CODEWITHSUNDEEP
pythonpython-3.x
nsdimeglio
nsdimeglio

Reputation: 13

Why do these two programs return different output?

I have two programs that are supposed to reverse a list of chars (strings with length 1). While the second program gives the correct output, the first one doesn't.

Here is program #1:

string = ['h', 'e', 'l', 'l', 'l', 'o']
y = len(string) / 2
for letter in string:
    x = string.index(letter)
    if x < y:
       string[x], string[-1-x] = string[-1-x], string[x]

Here is program #2:

string = ['h', 'e', 'l', 'l', 'l', 'o']
y = len(string) / 2
x = 0
while x < y:
    if x < y:
        string[x], string[-1-x] = string[-1-x], string[x]
    x += 1

My second program successfully reverses the string, but the first one returns ['o', 'e', 'l', 'l', 'l', 'h']. I'm glad I found a solution but would prefer to use .index() rather than counting each loop.

Upvotes: 0

Views: 137

Answers (2)

TrebledJ
TrebledJ

Reputation: 8987

There is an anti-pattern in your first program.

x = string.index(letter)

Consider when you're indexing for the 2nd or 3rd 'l'... x will always be 2. The condition x < y will kick in and reverse the positions of 'e' and the second 'l'.

Here's some debugging:
(Notice that x is 2 at the second, third, and fourth iterations when it should be 2, 3, and 4 respectively.)

Iter 0:
letter = 'h'; x = 0; swap √; string = ['o', 'e', 'l', 'l', 'l', 'h']
#                                       ^------------------------^

Iter 1:
letter = 'e'; x = 1; swap √; string = ['o', 'l', 'l', 'l', 'e', 'h']
#                                            ^--------------^    

Iter 2:
letter = 'l'; x = 2; swap √; string = ['o', 'l', 'l', 'l', 'e', 'h']
#                                                 ^----^    

Iter 3:           !
letter = 'l'; x = 2; swap √; string = ['o', 'l', 'l', 'l', 'e', 'h']
#                                                 ^----^    

Iter 4:           !
letter = 'l'; x = 2; swap √; string = ['o', 'e', 'l', 'l', 'l', 'h']
#                                            ^--------------^    

Iter 5:
letter = 'l'; x = 5; swap X; string = ['o', 'e', 'l', 'l', 'l', 'h']

Pythonically, you should use enumerate to keep track of the index:

y = len(string) / 2
for i, _ in enumerate(string):
    if i < y:
       string[i], string[-1-i] = string[-1-i], string[i]

(An underscore was substituted for letter following PEP8 style guides.)

Or even better:

string = string[::-1]

Upvotes: 2

Samy
Samy

Reputation: 649

I see that the variable "string" is list of strings. You can reverse it as simple as this:

string = ['h', 'e', 'l', 'l', 'l', 'o']
string.reverse()
string

This should print ['o', 'l', 'l', 'l', 'e', 'h']

Upvotes: 0

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