Comprehension and multidimensional arrays

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Review of list iterator

A = [
     'Fox',
     'Giraffe', 
     'Hippo'
    ]
A.append('Iguana')
A[:].reverse()
B = A
for anim in B:
    if anim[1] == 'i':
        B.remove(anim)
print(A)

List building with loops

• Commonly will make lists with a simple:

  even_digits = [ 2, 4, 6, 8 ]

• But in many cases, it is easier to specify the elements of a list using a sequence of values generated by a for loop. For instance

  even_digits = [ ]
  for i in range(0, 10, 2):
      even_digits.append(i)

• Python gives us a shorthand notation to achieve this:

  even_digits = [ i for i in range(0, 10, 2) ]

  • Called list comprehension

Comprehending Lists

• The simplest list comprehension syntax is:

  [ expression iterator ]

  where expression is any Python expression and iterator is a for loop header

• The iterator component can be followed by any number of additional modifiers

  • More for loop headers for nested loops
  • or if statements to select specific values

• Example: all even numbers to 20 not also visible by 3

  [i for i in range(0,20,2) if i % 3 != 0]

Useful Comprehension: example

• Consider the two codes below
• Both does the same thing but the right most is more comprehensible

list1 = [2,4,6,5,3]
list2 =[]
for i in range(len(list1)):
      if list1[i]%2==0: ## eliminating all odd elements
            list2.append(list1[i])
print(list2)

list1 = [2,4,6,5,3]
list2 = [list1[i] for i in range(len(list1)) if list1[i]%2==0]
print(list2)

Where Strings and Lists Meet

• There are a handful of methods that transition between strings and lists

Tabulation

• Arrays can also be useful when you have a set of values and you need to count how many values fall into a different ranges
  • Process is called tabulation
• The idea is that for each piece of data we encounter, we figure out a corresponding index in our tabular array and then increment the value of that element
• Your book shows this for seeing how many times each letter of the alphabet appears in a text sequence
• Let’s instead look at an example of determining how many students got different letters grades on a final

Multidimensional Arrays

• We know that elements of a list can be lists in and of themselves. If the lengths of all the lists making up the elements of a list remain fixed, then the list of lists is called a multidimensional array

• In Python, we can create multidimensional arrays just by creating lists of constant length as the elements to another list

  magic = [ [2, 9, 4], [7, 5, 3], [6, 1, 8] ]

• We can always get the individual element of one of the inner lists by using 2 indices.

  • magic[1][1] = 5
  • magic[-1][0] = 6

Picturing Multidimensional Arrays

• Multidimensional arrays are commonly pictured as each inner list being stacked beneath the previous
• In such a representation, the outermost/first elements/indices represent the row, and the inner/second elements/indices represent the column

[ [2, 9, 4], [7, 5, 3], [6, 1, 8] ]

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