# Erle Robotics Learning Python GitBook Free

###### Exercise 1

Use a list comprehension to create a list, cubes_by_four. The comprehension should consist of the cubes of the numbers 1 through 10 only if the cube is evenly divisible by four. Finally, print that list to the console. Note that in this case, the cubed number should be evenly divisible by 4, not the original number.

###### Exercise 2

Create a variable, backwards_by_tens, and set it equal to the result of going backwards through to_one_hundred by tens. Go ahead and print backwards_by_tens to the console.

###### Exercise 3

Create a list, to_21, that's just the numbers from 1 to 21, inclusive. Create a second list, odds, that contains only the odd numbers in the to_21 list (1, 3, 5, and so on). Use list slicing for this one instead of a list comprehension. Finally, create a third list, middle_third, that's equal to the middle third of to_21, from 8 to 14, inclusive.

###### Exercise 4

Create a list, squares, that consists of the squares of the numbers 1 to 10. A list comprehension could be useful here. Use filter() and a lambda expression to print out only the squares that are between 30 and 70 (inclusive).

###### Exercise 5

The string

garbled = "!XeXgXaXsXsXeXmX XtXeXrXcXeXsX XeXhXtX XmXaX XI"


is garbled in two ways:

• First, our message is backwards;
• Second, the letter we want is every other letter.

Use lambda and filter to extract the message and save it to a variable called message. Use list slicing to extract the message and save it to a variable called message.

## Solutions

###### Exercise 1
>>> cubes_by_four = [i**3 for i in range(1,11) if i**3 % 4 == 0 ]
>>>
>>> print cubes_by_four
[8, 64, 216, 512, 1000]

###### Exercise 2
>>> to_one_hundred = range(101)
>>>
>>> backwards_by_tens=to_one_hundred[::-10]
>>> print backwards_by_tens
[100, 90, 80, 70, 60, 50, 40, 30, 20, 10, 0]
>>>
>>>

###### Exercise 3
>>> to_21=range(1,22)
>>> print to_21
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]
>>>
>>> odds=to_21[0::2]
>>> print odds
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21]
>>>
>>> middle_third=to_21[7:14:1]
>>> print middle_third
[8, 9, 10, 11, 12, 13, 14]
>>>

###### Exercise 4
>>> squares= [i**2 for i in range(1,11) ]
>>> print squares
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
>>>
>>> print filter(lambda x: x<=70 and x>=30,squares)
[36, 49, 64]
>>>

###### Exercise 5
>>> garbled = "!XeXgXaXsXsXeXmX XtXeXrXcXeXsX XeXhXtX XmXaX XI"
>>>
>>> mess1=garbled[::-1]
>>> print mess1
IX XaXmX XtXhXeX XsXeXcXrXeXtX XmXeXsXsXaXgXeX!
>>>
>>> #Using filter
...
>>> message=filter(lambda let: let != "X",mess1)
>>> print message
I am the secret message!
>>>
>>>#Using list slicing
...
>>> message=mess1[::2]
>>> print message
I am the secret message!