---

Back

Python Chrestomathics

Ahead

---

 

Part 7 — Lists

Lest we go forward — Without knowing about lists — We would be wanting

---

Section 7.1 Basics

• A Python list is an ordered sequence of elements — there is a first element, second element, and so on. The word array is often used by programmers as a synonym for list.

• As with strings, the first element of a list has index 0, the second element has index 1, and so on. Therefore, the last element in a list with n elements has index n-1.

• You can create a new empty list by using empty brackets. The below statement makes values a list with no elements. The code snippets in this section are from program list_basics.py.

# create an empty list

values = []

• You can populate a new list by putting values within the brackets. The below snippet initializes stuff to be a three-element list and numbers to be a five-element list. Unlike some programming languages, Python does not require its list elements have the same type.

# create some non-empty lists

stuff = [ "abc", 1112, 2.71 ]

numbers = [ 14, 12, 27, 31, 24 ]

• The len() function works on lists. When used that way, it returns the number of elements in its list argument. The below snippet initializes nv, ns, and nv respectively to 0, 3, and 5.

# determine lengths

nv = len( values )

ns = len( stuff )

nn = len( numbers )

print( "nv:", nv )

print( "ns:", ns )

print( "nn:", nn )

Its output confirms this intitialization.

nv: 0

ns: 3

nn: 5

• The brackets operator works on lists in the same way as it does on strings. If there is a single integer within the brackets, then the integer is used as an index to determine the list value of interest. Suppose variables i and j are initialized to 1 and 2.

# set indices

i = 2

j = 4

Then the below code initializes x and y to 27 and 24.

# access elements

x = numbers[ i ]

y = numbers[ j ]

print( "x:", x )

print( "y:", y )

The output confirms the initialization.

x: 27

y: 24

• Unlike strings, lists are mutable — the values of individual elements can change, and elements can be deleted, inserted, or appended. The below statement resets the third value in numbers from 27 to 25.

numbers[ 2 ] = 25

• If a colon (:) appears within the brackets, then a new list is created through slicing. The below statement sets items to be a copy of those elements of numbers with indices 1 through 3; that is [12, 25, 31].

# slice

items = numbers[ 1 : 4 ]

• Built-in functions max() and min() can be used to determine the smallest and largest values in a list. The below code sets variables a and b to 12 and 31.

# get min and max

a = min( numbers )

b = max( numbers )

print( "a:", a )

print( "b:", b )

The snippet shows these values in its output.

a: 12

b: 31

Naming gotchas Sometimes programmers will write code like the following, where data is a list of numbers. min = min( data ) max = min( data ) When the code completes, variables min and max are respectively the minium and maximum values in data. They are no longer built-in functions and cannot be used to determine min and maxes of other lists. So, the code is better written as the following. data_min = min( data ) data_max = min( data )

• Lists can be displayed using function print(). The below code segment prints the example lists.

print( "values:", values )

print( "stuff:", stuff )

print( "numbers:", numbers )

print( "items:", items )

• The code segment outputs

values: []

stuff: ['abc', 1112, 2.71]

numbers: [14, 12, 25, 31, 24]

items: [12, 25, 31]

---

Section 7.2 — List methods for adding and counting elements

• Lists have many method functions to inspect and manipulate the list. The code demonstrations in this section are from program list_add_count.py. The program begins with a request for text from the user. The reply is split into a list of words and printed.

# get example text

reply = input( "Enter text: " )

words = reply.split()

print( "words:", words )

• Suppose the user responds as below.

Enter words: better today means better tomorrow

• Then words will be below the list.

words: ['better', 'today', 'means', 'better', 'tomorrow']

• To append a value to the end of a list, use list method function append(). The next statement updates words to have a sixth element equal to "okay".

# append a word

words.append( "okay" )

• The list words is now

words: ['better', 'today', 'means', 'better', 'tomorrow', 'okay']

• There is list method function insert() to place a new element at an indicated index. The function takes two arguments, the index and element value of interest.

# insert a word

words.insert( 3, "a" )

• The list words is now

words: ['better', 'today', 'means', 'a', 'better', 'tomorrow', 'okay']

• There is also a list method function count() that takes a single argument. The list returns the number of elements equal to the argument.

# search words

n1 = words.count( "better" )

n2 = words.count( "e" )

• The values of n1 and n2 are respectively 2 and 0.

n1: 2

n2: 0

Counting gotcha I suspect some of you might be surprised that the above code reports it did not find an "e" in words. You need to remember that count() was checking for elements equal to "e" and not elements that contained an "e".

---

Section 7.3 — List methods for deleting elements

• Lists have two methods for removing elements — pop() and remove(). The code demonstrations in this section are from program list_delete_methods.py. The program begins with some request for input from the user.

# get example text

reply = input( "Enter text: " )

words = reply.split()

reply = input( "Enter index: " )

i = int( reply )

w = input( "Enter word: " )

• The inputs are used to get a lists of words w, an index i, and a word of interest w.

• Suppose the user responds as below.

Enter text: abc acb bac bca cab cba

Enter index: 4

Enter word: bac

• Then the values of words, i, and a w are

words: ['abc', 'acb', 'bac', 'bca', 'cab', 'cba']

i: 0

w: bac

• List method pop() has an optional index parameter. If an index is supplied, then the element at that index is deleted from the list. If no index is supplied, then the last element in the list is deleted. Either way, the method returns the value of the deleted element.

• The below code segment shows both ways to use method pop().

# pop some elements

popped1 = words.pop( i )

popped2 = words.pop( )

• As variable i has value 0, the program run removes the first and last elements fron words and initializes variables popped1 and popped2 to those former list values.

popped1: abc

popped2: cba

words after removing popped1 and popped2: ['acb', 'bac', 'bca', 'cab']

• List method remove() expects as its argument one of the values from its list. If such a value is given, the first occurence of the value is deleted from the list.

# remove element with value w

words.remove( w )

• After the above statement executes words now equals

words after removing w: ['acb', 'bca', 'cab']

Removing gotchas If method remove() is not given a list value as its argument, then the statement generates an error when executed.

• If it becomes necessary to remove all elements from a list, then use method clear() as shown below.

# clear out all the elements

words.clear()

• After the statement runs words has no elements.

words after clearing: []

---

Section 7.4 What’s next

• We next turn our attention to statements that support repeated execution of their actions

---

Back

Python Chrestomathics

Ahead

 

---

© Jim Cohoon. For now, all rights reserved.