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
– BrenBarn, Commented Jul 18, 2014 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
– sundar nataraj, Commented Jul 18, 2014 at 18:20

Community · Accepted Answer · 2017-05-23 12:05:34Z

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, 2017 at 12:05

CommunityBot

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answered Jul 18, 2014 at 18:20

beiller

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1 Comment

nam Over a year ago

thx beiller, I learn from your statement, fruit[::-1], it is neat and sound pretty for me~!

sundar nataraj · Accepted Answer · 2014-07-18 18:35:03Z

0

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.

edited Jul 18, 2014 at 18:35

answered Jul 18, 2014 at 18:29

sundar nataraj

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1 Comment

nam Over a year ago

Thank you very much for your detailed explanation on local and global variables. It does help a lot!

Padraic Cunningham · Accepted Answer · 2014-07-18 18:24:06Z

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")
h
c
a
e
p

answered Jul 18, 2014 at 18:24

Padraic Cunningham

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1 Comment

nam Over a year ago

Thank you very much for your comment on assign value to index. My program gets run. Thx =]

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