def reverse(x):
output = ""
for c in x:
output = c + output
return output
print(reverse("Hello"))
This code works fine to reverse a string in Python, I just can't seem to understand why it is working and how.
If, for example, I iterate through a string, usually it will iterate and print starting from "H" and work its way down to "O." How is it that here, it's going backwards?
If you put some output to your loop you can see how the code is filling up the string.
def reverse(x):
output = ""
for c in x:
output = c + output
print(output)
return output
print(reverse("Hello"))
So you can see how it works:
H
eH
leH
lleH
olleH
olleH
So you ad the last one to the end and that is how you reverse it. If you switch the following line in your loop:
output = output + c
Then the order is not reversed:
H
He
Hel
Hell
Hello
Hello
This loop goes over the characters of the string, and for each character prepends it to the current result.
Let's track the value of output as we go over the characters (the c values):
c output
H H
e eH
l leH
l lleH
o olleH
Every time a character goes into the loop, it gets appended into the output. Initially output = ""
So for iteration 1: output = ""
c = "H"
output = "H" + "" = "H"
Iteration 2: output = "H"
c = "e"
output = "e" + "H" = "eH"
Iteration 3: output = "eH"
c = "l"
output = "l" + "eH" = "leH"
Iteration 4: output = "leH"
c = "l"
output = "l" + "leH" = "lleH"
Iteration 5: output = "lleH"
c = "o"
output = "o" + "lleH" = "olleH"
So the final result comes out to "olleH"
output = c + output. The character is concatenated to the beginning of the output string instead of to the end, resulting in a reversed string. The easiest way to reverse a string, however, isx[::-1].