Go through array based on previous value

Question

I have an array ['green', 'red', 'yellow', 'blue'].

I want to change the background color of my website using these colors. But I don't want to just take a random color from this array. I want to iterate it in order.

So if I get the background color of my website with getBgColor() and it prints red, then I want a function setBgColor(currentColor) to print yellow. How do I do this?

I guess I should do something like

function setBgColor(currentColor) {
  var array = ['green', 'red', 'yellow', 'blue'];

  newColor = array[array.indexOf(currentColor) + 1];
}

but is this the right approach? And how do I make sure that I go from blue to green and thus not exceeding the length of the array?

Nina Scholz · Accepted Answer · 2016-09-23 21:40:08Z

5

You could use modulo % with the length of the array.

function setBgColor(currentColor) {
    var array = ['green', 'red', 'yellow', 'blue'],
        newColor = array[(array.indexOf(currentColor) + 1) % array.length ];
    // set the new color
    // ...
}

edited Sep 23, 2016 at 21:40

answered Sep 23, 2016 at 21:29

Nina Scholz

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1 Comment

The One and Only ChemistryBlob Over a year ago

nice answer - newColor needs to be returned

The One and Only ChemistryBlob · Accepted Answer · 2016-09-23 21:37:32Z

0

Just include a conditional in your number to add:

var array = ['green', 'red', 'yellow', 'blue'],
    last_index = array.length - 1,
    thisIndex = array.indexOf(currentColor);

return array[thisIndex + (thisIndex !== last_index ? 1 : (-1 * last_index))];

edited Sep 23, 2016 at 21:37

answered Sep 23, 2016 at 21:31

The One and Only ChemistryBlob

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Comments

Barmar · Accepted Answer · 2016-09-23 22:45:20Z

0

Use a variable that contains the current index, rather than searching for the color in the array. Use modular arithmetic to increment the index, wrapping around at the array size.

var colorIndex = 0;
var colorArray = ['green', 'red', 'yellow', 'blue'];

function setBgColor() {
    colorIndex = (colorIndex + 1) % array.length;
    return colorArray[colorIndex];
}

answered Sep 23, 2016 at 22:45

Barmar

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