

Hermes

Answer 7

There is an operator just for this. For example, if I wanted to change a variable i by 3 then:

var someValue = 9;
var Increment  = 3;
for(var i=0;i<someValue;i+=Increment){
//do whatever
}

to decrease, you use -=

var someValue = 3;
var Increment  = 3;
for(var i=9;i>someValue;i+=Increment){
//do whatever
}

Invective

Answer 8

For those who are looking to increment pair of numbers (like 1-2 to 3-4):

Solution one:

//initial values
var n_left = 1;
var n_right = 2;
    
for (i = 1; i <= 5; i++) {
        
    console.log(n_left + "-" + n_right);   
        
    n_left =+ n_left+2;
    n_right =+ n_right+2; 
}
//result: 1-2 3-4 5-6 7-8 9-10

Solution two:

for (x = 0; x <= 9; x+=2) {
    
    console.log((x+1) + "-" + (x+2));  

}
//result: 1-2 3-4 5-6 7-8 9-10

Pion

Answer 9

The last part of the ternary operator allows you to specify the increment step size. For instance, i++ means increment by 1. i+=2 is same as i=i+2,... etc. Example:

let val= [];

for (let i = 0; i < 9; i+=2) {
  val = val + i+",";
}


console.log(val);

Expected results: "2,4,6,8"

'i' can be any floating point or whole number depending on the desired step size.

Kimono