Lists

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Python

Python Lists

Loops let us do things many times

Python Lists

Collections let us store many values together

Python Lists

Collections let us store many values together

Most popular collection is a list

Python Lists

Create using [value, value, ...]

Python Lists

Get/set values using var[index]

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print gases['He', 'Ne', 'Ar', 'Kr']

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print gases['He', 'Ne', 'Ar', 'Kr']

print gases[1]Ne

Python Lists

Index from 0, not 1

Python Lists

Index from 0, not 1

Reasons made sense for C in 1970...

Python Lists

Index from 0, not 1

It's an error to try to access out of range

Python Lists

Index from 0, not 1

It's an error to try to access out of range

gases = ['He', 'Ne', 'Ar', 'Kr']print gases[4]IndexError: list index out of range

Python Lists

Use len(list) to get length of list

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print len(gases)4

Python Lists

Returns 0 for the empty list

etheric = []print len(etheric)0

Python Lists

Some negative indices work

Python Lists

values[-1] is last element, values[-2] next-to-last, ...

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print gases[-1], gases[-4]Kr He

Python Lists

values[-1] is much nicer than values[len(values)-1]

Python Lists

less error prone

values[-1] is much nicer than values[len(values)-1]

Python Lists

Mutable : can change it after it is created

Python Lists

gases = ['He', 'Ne', 'Ar', 'K'] # last entry misspelled

Python Lists

gases = ['He', 'Ne', 'Ar', 'K'] # last entry misspelledgases[3] = 'Kr'

Python Lists

gases = ['He', 'Ne', 'Ar', 'K'] # last entry misspelledgases[3] = 'Kr'print gases['He', 'Ne', 'Ar', 'Kr']

Python Lists

Location must exist before assignment

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']gases[4] = 'Xe'IndexError: list assignment index out of range

Python Lists

Heterogeneous : can store values of many kinds

Python Lists

helium = ['He', 2]neon = ['Ne', 8]

Python Lists

[string, int]

Python Lists

helium

Python Lists

helium = ['He', 2]neon = ['Ne', 8]gases = [helium, neon]

Python Lists

helium

Python Lists

helium

Devote a whole

episode to this

Python Lists

Loop over elements to "do all"

Python Lists

Use while to step through all possible indices

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']i = 0while i < len(gases): print gases[i] i += 1

Python Lists

First legal index

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']i = 0while i < len(gases): print gases[i] i += 1 Next index

Python Lists

Defines set of legal indices

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']i = 0while i < len(gases): print gases[i] i += 1HeNeArKr

Python Lists

Tedious to type in over and over again

Python Lists

Tedious to type in over and over again

And it's easy to forget the "+= 1" at the end

Python Lists

Use a for loop to access each value in turn

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']for gas in gases: print gasHeNeArKr

Python Lists

Loop variable assigned each value in turn

Python Lists

Not each index

Python Lists

Not each index

Because that's the most common case

Python Lists

Can delete entries entirely (shortens the list)

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']del gases[0]

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']del gases[0]print gases['Ne', 'Ar', 'Kr']

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']del gases[0]print gases['Ne', 'Ar', 'Kr']del gases[2]

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']del gases[0]print gases['Ne', 'Ar', 'Kr']del gases[2]print gases['Ne', 'Ar']

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']del gases[0]print gases['Ne', 'Ar', 'Kr']del gases[2]print gases['Ne', 'Ar']

Yes, deleting an index that doesn't exist is an error

Python Lists

Appending values to a list lengthens it

Python Lists

gases = []

Python Lists

gases = []gases.append('He')

Python Lists

gases = []gases.append('He')gases.append('Ne')

Python Lists

gases = []gases.append('He')gases.append('Ne')gases.append('Ar')

Python Lists

gases = []gases.append('He')gases.append('Ne')gases.append('Ar')print gases['He', 'Ne', 'Ar']

Python Lists

Most operations on lists are methods

Python Lists

A function that belongs to (and usually operates on)

specific data

Python Lists

A function that belongs to (and usually operates on)

specific data

thing . method (args)

Python Lists

Some useful list methods

Python Lists

gases = ['He', 'He', 'Ar', 'Kr'] # 'He' is duplicated

Python Lists

gases = ['He', 'He', 'Ar', 'Kr'] # 'He' is duplicatedprint gases.count('He')2

Python Lists

gases = ['He', 'He', 'Ar', 'Kr'] # 'He' is duplicatedprint gases.count('He')2print gases.index('Ar')2

Python Lists

gases = ['He', 'He', 'Ar', 'Kr'] # 'He' is duplicatedprint gases.count('He')2print gases.index('Ar')2gases.insert(1, 'Ne')

Python Lists

gases = ['He', 'He', 'Ar', 'Kr'] # 'He' is duplicatedprint gases.count('He')2print gases.index('Ar')2gases.insert(1, 'Ne')print gases['He', 'Ne', 'He', 'Ar', 'Kr']

Python Lists

Two that are often used incorrectly

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print gases.sort()None

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print gases.sort()Noneprint gases['Ar', 'He', 'Kr', 'Ne']

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print gases.sort()Noneprint gases['Ar', 'He', 'Kr', 'Ne']print gases.reverse()None

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print gases.sort()Noneprint gases['Ar', 'He', 'Kr', 'Ne']print gases.reverse()Noneprint gases['Ne', 'Kr', 'He', 'Ar']

Python Lists

A common bug

Python Lists

A common bug

gases = gases.sort() assigns None to gases

Python Lists

Use in to test for membership

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print 'He' in gasesTrue

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print 'He' in gasesTrueif 'Pu' in gases: print 'But plutonium is not a gas!'else: print 'The universe is well ordered.'

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print 'He' in gasesTrueif 'Pu' in gases: print 'But plutonium is not a gas!'else: print 'The universe is well ordered.'The universe is well ordered.

Python Lists

Use range to construct lists of numbers

Python Lists

print range(5)[0, 1, 2, 3, 4]

Python Lists

print range(5)[0, 1, 2, 3, 4]print range(2, 6)[2, 3, 4, 5]

Python Lists

print range(5)[0, 1, 2, 3, 4]print range(2, 6)[2, 3, 4, 5]print range(0, 10, 3)[0, 3, 6, 9]

Python Lists

print range(5)[0, 1, 2, 3, 4]print range(2, 6)[2, 3, 4, 5]print range(0, 10, 3)[0, 3, 6, 9]print range(10, 0)[]

Python Lists

So range(len(list)) is all indices for the list

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print len(gases)4print range(len(gases))[0, 1, 2, 3]

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print len(gases)4print range(len(gases))[0, 1, 2, 3]for i in range(len(gases)): print i, gases[i]

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print len(gases)4print range(len(gases))[0, 1, 2, 3]for i in range(len(gases)): print i, gases[i]0 He1 Ne2 Ar3 Kr

Python Lists

gases = ['He', 'Ne', 'Ar', 'Kr']print len(gases)4print range(len(gases))[0, 1, 2, 3]for i in range(len(gases)): print i, gases[i]0 He1 Ne2 Ar3 Kr

A very common idiom in Python

October 2010

narrated by

Dominique Vuvan

This work is licensed under the Creative Commons Attribution License

See http://software-carpentry.org/license.html for more information.

Lists

Technology

Transcript of Lists

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