How to count down in for loop?

Question

In Java, I have the following for loop and I am learning Python:

for (int index = last-1; index >= posn; index--)

My question is simple and probably obvious for most of people who are familiar with Python. I would like to code that ‘for’ loop in Python. How can I do this?

I tried to do the following:

for index in range(last-1, posn, -1):

I think it should be range(last-1, posn + 1, -1). Am I right?

I am thankful to anyone especially who will explain to me how to understand the indices work in Python.

Asked By: Jika

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Source

Answer 1

The range function in python has the syntax:

range(start, end, step)

It has the same syntax as python lists where the start is inclusive but the end is exclusive.

So if you want to count from 5 to 1, you would use range(5,0,-1) and if you wanted to count from last to posn you would use range(last, posn - 1, -1).

Answered By: Loocid

Answer 2

If you google. "Count down for loop python" you get these, which are pretty accurate.

how to loop down in python list (countdown)
Loop backwards using indices in Python?

I recommend doing minor searches before posting. Also "Learn Python The Hard Way" is a good place to start.

Answered By: user4421333

Answer 3

First I recommand you can try use print and observe the action:

for i in range(0, 5, 1):
    print i

the result:

You can understand the function principle.
In fact, range scan range is from 0 to 5-1.
It equals 0 <= i < 5

When you really understand for-loop in python, I think its time we get back to business. Let’s focus your problem.

You want to use a DECREMENT for-loop in python.
I suggest a for-loop tutorial for example.

for i in range(5, 0, -1):
    print i

the result:

Thus it can be seen, it equals 5 >= i > 0

You want to implement your java code in python:

for (int index = last-1; index >= posn; index--)

It should code this:

for i in range(last-1, posn-1, -1)

Answered By: Burger King

Answer 4

In python, when you have an iterable, usually you iterate without an index:

letters = 'abcdef' # or a list, tupple or other iterable
for l in letters:
    print(l)

If you need to traverse the iterable in reverse order, you would do:

for l in letters[::-1]:
    print(l)

When for any reason you need the index, you can use enumerate:

for i, l in enumerate(letters, start=1): #start is 0 by default
    print(i,l)

You can enumerate in reverse order too…

for i, l in enumerate(letters[::-1])
    print(i,l)

ON ANOTHER NOTE…

Usually when we traverse an iterable we do it to apply the same procedure or function to each element. In these cases, it is better to use map:

If we need to capitilize each letter:

map(str.upper, letters)

Or get the Unicode code of each letter:

map(ord, letters)

Answered By: chapelo

How to count down in for loop?

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