[FIXED] I do not understand why this function reverses the string

Issue

This function requests a string input and reverses it. For some reason, I just cannot wrap my head around the logic behind it.

def reverse(s):
    new = ""
    for i in s:
        print(new)
        new = i + new
    return new
oldStr = input("String?")
newStr = reverse(oldStr)
print(newStr)
print(reverse("good bye"))

A friend suggested I print the variable new in the string which I added and it helped a little, but I just don't understand it.

Solution

It looks to me as if you are in a stage where you want to learn how to debug small programs.

For questions on StackOverflow, you want to provide a minimal reproducible example. What does that mean in your case?

1. "repoducible" means that you should not depend on user input. Replace user input by hardcoded values.
2. "minimal" means that you don't call the function twice. Once is enough.

For that reason, I'll not walk you through your original program, but through this one instead:

def reverse(s):
    new = ""
    for i in s:
        print(new)
        new = i + new
    return new

print(reverse("abcdef"))

I'll use a free program called PyCharm Community Edition. You set a breakpoint where you want to understand how things work. Since you don't understand the reverse() method, put it right at the beginning of that method. You do that by clicking between the line number and the text:

Even if your code has no bug, go to the Run/Debug menu:

Execution will stop at the breakpoint and you'll now be able to see all the variables and step through your code line by line.

Look at all the variables after each single line. Compare the values to what you think the values should be. If there is a mismatch, it's likely a misunderstanding on your side, not by Python. Do that a few times and it'll be obvious why and how the string is reversed.

Answered By - Thomas Weller