Array.length在for循环中未产生预期结果

Question

So I was doing a question which involves reversing an array. It takes an array as an argument and produces a new array that has the same elements in the inverse order. This is what I came up with at first.

var fruits = ["Banana", "Orange", "Apple", "Mango"];
var array = [];

for(var i = 0; i < fruits.length; i++){
 array.push(fruits.pop(i))
}
console.log(array);

The result i ended up getting was ["Mango", "Apple"] which is not what i was hoping for. So i tried this instead:

var fruits = ["Banana", "Orange", "Apple", "Mango"];
var array = [];
var count = fruits.length;

for(var i = 0; i < count; i++){
 array.push(fruits.pop(i))
}
console.log(array);

This version gave me what I wanted ["Mango", "Apple", "Orange", "Banana"] . So the only difference between them is that in the first one I use fruits.length directly in the for loop and I assign the value of fruits.length to count in the second one which I then use in the for loop. I don't understand why they produce two different results. Aren't they the same thing? Can someone please help understand what's going on here?

Answer 1

You are removing a fruit from your array ie fruits.pop(i) , which will decrement the fruits.length . Which is then evaluated every time in the loop.

So it makes perfectly sense that your second example works correctly. Here you set the length once and once only.

Answer 2

Using fruits.pop() removes the last element from the array, so on the first pass through, i = 0 and fruits.length = 4, next pass i = 1 and fruits.length = 3, next pass i = 2 and fruits.length also = 2 so your i < fruits.length check is no longer passing and the loop ends.

In your second example, you are pre-defining the length to equal 4 and that doesn't change as you are removing elements from the array, so the loop continues while i < 4.

Answer 3

Basically, array.length will be evaluated in every loop, and you're changing the array size every time by -1. Since you're assigning the count to a variable before entering the loop, the variable won't change.

Let's break it down:

Your array has 4 items, your counter is 0. You iterate once, remove one item from the array and the counter goes up by 1. Now array.length is 3.

Same thing happens, now counter is 2 and array.length is 2.

When trying to run for the third time, the conditional will not match and the code won't run. That's why the array is having two elements only.

Answer 4

Hm, You are using pop() method which removes an accessed element from an array after it has been read.

In the beginning, you have 4 elements.

So loop starts to access the last element and removes it from fruits and then add to a new array.
At next loop length is 3 instead 4.
then length is 2 and this is a place where the loop has not meet the condition for continue.

If you store count number in a variable it will not be affected in latter array count.

Answer 5

Firstly, Array.pop does not acknowledges any parameter. Hence, fruits.pop(i) & fruits.pop() are one and the same thing.

Understanding 1st Loop

for(var i = 0; i < fruits.length; i++){
 array.push(fruits.pop(i))
}

For i = 0 , fruits = ["Banana", "Orange", "Apple", "Mango"] , fruits.length = 4 -> i < fruits.length is TRUE , enters loop and pops out 1 value

For i = 1 , fruits = ["Banana", "Orange", "Apple"] , fruits.length = 3 -> i < fruits.length is TRUE , enters loop and pops out 1 value

For i = 2 , fruits = ["Banana", "Orange"] , fruits.length = 2 -> i < fruits.length is FALSE , breaks the loop

---

Understanding 2nd Loop

for(var i = 0; i < count; i++){
 array.push(fruits.pop(i))
}

For i = 0 , fruits = ["Banana", "Orange", "Apple", "Mango"] , count = 4 -> i < count is TRUE , enters loop and pops out 1 value

For i = 1 , fruits = ["Banana", "Orange", "Apple"] , count = 4 -> i < count is TRUE , enters loop and pops out 1 value

For i = 2 , fruits = ["Banana", "Orange"] , count = 4 -> i < count is TRUE , enters loop and pops out 1 value

For i = 3 , fruits = ["Banana"] , count = 4 -> i < count is TRUE , enters loop and pops out 1 value

For i = 4 , fruits = [] , count = 4 -> i < count is FALSE , breaks the loop