Notebook 1¶

Run each commands and try to understand why certain operations behave the way they do before looking at the answers.

There are 3 parts to this exercise: Numbers, Strings, and Lists, with the answers written under each section.

Numbers¶

2 + 2

10 / 4

50 - 5*6

(50 - 5)*6

5 ** 2

7 % 2

7 // 2

3.14 + 2

Assign the integer 20 to the variable width

Assign the float 5 to the variable height

Save width * height to the variable area

What is the area?

What happens if you assign 30 to width and print area again?

Re-assign width * height to area and see if the results changes now.

Assign area * 2 to 2timesArea

Assign area + 2 to return

Numbers - Answers¶

In [3]:

2 + 2         

Out[3]:

In [4]:

10 / 4          # Note that division of two integers becomes a float

Out[4]:

2.5

In [5]:

50 - 5*6        # multiplication is evaluated before the minus

Out[5]:

In [6]:

(50 - 5)*6      # Order of operations, parenthesis first

Out[6]:

In [7]:

5 ** 2          # To the power of

Out[7]:

In [8]:

7 % 2           # Modulo yields the remainder of the division of 7 by 2 (6/2 has the remainder 1)

Out[8]:

In [9]:

7 // 2          # Floor division returns the highest integer

Out[9]:

In [10]:

3.14 + 2

Out[10]:

5.140000000000001

So why is the example above not 5.14? Python represents floats as base 2 (binary) fractions. This causes some of the numbers to having to be approximated instead of exact. If you need very precise calculations you need to be aware that python behaves this way. To read more about this, click here

In [11]:

width  = 20
height = 5.0
area   = width * height
area                                 # Multiplication of a float and an integer results in a float

Out[11]:

100.0

In [12]:

width = 30
area

Out[12]:

100.0

When assigning width * height to area, python stores the actual value assigned to area in memory, ie area = 100.0. It does not store the pointer to width * height. To update area with the new inputs we have to actively assign area a value again.

In [13]:

area = width * height
area

Out[13]:

150.0

In [14]:

2timesArea = area * 2

  File "<ipython-input-14-4bdf2ca84b6b>", line 1
    2timesArea = area * 2
             ^
SyntaxError: invalid syntax

In [15]:

return = area + 2

  File "<ipython-input-15-da7854f8d5e4>", line 1
    return = area + 2
           ^
SyntaxError: invalid syntax

Why does the above fail? In the first case we are trying to assign a value to a variable name starting with a number, which is not allowed. In the second case we are trying to assign a value to a reserved keyword. However, both of them results in the same error message, a SyntaxError, which sometimes can be tricky to track, so be aware.

Strings¶

'Hello World!'
"Hello World!"

'Oh, no, you didn\\'t...'
"Oh, no, you didn't..."

"\\"Yes\\", I did!"
'"Yes", I did'

'Hello' ' ' 'World' '!'

s = 'Hello\tWorld'
s
print(s)

s = 'First line.\nSecond line.'
s
print(s)

folder = 'C:\some\name'
print(folder)

folder = r'C:\some\name'
print(folder)

'hmm ' + 3 * 'miam'

'Py' 'thon'

prefix = 'Py'
prefix + 'thon'

text = ('Put several strings within parentheses ' +
'to have them joined together, ' +
'with the use of ' +
'concatenation')
text

Strings - Answers¶

In [16]:

'Hello World!'          # "" and '' can be used interchangeably

Out[16]:

'Hello World!'

In [17]:

"Hello World!"

Out[17]:

'Hello World!'

In [18]:

'Oh, no, you didn\'t...'

Out[18]:

"Oh, no, you didn't..."

In the example above we have to escape the special character ' to prevent python as interpreting it as end of the string. Another way of inserting ' into a string would be to enclose the string in " instead, as in the example below. That way the ' is interpreted as a normal character.

In [19]:

"Oh, no, you didn't..."

Out[19]:

"Oh, no, you didn't..."

In [20]:

"\"Yes\", I did!"

Out[20]:

'"Yes", I did!'

In [21]:

'"Yes", I did'

Out[21]:

'"Yes", I did'

In [22]:

'Hello'          ' '         'World'             '!'    # Ignores spaces between strings

Out[22]:

'Hello World!'

In [23]:

s = 'Hello\tWorld'                                      # Adds a tab between the words
s                                                       # Notice that s only shows the content

Out[23]:

'Hello\tWorld'

In [24]:

print(s)                                                # While print() renders the content of s

Hello	World

In [25]:

s = 'First line.\nSecond line.'                         # \n adds a newline
s                                                       # Without print(), \n is included in the output

Out[25]:

'First line.\nSecond line.'

In [26]:

print(s)                                                # With print(), \n produces a new line

First line.
Second line.

In [27]:

folder = 'C:\some\name'                                 # Here the \n will be interpreted as a newline
print(folder)

C:\some
ame

In [28]:

folder = r'C:\some\name'
print(folder)

C:\some\name

If you don’t want characters prefaced by \ to be interpreted as special characters, you can use raw strings by adding an r before the first quote as above

In [29]:

'hmm ' + 3 * 'miam'

Out[29]:

'hmm miammiammiam'

In [30]:

'Py' 'thon'

Out[30]:

'Python'

In [31]:

prefix = 'Py'
prefix + 'thon'

Out[31]:

'Python'

In [32]:

text = ('Put several strings within parentheses ' +
        'to have them joined together, ' +
        'with the use of ' +
        'concatenation')
text

Out[32]:

'Put several strings within parentheses to have them joined together, with the use of concatenation'

Lists¶

squares = [1, 4, 9, 16, 25]
cubes = [1, 8, 27, 64, 125]

squares + cubes
squares * 2
squares * cubes

Lists - Answers¶

Lists are a collection of items grouped together. A list is denoted using comma-separated values between square brackets. A list might contain items of different types, but usually the items all have the same type.

In [33]:

squares = [1, 4, 9, 16, 25] 
cubes = [1, 8, 27, 64, 125]

squares + cubes     # Even though both contains integers and are of equal length they cannot be added together, only concatenated

Out[33]:

[1, 4, 9, 16, 25, 1, 8, 27, 64, 125]

In [34]:

squares * 2         # Multiplication with 2 will produce a new list with a duplicate of the first list concatenated to the end

Out[34]:

[1, 4, 9, 16, 25, 1, 4, 9, 16, 25]

In [35]:

squares * cubes     # Multiplication of lists can only be done with integers, not lists

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-35-27f4ae718739> in <module>()
----> 1 squares * cubes     # Multiplication of lists can only be done with integers, not lists

TypeError: can't multiply sequence by non-int of type 'list'