## What does "array.length -1" mean in JavaScript?

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I know how to use the JavaScript for loops to cycle through arrays for example, but I still didn't understand what the `array.length -1` means, specifically the `-1` part.

When using a for loop over an array we have something like this:

```for (i = 0; i < array.length; i++) {...}
```

But I've seen also something like this, sometimes:

```for (i = 0; i < array.length - 1; i++) {...}
```

In the second case, why there is the "-1" in the array.length and what does it do? Also why sometimes there is and sometimes isn't it shown?

It's to prevent fencepost errors, aka "off-by-one".

Common exam question:

You are required build 100 meters of fence, with a fence post every meter. How many fenceposts will you need?

```    100 meters
--------------- = 100 posts
1 post
------
meter
```

Because for 100 meters of fence, you need 101 posts:

```Distance:    1 2  ....  99 100
|-|-|.....| - | - |
Post:       1 2 3 ....99  100 101
```

Now with arrays, the same thing happens. Let's say it's an array with 5 items:

```for (i = 0 ; i <= 5; i++)
^--length of array
```

You end up doing

```        i:    0, 1, 2, 3, 4, 5
iteration:    1  2  3  4  5  6
```

Oops. 5 item array, but you've executed your loop 6 times - one times too many.

You can fix the error in two ways:

```for (i = 0; i < length; i++)
^---change from "<=" to "<"
```

or

```for (i = 0; i <= length - 1; i++)
^---change the upper limit value.
```

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Javascript arrays are 0-based, meaning if you have an array of 5 items, you would use the indices 0 through 4 to access them. They're subtracting one (there should be a space between '-' and '1' to make it more clear) to find the last index.

Edited: If they're subtracting one, the test should be '<='.

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If you access multiple indices at once you generally want to avoid a overflow by making sure every accessed index is `< array.length`

so if you do something like this

```array[x+2] = array[x] + array[x+1];
```

you generally want to make sure that the highest occuring value in the `[]` is lower than array.length

then your for in the upper example would probably look like

```for(int x = 0; x+2 < array.length; x++) {
array[x+2] = array[x] + array[x+1];
}
```

which is the mathematically equlivalent to

```for(int x = 0; x < array.length - 2; x++) {
array[x+2] = array[x] + array[x+1];
}
```

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• `i < length` and `i <= length - 1` are equivalent
• remember that `for` keeps going until that check condition goes false, so... `i<=5` will be TRUE for i=5 (array element #6). if you switch to `i<5`, then when `i` becomes 5, `i<5` becomes `false` and the loop aborts. for integers, `i<5` and `i<=4` (aka `i<=5-1`) are effectively identical
• choosing which to use is entirely up to the programmer. Personally, I'll with the `<= count minus 1` variation, since it makes it clear that you're stopping at one below the count. `< count` is easy to misinterpret. but that's purely a stylistic choice, and therefore opinion, and therefore off-topic.