python - start from a number and then subtract x in a for

Question

I have a code like this

for x in range(1,20):
    print(x)

it works perfectly, but when I want to start from the end like => 60 to 1 i tried this first

for x in range(20,1):
    print(x)

but it prints nothing! also, I tried this,

for x in range(1,20):
    count = 20
    count -= 1
    print(count)

but -= 1 only printed twenty samples of 19. any help?

Take a look at the first statement INSIDE your for loop (count =20). Think about what is happening to count every time the for loop increments. — Eris Cacigieri, Aug 26 '22 at 17:12
Does this answer your question? [Print a list in reverse order with range()?](https://stackoverflow.com/questions/7286365/print-a-list-in-reverse-order-with-range) — Abdul Aziz Barkat, Aug 26 '22 at 17:26

Massifox · Accepted Answer · 2022-08-26T17:19:12.180

1

First answer: Use range() built-in function whit negative step:

for x in range(20, 1, -1):
    print(x)

Second answer: You have to declare the count variable outside the for loop, otherwise it will be initialized to 20 each loop, and then subtracting 1 will always make 19 for each loop.

count = 20
for x in range(1,20):
    count -= 1
    print(count)

edited Aug 26 '22 at 17:19

answered Aug 26 '22 at 17:11

Massifox

4,369
11
31

1

This works, but the OP needs to see what they are doing in the for loop that is causing the 19s to repeat – Eris Cacigieri Aug 26 '22 at 17:16

score 1 · Answer 2 · answered Aug 26 '22 at 17:24

@Massifox already pointed out the -1 step but I can add more context. The optional third parameter in range() is step. If you did range(1, 20, 2) it would print 2, 4, 6, ..., 18 because you specified steps of 2.

For your example, you started 20, and incremented until you hit 1, but that will never happen, so nothing gets printed, so you specify a step of -1 like range(20, 1, -1), so that it will step backwards each iteration printing 20, 19, ..., 2 (second parameter is exclusive).

When you tried this

for x in range(1,20):
    count = 20
    count -= 1
    print(count)

You created a temp variable named count, initialized to 20, subtracted one from it, and then printed it, and the result was the 19's you were getting. Everything inside the for loop will be a temporary variable and cannot affect the iteration of the for loop. LMK if this is too verbose.

python - start from a number and then subtract x in a for

2 Answers2