slicing strings with variables

Question

I am making a python program where I need to inspect individual four letter parts of a variable. If the variable is for example help_me_code_please it should output ease then e_pl etc , I attempted it with

a=0
b=3
repetitions=5
word="10011100110000111010"
for x in range (1,repetitions):        
    print(word[a:b])
    a=a+4
    b=b+4

however it just outputs empty lines. Thanks so much for any help in advanced.

Hey @Uriel, what is `repitions` and `word` in the code above? These don't appear to have been defined which would explain your problem — Peter Dolan, Mar 22 '18 at 18:57
This answer has worked for me in the past: https://stackoverflow.com/a/312464/3282436 — 0x5453, Mar 22 '18 at 18:58

Peter Dolan · Accepted Answer · 2018-03-22T21:15:55.303

0

This should work:

words = "help_me_code_please"
size = 4
start = len(words)%size - size

for i in reversed(range(start, len(words), size)):
    print(words[max(0,i):i+size])

output is

ease
e_pl
_cod
p_me
hel

Explanation: start gives us a starting point, that is some divisible block away from the length of our string (words). In our example, it's -1, as that is 20 (5*4) away from 19, the length of words.

Now we do a reverse loop, from 19 -> -1 at size four. So we go 15, 11, 7, 3, -1. Within each loop, we print from our position to the position + 4. So the first loop iteration prints out words[15:19] which outputs ease, the second is words[11:15], etc etc. This results in the output we see.

Hopefully this explanation makes sense!

edited Mar 22 '18 at 21:15

answered Mar 22 '18 at 19:08

Peter Dolan

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Thank you so much, would it be possible to be – Uriel Sasieni Mar 22 '18 at 19:13
1

I think you didn't finish your sentence @UrielSasieni! – Peter Dolan Mar 22 '18 at 19:21
Thank you so much, would it be possible to be outputed as: ease , e_pl , _cod , p_me , _hel – Uriel Sasieni Mar 22 '18 at 19:24
hey @UrielSasieni, updated my answer. Now outputs as you want it. – Peter Dolan Mar 22 '18 at 19:31
thats perfect, would you mind explaining it to me , thank you so much for your help – Uriel Sasieni Mar 22 '18 at 21:03
Added an explanation, hopefully that helps – Peter Dolan Mar 22 '18 at 21:16

score 0 · Answer 2 · answered Mar 22 '18 at 19:13

Assuming you want print the word backwards with a repetition of 4.

word = 'help_me_code_please'

# Looping from backwards on the word with a step of 4
for i in range(len(word)-1, -1, -4):
    # j is the starting point from where we have to print the string.
    j = i-3 if i > 3 else i-1
    print word[j:i+1]

Xavier · Answer 3 · 2018-03-22T21:11:48.777

For forward:

[word[4*l:4*(l+1)] for l in range(0, int(len(word)/4)+1)]
['help', '_me_', 'code', '_ple', 'ase']

For backward:

[word[::-1][4*l:4*(l+1)][::-1] for l in range(0, int(len(word)/4)+1)]
['ease', 'e_pl', '_cod', 'p_me', 'hel']

This is list comprehensions (https://docs.python.org/3.6/tutorial/datastructures.html#list-comprehensions), you can iterate the resulting list.

Its var[start:ends:step]. word[::-1] means get a letter, start at the beginning, finish at the end, and advance -1 steps, so it reverse the string

For the new values:

word="10011100110000111010"
[word[4*l:4*(l+1)] for l in range(0, int(len(word)/4)+1)]
['1001', '1100', '1100', '0011', '1010']

[word[::-1][4*l:4*(l+1)] for l in range(0, int(len(word)/4)+1)]
['0101', '1100', '0011', '0011', '1001']

it says s is not defined – Uriel Sasieni Mar 22 '18 at 20:59 — Uriel Sasieni, Mar 22 '18 at 20:59
Sorry, i used a var s and didn't change to word. Corrected – Xavier Mar 22 '18 at 21:12 — Xavier, Mar 22 '18 at 21:12

slicing strings with variables

3 Answers3