working-with-lists.md

Working with Lists

**Lists **consist of a sequence of elements. Each element is accessed using an **index **value, as you access characters in a string. For example,

marks = [75, 91, 82, 70, 88, 77, 90, 86]

**Notice **the square brackets and that the **elements **in the list are separated by commas. The following program

print( marks[0] )
print( marks[7] )
print( marks[3] )

produces the output

75
86
70

As with strings, the **index **of the **element **at the beginning of the list is 0 and increases by 1 as you move across the list. Also, the code

print( len( marks ) )

outputs the length of the list

8

as with strings.

Differences between Strings and Lists

• Strings contain a sequence of characters, whereas **lists **can contain a mixture of different data types (integers, floats, strings, booleans, lists, etc.). For example,

info = [ "Mr. Guse", 47, "Albert College", ["Calculus", "Advanced Functions", "Computer Science", "Physics"] ]

• Strings are **immutable **- characters cannot be replaced in a string using the index. Lists are mutable - elements can be replaced with other values (and rearranged). For example,

info[1] = 48

will change the second element of the **list **above. However, the following code involving a **string **would generate an error.

name = "albert college"
name[0] = "A"

Why Use Lists?

Lists are used when you want to store a sequence of values in memory, so that they can be used later in a program and modified if necessary.

How do you Create a List?

• Assign the initial values of a list to a variable in your program

marks = [75, 91, 82, 70, 88, 77, 90, 86]

• Use a loop to assign values to a list. For example, you may want the user to enter multiple values into a list. For example,

marks = []                              # An empty list
for i in range(8):
    n = int( input("Enter a mark: ") )
    marks.append(n)                     # append() is a list method that appends values to an existing list

• Use a generator. Generators are expressions that allow you to fill a list according to a formula. The general form of a generator is as follows:

  [ expression for variable in sequence ]
  

  wherevariableis the ID of some variable,sequenceis a sequence of values, which takes the variable (this can be a list, a string, or an object obtained using the functionrange),expression_ — some expression, usually depending on the variable used in the generator. The list elements will be filled according to this expression.

Here are some examples of using a generator,

a = [ i**2 for i in range(5) ]
print(a)

produces a list of squared whole numbers from 0 to 4.

[0, 1, 4, 9, 16]

or

b = [ 2*i for i in [-5, 3, 11, 15] ]
print(b)

produces a list that has elements which are double the values from the initial list

[-10, 6, 22, 30]

or

c = [ int(i) for i in [ '3', '6', '9', '11'] ]

produces a list of integer values converted from the initial list of strings.

• Use the * operator to repeat a set of values in an existing list. For example,

a = [0] * 5
print(a)

produces a list containing 5 zeros, which may act as initial values.

[0, 0, 0, 0, 0]

How do you read values from a List?

If you want to scan through all of the values (elements) in a list, a for-loop can be used in two different ways.

Example #1: Without the range() function

for value in [4, 3, -6, -8]:
    print(value, end=":")

produces the output

4:3:-6:-8

Example #2: Using the range() function, an index and len()

    a = [4, 3, -6, -8]
    for i in range( len(a) ):
         print(a[i], end=":")

produces the output

4:3:-6:-8

The advantage of this method is that the variable i keeps track of the position (index) of each element in the list. This index information is important in certain situations and problems.

Converting a String to a List using split()

Often in programs you want to convert a string into a list especially when getting data input from the keyboard or from a file. For example,

a = "75 91 82 70 88 77 90 86"
b = a.split()
print(b)

produces the output

['75', '91', '82', '70', '88', '77', '90', '86']

where the split() function uses the spaces to separate the string into a list of strings. Now, in this situation we probably want to convert each element in the list to an integer. This can be done using a generator (described above) as follows:

c = [ int(value) for value in b ]
print(c)

This produces the output

[75, 91, 82, 70, 88, 77, 90, 86]

Example: Input from the Keyboard

You can input multiple values from the keyboard using the following code:

a = input("Enter your marks: ")            # Input a single string
b = a.split()                              # Convert the string to a list of strings
c = [ int(value) for value in b ]          # Create a list of integers from a list of strings
print(a)                                   # Output the single string
print(b)                                   # Output the list of strings
print(c)                                   # Output the list of integers

Here is an example of how this could run:

>>Enter your marks:  75 91 82 70 88 77 90 86
'75 91 82 70 88 77 90 86'
['75', '91', '82', '70', '88', '77', '90', '86']
[75, 91, 82, 70, 88, 77, 90, 86]

This can also be done in a single line as follows:

marks = [ int(value) for value in input().split() ]

where input() returns a single string, split() then returns a list of strings, and then the generator returns the list of integers.