Indexing

Python for Linguists

Axel Bohmann

Language data as strings and lists

mystring = "I am a text, but in Python does not know that. To Python, I am just a sequence of characters."
myWordList = ["I", "am", "a", "list", "of", "words."]

• Python does not understand language; that task is yours

• Both strings and lists are ordered sequences:
  • Position matters: Python understands that "a text" comes after "I am " in mystring and "list" is the fourth item in myWordList.
  • We can access elements in an ordered sequence via their index position.

Indexing

• Get item at index position 1 in the string "abcd":

"abcd"[1]

• The first item in an ordered sequence is in index 0.

• What happens when you try this?

"abcd"[4]

• What about this?

"abcd"[-1]

Index positions

• Indexing from the left:

element	A	B	C	D	E	F
index	0	1	2	3	4	5
	0	…	…	…	…	len(s)-1

• Indexing from the left:

element	A	B	C	D	E	F
index	-6	-5	-4	-3	-2	-1
	-len(s)	…	…	…	…	-1

Slicing

• Not just one element, but a sub-sequence:

"Mississippi"[3:8]

• The left index is inclusive: It is the position of the first element in the slice.

• The right index is exclusive: It is the index position of the first element outside of the slice.

• Try these:

"Mississippi"[3:1000]
"Mississippi"[2:-2]
"Mississippi"[-3:-1]

Slicing rules

• Trying to access a single index outside of the actual range of of indices throws an IndexError: string/list index out of range.

• Going outside the range when slicing is more forgiving.

• Also, when your slice coincides with the beginning/end of a sequence, you can omit the corresponding index:

"Mississippi"[3:]
"Mississippi"[:3]
"Mississippi"[:-2]

Let’s practice

Exercise

• Define a string called test containing the word "hullaballoo".

• Use slicing to extract the sequence "ball" from test.

• Define a list called chomsky containing the words in the sentence "colorless green ideas sleep furiously".

• Using slicing methods and other list operations, can you produce the list ["coloress", "green", "sleep", "furiously"]?

• Challenge: What about ["furious", "sleep"]?

Better than sliced bread!

Image Credits

• Title slide and slide background: Coiled snake png sticker illustration, image in the public domain (CC0)
• Side 7: Yellow rock python snake photo, image in the public domain (CC0)
• Slide 9: Benjamin Franklin’s Join, or Die cartoon image in the public domain (CC0)

Thank you

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For the presentation slides, visit

https://pylx-4-indexing.netlify.app/

or scan QR code.

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