Python string range does not produce the last character

Question

The word Hello contains 5 characters.

>>> x = 'Hello'
>>>
>>> len(x)
5
>>>

x[5] will produce an error as Python number is started from 0. I can understand this.

>>> x[5]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range
>>> 

>>> x[0]
'H'
>>>

And last charactor is at x[4]

>>> x[4]
'o'
>>>

However, when I try a range which is from x[0:4], the last letter is not there.

>>> x[0:4]
'Hell'
>>>

May I know why?

range's upper bound is **excluded** – RomanPerekhrest Aug 08 '19 at 13:41 — RomanPerekhrest, Aug 08 '19 at 13:41

score 1 · Answer 1 · answered Aug 08 '19 at 13:41

1

Slicing from 0 to 4 will produce items 0, 1, 2, and 3. If you want to include the las one use

x[0:5]

answered Aug 08 '19 at 13:41

josoler

score 1 · Answer 2 · answered Aug 08 '19 at 13:41

1

x[0:4]

means

index 4 is excluded.

There is a rationale behind that. You can just substract the two numbers to calculate the length of outcome. :)

answered Aug 08 '19 at 13:41

Saurav Sahu

The rationale is that you can do `x` corresponds to `x[0:len(x)]` and not `len(x)-1` – WiseDev Aug 08 '19 at 13:47

score 1 · Answer 3 · answered Aug 08 '19 at 13:43

1

In slicing, first int is included while last is excluded

answered Aug 08 '19 at 13:43

score 1 · Answer 4 · answered Aug 08 '19 at 13:46

1

I believe the reason is the way most of those operations are performed:

for(x=start;x<end;x++){
   output[x]=input[x]
}

(pardon my C).

Another reason is consistency between slice operator and len. It's believed that a[0:len(a)] should return a.

answered Aug 08 '19 at 13:46

George Shuklin

Thanks @George. Would it be possible to view the actual source code of this slicing technique in Python? If yes, please let me know how or where is the actual code. – Aug 08 '19 at 23:56

4 Answers4