How does array.length-- work?

Question

I'm having trouble understanding why people call
array.length--

after deleting an element in an array (as seen in this answer and many others). Why is there need for doing so? When is the length of an array evaluated? Or maybe I'm missing something obvious, what does array.length-- actually do?

Shane Fontaine · Answer

You must use array.length-- in order to accurately persist the length of the array.
The confusion comes from the fact that delete in Solidity does not actually delete an element from an array, but rather it just sets the element to it's default value (0 for a uint256, for example).
Observe the following code as an example:
uint256[] public testArray = [1,2,3,4,5];

// testArray = [1,2,3,4,5]
// testArray.length = 5

delete testArray[4];

// testArray = [1,2,3,4,0]
// testArray.length = 5

testArray.length--

// testArray = [1,2,3,4]
// testArray.length = 4

How does array.length-- work?

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