Python: Passing Function Arguments for strings

Question

I am practicing with "Think Python", Exercise 8.1 that:

"Write a function that takes a string as an argument and displays the letters backward, one per line."

I am able to do this question, by using banana as an example to print each letter per line.

index = 0
fruit = "banana"
while index < len(fruit):
    letter = fruit[len(fruit)-index-1]
    print letter
    index = index + 1

However, I would like to generalize the situation to any input words and I got the problem, my code is

index = 0
def apple(fruit):
    while index < len(fruit):
        letter = fruit[len(fruit)-index-1]
        print letter
        index = index + 1

apple('banana')

The corresponding errors are:

Traceback (most recent call last):
  File "exercise8.1_mod.py", line 21, in <module>
    apple('banana')
  File "exercise8.1_mod.py", line 16, in apple
    while index < len(fruit):
UnboundLocalError: local variable 'index' referenced before assignment

I think there should be problems concerned with the function arguments used. Any helps will be appreciated.

Just put your `index = 0` inside your function as well (at the beginning of it). — BrenBarn, Jul 18 '14 at 18:18
@BrenBarn if you explain him y he need to keep inside so that he will learn about local and global variables right? :) — sundar nataraj, Jul 18 '14 at 18:20

score 1 · Answer 1 · edited May 23 '17 at 12:05

1

This should probably work better:

def apple(fruit):
    for letter in fruit[::-1]:
        print letter

apple('banana')

This works by indexing the string in reverse, a built in python function known as slicing.

Reverse a string in Python

edited May 23 '17 at 12:05

Community

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answered Jul 18 '14 at 18:20

beiller

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thx beiller, I learn from your statement, fruit[::-1], it is neat and sound pretty for me~! – nam Jul 19 '14 at 13:59

score 0 · Answer 2 · answered Jul 18 '14 at 18:24

0

You need to assign a value to index before you use it.

def apple(fruit):
    index = 0  # assign value to index
    while index < len(fruit):
        letter = fruit[len(fruit)-index-1]
        print letter
        index = index + 1
apple("peach")
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answered Jul 18 '14 at 18:24

Padraic Cunningham

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Thank you very much for your comment on *assign value to index*. My program gets run. Thx =] – nam Jul 19 '14 at 13:43

sundar nataraj · Accepted Answer · 2014-07-18T18:35:03.287

your program got error due to your accessing a global variable in your method and trying to change its value

index = 0
def apple(fruit):
    .....
    index = index + 1
    ....    
apple('banana')

this give you error UnboundLocalError: local variable 'index' referenced before assignment

but if you give

def apple(fruit):
        global index
        .....
        index = index + 1
        ....

this produces correct result

in python we have Global variable and Local variables

please go throught this

In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a new value anywhere within the function’s body, it’s assumed to be a local. If a variable is ever assigned a new value inside the function, the variable is implicitly local, and you need to explicitly declare it as global.

Thank you very much for your detailed explanation on local and global variables. It does help a lot! — nam, Jul 19 '14 at 13:42

Python: Passing Function Arguments for strings

3 Answers3