Range, Dictionaries and Comprehensions

Range

for i in range(5):
    print(i)

0
1
2
3
4

for i in range(50, 101, 10):
    print(i)

50
60
70
80
90
100

Often, we just want to do something 5 times without looping over a collection.

The most common way to do that is to use range(), which automatically generates a collection of the integers in some sort of sequence, generally 0, to N-1.

We can also specify a start value, an end value and a skip-by value with range.

range(start, end, skip)

for i in range(7,10):
    print(i)

7
8
9

for i in range(4):
    print(i)

0
1
2
3

for i in range(0.5,1.0,0.1):
    print(i)

TypeError: 'float' object cannot be interpreted as an integer.

Detailed traceback: 
  File "<string>", line 1, in <module>

The range function specifies the starting integer first, the ending integer next, and the skip by integer last.

Similarly to slicing with .iloc[] in pandas, the starting point of range() is included in the sequence, and the ending number is excluded.

The skip-by variable is optional.

In range(), the default skip number is 1, so if we don’t include it, Python will assume to increase in increments of 1.

If we only include a single integer in the function, a sequence up to the given number (but not including it) will be generated starting from 0.

range() only works for integers. If we attempt a loop with type float, an error will occur:

Looping in a dictionary

cereals = {'Special K': 4, 'Lucky Charms': 7, 'Cheerios': 2, 'Wheaties': 3}
cereals.items()

dict_items([('Special K', 4), ('Lucky Charms', 7), ('Cheerios', 2), ('Wheaties', 3)])

cereals = {'Special K': 4, 'Lucky Charms': 7, 'Cheerios': 2, 'Wheaties': 3}

for cereal, stock in cereals.items():
    print( cereal  + " has " + str(stock) + " available")

Special K has 4 available
Lucky Charms has 7 available
Cheerios has 2 available
Wheaties has 3 available

We can also loop over the key-value pairs of a dictionary using .items().

We saw this verb back in module 4 when we learned about dictionaries.

Since each key-value pair has 2 elements in it, we need to specify a variable for each item in the tuple:

• One for the dictionary key
• One for the dictionary values

Here we assign an object named cereal for the items in the first position of the tuple, which are the dictionary keys, and an object named stock for the second index in a tuple, which are the dictionary values.

Comprehensions

numbers = [2, 3, 5]

squared = list()

for number in numbers: 
    squared.append(number ** 2)
    
squared

[4, 9, 25]

squared = [number ** 2 for number in numbers]
squared

[4, 9, 25]

We learned in the last Module that we can create if and else conditions in a single line of code, and we can do something similar with basic loops.

Comprehensions allow us to build lists/sets/dictionaries in one convenient, compact line of code.

In the last set of slides, we made a loop that calculates the square of each element from a list and adds them to a new list name squared.

This can be done using comprehension, so now it executes using the single line of code.

numbers = [2, 3, 5]

word_length = {number : number ** 2 for number in numbers}
word_length

{2: 4, 3: 9, 5: 25}

Dictionary comprehensions can be done in a similar way, but this time we wrap the line with curly brackets and specify what our keys and values are.

In this case, we name the element in the list number.

We assign number as the dictionary key and the square of number as the dictionary value.

Both methods of building lists and dictionaries work, and it’s down to you to decide what works better for you.

Let’s apply what we learned!