CODEWITHSUNDEEP
pythonstringlistpython-3.xtype-conversion
hakuna121
hakuna121

Reputation: 243

Python-Unable to convert a list to string

What I was trying to do:

Take a string and append a backwards copy of that string, making a palindrome

What I came up with: 

# take an input string
a = input('Please enter a string: ')
a = list(a)

# read the string backwards
b = list(reversed(a))

# append the backward-ordered string to the original string, and print this new string
c = a + b
c = str(c)

print(c)

Question: When given a run, this script takes a string, for example "test", and returns ['t', 'e', 's', 't', 't', 's', 'e', 't']; I'm confused about this result since I explicitly converted c, as a result of concatenation of a and b, to a string. (c = str(c)) I know I must have missed some basic stuff here, but I wasn't able to figure out what. Could someone throw some light on this? Thank you!

And would anyone care to elaborate on why my c = str(c) didn't work? Thanks!

Upvotes: 1

Views: 4228

Answers (4)

Ricky H
Ricky H

Reputation: 117

def make_palindrome(string):
    return string + string[::-1]

make_palindrome(input('Please enter a String: '))

Upvotes: 0

dawg
dawg

Reputation: 103754

You can do this:

in_str = input('Please enter a string: ')
a = list(in_str)
b=a+a[::-1]
print (''.join(b))

Prints:

Please enter a string: test
testtset

And there is actually no reason to convert to a list first for this case since you can index, reverse and concatenate the string directly in Python:

>>> s='test'
>>> s+s[::-1]
'testtset'

Which shows a common idiom in Python to test if a string is a palindrome:

>>> pal='tattarrattat'
>>> pal==pal[::-1]
True

Upvotes: 1

abarnert
abarnert

Reputation: 365657

It's worth understanding how to use join—and nrpeterson's answer does a great job explaining that.

But it's also worth knowing how not to create problems for yourself to solve.

Ask yourself why you've called a = list(a). You're trying to convert a string to a sequence of characters, right? But a string is already a sequence of characters. You can call reversed on it, you can loop over it, you can slice it, etc. So, this is unnecessary.

And, if you've left a as a string, the slice a[::-1] is also a string.

That means your whole program can reduce to this:

a = input('Please enter a string: ')

# read the string backwards
b = a[::-1]

# append the backward-ordered string to the original string, and print this new string
c = a + b

print(c)

Or, more simply:

a = input('Please enter a string: ')

print(a + a[::-1])

Upvotes: 1

Nick Peterson
Nick Peterson

Reputation: 771

The problem with saying c = str(c) is that applying str to a list simply gives a string representation of that list - so, for instance, str([1,2,3]) yields the string '[1, 2, 3]'.

The easiest way to make a list of strings in to a string is to use the str.join() method. Given a string s and a list a of strings, running s.join(a) returns a string formed by joining the elements of a, using s as the glue.

For instance:

a = ['h','e','l','l','o']
print( ''.join(a) ) # Prints: hello

Or:

a = ['Hello', 'and', 'welcome']
print( ' '.join(a) ) # Prints: Hello and welcome

Finally:

a = ['555','414','2799']
print( '-'.join(a) ) # Prints: 555-414-2799

Upvotes: 5

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