Reversing string in a for-loop

Question

I have the following function:

s = 'hello'
rev = ''

for char in s:
    rev = char + rev
return rev

Why does the for loop above add the characters in reverse? I tried using rev += char and it returned the string in the original order, s. This was mentioned in a video and they didn't explain properly, just said "Python's for loop over string doesn't work that way". I want to know why it does this, so in the future I will be more inclined to avoid these mistakes.

Another question is, can I implement this type of line in the for-loop over lists and will it output the reverse of the list? Since lists are mutable.

Because you're adding the character the the beginning of the list. Do rev = rev + char or simply rev += char (which means the same thing) instead. — user3064538
– user3064538, Commented Nov 2, 2019 at 1:03
Surely the purpose of this code is to reverse the string? Otherwise you would just return s directly. — kaya3
– kaya3, Commented Nov 2, 2019 at 1:05
This has nothing to do with "Python's for loop over string doesn't work that way". It's just because you are doing rev = ch + rev, that is: iterating over every character and adding it at the beginning of rev. — Marco Bonelli
– Marco Bonelli, Commented Nov 2, 2019 at 1:05
On the first iteration you have rev = "h" + "" so rev is set to "h". Then you have rev = "e" + "h" so rev is set to "eh" then you have rev = "l" + "eh" and rev becomes "leh" and so on. — user3064538
– user3064538, Commented Nov 2, 2019 at 1:08
Also, if you want to reverse the order, use string slicing 'hello'[::-1] = 'olleh'. See Strings and Character Data in Python. ['t', 'e', 's', 't'][::-1] = ['t', 's', 'e', 't'] lists too. — Trenton McKinney
– Trenton McKinney, Commented Nov 2, 2019 at 1:08

ascripter · Accepted Answer · 2019-11-02 01:05:41Z

3

The +-operator on strings means concatenation which is not commutative. So there's a difference between char + rev and rev + char.

Successively prepending the next character in front effectively reverses the string.

answered Nov 2, 2019 at 1:05

ascripter

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Comments

Austo · Accepted Answer · 2019-11-02 01:13:59Z

1

In the loop you have, your for loop is defined as:

s = 'happy'
rev = ''

for char in s:
    rev = ch + rev

If we look at this going through the first few iterations, we would get: h ah pah ppah yppah

This is because as you update the variable rev, you are adding the next character (char) in front of rev through your definition.

rev = NEW CHARACTER + CURRENT REV so looking at the final iteration, where we add y, you're adding:

rev = NEW CHARACTER (Y) + CURRENT REV (PPAH), basically stating you are adding PPAH to the letter y, instead of adding y to PPAH.

The loop can be easily fixed by swapping ch and rev, making it:

for char in s:
    rev = rev + char

answered Nov 2, 2019 at 1:13

Austo

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