Handout1: Types and variables

Handout1.pdf

We can use the Python interpreter to check our work

pimento[w02]$ python3
Python 3.6.9 (default, Nov  7 2019, 10:44:02)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> type("0.0")
<class 'str'>
>>> type(0.0)
<class 'float'>
>>> a = 0.0
>>> type(a)
<class 'float'>
>>> 4
4
>>> type(4)
<class 'int'>
>>> type(-4.5)
<class 'float'>
>>> -4.5
-4.5
>>> "6.6"
'6.6'
>>> 1 + 3.0
4.0
>>> type(1 + 3.0)
<class 'float'>
>>> "cup" + "cake"
'cupcake'
>>> type("cup" + "cake")
<class 'str'>

To test whether a variable name is valid, try assigning it a value in the Python interpreter!

>>> def = 1
  File "<stdin>", line 1
    def = 1
        ^
SyntaxError: invalid syntax
>>> h = 1
>>> h
1
>>> Amount# = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'Amount' is not defined
Handout1 Ans

Operator precedence

Terms to know (see reading):

  • operator

  • operand

  • statement

  • expression

Rules

  • operands with higher precedence are evaluated first

  • operands with same precedence are evaluated left to right

  • if you’re not sure, use ()

highest

()

**

/,//,*,%

lowest

+,-

Examples:

  • 4 * 2 + 3 = 11

  • 4 * (2 + 3) = 20

  • "c" + "b" * 3 = "cbbb"

  • ("c" + "b") * 3 = "cbcbcb"

Exercise:

  1. 1 + 1 ** 5 - 2 = 0

  2. (1 + 1) ** (5 - 2) = 8

The data type of a variable is determined by the value stored in the variable. When evaluating expressions involving variables, we first substitute the variable with its value and then compute the operations.

Exercise: inchesToFeet.py

What is the output of the following program?

wk02 inchesToFeet.py

The program defines 4 variables

variable

type

value

height

int

60

feet_to_inches

int

12

inches_to_feet

float

1/12 (e.g. 0.08333)

mystery

float

(60 + 12) * (1/12) = 72/12 = 6.0

 
$ python3 inchesToFeet.py
The value is 6.0

String indexing and len()

  • indexing allows us to get a single character

    • valid indexes for a string range from 0 to len-1

    • indexes represent the offset from the start of the string. The first character is 0 offset from the start.

    • indexes out of the range are invalid and result in an error: IndexError: string index out of range

    • indexes must be integers!

  • the len() function tells us the length of a string

pimento[w02]$ python3
Python 3.6.9 (default, Nov  7 2019, 10:44:02)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> name = "Mr. Potato"
>>> name
'Mr. Potato'
>>> name[0]
'M'
>>> name[1]
'r'
>>> name[2]
'.'
>>> name[3]
' '
>>> name[4]
'P'
>>> len(name)
10
>>> name[10]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range
>>> name[0]
'M'
>>>

Exercise: requestChar.py

wk02 requestChar.py

Lists

Lists are a data type for storing multiple values. Indexing for lists work similary to strings. Lists also support the + and * operators for concatenation and repetition.

pimento[w02]$ python3
Python 3.6.9 (default, Nov  7 2019, 10:44:02)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> values = [0, 1, 5, -4]
>>> values
[0, 1, 5, -4]
>>> values[0]
0
>>> num_values = len(values)
>>> num_values
4
>>> values[num_values-1]
-4
>>> values[10]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range
>>> values = [9.0, "cat", 3, 5]
>>> values
[9.0, 'cat', 3, 5]
>>> values[0]
9.0
>>> values[1]
'cat'
>>> values = []
>>> len(values)
0
>>> values = [] + [4,5]
>>> values
[4, 5]
>>>

for loops

Our programs will often need to repeat instructions. For example, suppose we would like to print every character from a word on its own line.

