Python del operator not working in my for loop

Question

I'm very new to python and have read what I could find on for loops and the del operator, but I still don't understand why my solution to the problem below isn't working:

PROBLEM

Given a string, return a new string made of every other char starting with the first, so "Hello" yields "Hlo".

string_bits('Hello') → 'Hlo'
string_bits('Hi') → 'H'
string_bits('Heeololeo') → 'Hello'

SOLUTION

def string_bits(str):
    for i in range(len(str)):
      if i % 2 != 0:
        del[i]
    return str

This just returns a replica of the original string and doesn't delete anything - why aren't the odd numbers in the range being deleted?

Note: even if that were possible, del[i] wouldn't be the correct syntax; you're deleting the name i, not a piece of the sequence str. Also, don't name a variable str. — Wooble
– Wooble, Commented Jan 9, 2014 at 16:22

Ffisegydd · Accepted Answer · 2014-01-09 16:14:07Z

4

Strings are immutable, meaning you can't just delete arbitrary characters.

To solve your problem I would use slice notation

x = 'Heeololeo'

y = x[::2]

print(y) # Hello

You can find the details here.

answered Jan 9, 2014 at 16:14

Ffisegydd

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Comments

kylieCatt · Accepted Answer · 2014-01-09 16:12:49Z

1

You shouldn't change something that you are iterating over. You should extract the data you need and save it to a new variable and return that.

answered Jan 9, 2014 at 16:12

kylieCatt

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aIKid · Accepted Answer · 2014-01-09 16:13:06Z

1

First, strings are immutable. Second, you're not supposed to modify an iterable while iterating over it.

Make a copy of it, or use a filter:

new_string = ''.join(c for i, c in enumerate(old_string) if i%2 != 0)

Simplified version:

result = []
for i, c in enumerate(old_string):
    if i%2 != 0:
        result.append(c)
new_string = ''.join(result)

An easier, cooler way is to make a slice of it, with the extended slice notation:

new_string = old_string[::2]

Also, avoid using names like str, you'll shadow the useful built-in function.

answered Jan 9, 2014 at 16:13

aIKid

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jeffknupp · Accepted Answer · 2014-01-09 16:13:16Z

1

strings in Python are immutable, meaning that once they are created, they can't be changed. All functions that seem to "change" strings actually return a new version of the string.

Regardless, a good rule of thumb is to not change things you're iterating over. You could use a list comprehension to create a new string with only the character that you want.

answered Jan 9, 2014 at 16:13

jeffknupp

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