Comprehension and multidimensional arrays

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Review of list iterator

Given the code to the right, what would be the final printed value of A?

['Fox', 'Giraffe', 'Hippo', 'Iguana']
['Fox', 'Hippo', 'Iguana']
['Iguana', 'Fox']
['Fox', 'Iguana']

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

The code below will print list2 consisting even elements

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)

The code below replicate the same as in the left

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

Method	Description
`str.split()`	Splits a string into a list of its components using whitespace as a separator
`str.split(sep)`	Splits a string into a list using the specified separator `sep`
`str.splitlines()`	Splits a string into list of strings at the newline character
`str.join(list)`	Joins the elements of the `list` into a string, using `str` as the separator

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] ]

Comprehension and multidimensional arrays

Announcements

Review of list iterator

List building with loops

Comprehending Lists

Useful Comprehension: example

Where Strings and Lists Meet

Tabulation

Multidimensional Arrays

Picturing Multidimensional Arrays

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