Trouble with basic JavaScript loop

Question

I'm a newbie in JavaScript programming. At our university we were just beginning to learn about JavaScript loops. So here's a code using for loop:

var i;
var x = new Array();
var y = new Array(1, 1, 0, 3, 5);
for (i = 0; i < y.length; i++) {
 x[y[i]] = y[i];
}

I know that such code produces variable x to be an array with values

0, 1, undefined, 3, undefined, 5

I do not understand the logic behind it. Can somebody please help me?

• Yann Chabot: What do you not understand exactly?
• mplungjan: i=0->x[1]=1, i=1->x[1]=1, i=2->x[0]=0, i=3->x[3]=3, i=4->x[5]=5 so x[2] and x[4] is never set
• somethinghere: @mplungjan Waw, if the OP was having trouble deciphering the code he was looking at, this will be even more perplexing! :)
• mplungjan: Nah. He is a university student

Answer 1

Let's first simplify this a bit and expand to make it more legible:

var x = new Array();
var y = new Array(1, 1, 0, 3, 5);
for(let i = 0; i < y.length; i++){
 let key = y[i];
 x[ key ] = y[i];
}

On first evaluation, i === 0, so key === y[0] or 1. Now that we have the key, we will assign the key as a value to the x array at the index that was the number stored in your y array at index i. So x[y[0]] = y[0], therefor x[1] === 1.

Wrince, repeat.

• Pero: Tried your code and after some mental effort I understood. Thank you!

Answer 2

Okay let's loop through that one by one:

i=0

x[1] = 1 //because y[0] = 1

i=1

x[1] = 1 //because y[1] = 1

i=2

x[0] = 0 //because y[2] = 0

i=3

x[3] = 3 //because y[3] = 3

i = 4

x[5] = 5 //because y[4] = 5

So you've set values in x for indices 0, 1, 3 and 5, but not 2 and 4, that's why those values are undefined

• Pero: Thank you so much for this explanation!

Answer 3

You have to realize that the positions where data is put in Array X are taken from Array Y.

As Array Y only have the following numbers (it means, positions): 0, 1, 3, 5, the positions 2 and 4 will be undefined in the Array X

var i;
var x = new Array();
var y = new Array(1, 1, 0, 3, 5);
for (i = 0; i < y.length; i++) {
 console.log("Index taken from ARRAY Y where number will be put in ARRAY X: " + y[i]);
 x[y[i]] = y[i];
}

• Pero: Informative! Took me awhile to get it, but it's crystal clear now. Thank you!

Answer 4

Because you are not incrementing it by the index. You are setting the index based on the value of the index of the y array.

So what you are doing in your loop is

x[1] = 1 // i = 0;
x[1] = 1 // i = 1;
x[0] = 0 // i = 2;
x[3] = 3 // i = 3;
x[5] = 5 // i = 4;

There is no value of 2 or 4, so those indexes were never defined, so it it is undefined.

What you want is

for (i = 0; i < y.length; i++) {
 x[i] = y[i];
}

• Pero: I read post by Thomas and then returned to yours. Thank you so much!

Answer 5

Try to write down each iteration and see how x changes in successive steps.

START // []
x[y[0]] = y[0]; // x[1] = 1; [undefined, 1]
x[y[1]] = y[1]; // x[1] = 1; [undefined, 1]
x[y[2]] = y[2]; // x[0] = 0; [0, 1]
x[y[3]] = y[3]; // x[3] = 3; [0, 1, undefined, 3]
x[y[4]] = y[4]; // x[5] = 5; [0, 1, undefined, 3, undefined, 5]