3.1.4.12 Lists - collections of data | lists and loops

Lists in action

Now you can easily swap the list's elements to reverse their order:

myList = [10, 1, 8, 3, 5] myList[0], myList[4] = myList[4], myList[0] myList[1], myList[3] = myList[3], myList[1] print(myList)

Run the snippet. Its output should look like this:

[5, 3, 8, 1, 10]

It looks fine with five elements.

Will it still be acceptable with a list containing 100 elements? No, it won't.

Can you use the for loop to do the same thing automatically, irrespective of the list's length? Yes, you can.

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This is how we've done it:

myList = [10, 1, 8, 3, 5] length = len(myList) for i in range(length // 2): myList[i], myList[length - i - 1] = myList[length - i - 1], myList[i] print(myList)

Note:

• we've assigned the length variable with the current list's length (this makes our code a bit clearer and shorter)
• we've launched the for loop to run through its body length // 2times (this works well for lists with both even and odd lengths, because when the list contains an odd number of elements, the middle one remains untouched)
• we've swapped the i^th element (from the beginning of the list) with the one with an index equal to (length - i - 1) (from the end of the list); in our example, for i equal to 0 the (l - i - 1) gives 4; for i equal to 1, it gives 3 - this is exactly what we needed.

Lists are extremely useful, and you'll encounter them very often.

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