for <var> in <sequence>:
  <body>

  • <var> variable which stored the current value of the iteration

  • <sequence> the set of values to iterate over

  • <body> one or more statements to be repeated

def main()
  for i in range(5):
    print(i) # this line is repeated 5 times!
main()

In the above example, range(5) gives us the sequence of values 0,1,2,3,4.

iteration

i

1

0

2

1

3

3

4

3

5

4

range()

The range function generates sequences of integers. Range can be called with 1, 2, or 3 arguments

range(<end>) # one argument
range(<start>, <end>) # two arguments
range(<start>, <end>, <step>) # three arguments

  • <end> the last integer in the sequence will be <end>-1

  • <start> the first integer in the sequence

  • <step> the increment between values

Demo:

In the Python interpreter, we can convert range to a list to display the sequence of values. 
pimento[w02]$ python3
Python 3.6.9 (default, Nov  7 2019, 10:44:02)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> range(5)
range(0, 5)
>>> list(range(5))
[0, 1, 2, 3, 4]
>>> list(range(0, 5))
[0, 1, 2, 3, 4]
>>> list(range(1, 5))
[1, 2, 3, 4]
>>> list(range(2, 10))
[2, 3, 4, 5, 6, 7, 8, 9]
>>> list(range(2, 10, 2))
[2, 4, 6, 8]
>>> list(range(10, 2))
[]
>>> list(range(10, 2, -1))
[10, 9, 8, 7, 6, 5, 4, 3]
>>>

Example: letters.py

wk02 letters.py

Handout2: loops

Handout2.pdf

We can write Python code to check out work!

wk02 loop.py 
cheese[w02]$ python3 loop.py
0 0
1 2
2 4
3 6
4 8

0 a aaa
1 h hhh
2 o ooo
3 y yyy

0 8
1 20
2 22
3 15

0 Yay Yay!
1 What What!
2 hmmm hmmm!
Handout2 Ans

Example: countdown.py

wk02 countdown

Example: square.py

wk02 square.py

Accumulator pattern

The accumulator pattern allows us to update a variable using a loop. For example, imagine you are computing a total for a sequence of values without writing down values. A good way to do it is to remember the total so far and then update it with each new value. This is how accumulators work in code.

initialize the accumulator value
loop until done
   update accumulator

Example: total.py

wk02 total

In the table below, assume the user gives the values 0, 10, 4, -3, and 2. Note that total starts with value 0.

iteration

i

val

total

1

0

0

0

2

1

10

10

3

3

4

14

4

3

-3

11

5

4

2

13

Example: countup-string.py

wk02 countup string

iteration

i

text

1

0

"0-"

2

1

"0-1-"

3

3

"0-1-2-"

4

3

"0-1-2-3-"

5

4

"0-1-2-3-4-"

Example: pies.py

wk02 pies

iteration

i

printed phrase

ancestor

1

0

"My grandmother makes the best pies."

"great grandmother"

2

1

"My great grandmother makes the best pies."

"great great grandmother"

3

2

"My great great grandmother makes the best pies."

"great great great grandmother"

4

3

"My great great great grandmother makes the best pies."

"great great great great grandmother"

Exercise: average.py

wk02 average

Exercise: banner.py

wk02 banner

Exercise: factorial.py

There are many ways to implement factorial!

wk02 factorial1
wk02 factorial2
wk02 factorial3
wk02 factorial4

Handout3: accumulator

Handout3.pdf

Again, we can check our answers using code and the Python interpreter!

wk02 accumulators 
pimento[w02]$ python3
Python 3.6.9 (default, Nov  7 2019, 10:44:02)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> list(range(5,10))
[5, 6, 7, 8, 9]
>>> list(range(5,1,-1))
[5, 4, 3, 2]
>>> list(range(0, 15,3))
[0, 3, 6, 9, 12]
>>> list(range(5,5))
[]
>>> list(range(5,2))
[]
>>> list(range(5,2, -1))
[5, 4, 3]
>>>
Handout3 Ans