Branching if/elif/else

Branching statements allow us to express

if it's cold outside
    wear a jacket
if you're happy and you know it
    clap your hands

Single if statement

if <condition>:
    <body>

  • <condition> is an expression that evaluates to either True or False.

  • <body> is one or more statements that execute only when <condition> is true

wk03 height if

Two-way if/else statement

if <condition>:
    <body1>
else:
    <body2>

  • <body1> executes when <condition> is True

  • <body2> executes when <condition> is False

wk03 height ifelse

Multi-way if/else statement

if <condition>:
    <body1>
elif <condition2>:
    <body2>
.
.
.
elif <conditionN>:
    <bodyN>
else:
    <bodyN+1>

  • All conditions are tested in order, top to bottom. The first condition to be True will have its <body> executed.

  • The else statement is optional

  • <bodyN+1> when all conditions are False

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Exercise: Bug1

The following code doesn’t execute correctly. What’s wrong?

wk03 height bug1

Exercise: Bug2

The following code doesn’t execute correctly. What’s wrong?

wk03 height bug2

Conditionals

Booleans

Boolean data types can have one of two possible values

  • True

  • False

Relational operators

>, >=, ⇐, == (don’t confuse with assignment!!), !=

Relational operators for strings

[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> "apple" < "bananas"
True
>>> "apple" < "apricots"
True
>>> "apple" < "kiwi"
True
>>> "apple" > "kiwi"
False
>>> ord("a")
97
>>> ord("b")
98
>>> ord("c")
99
>>> ord("d")
100
>>> chr(100)
'd'
>>> chr(10000)
'✐'
>>> chr(20000)
'丠'
>>> chr(2000000)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: chr() arg not in range(0x110000)
>>> "apple" < "apples"
True

link::https://www.asciitable.com/[ASCII Table]

Logical operators

Syntax:

<bool expr1> and <bool expr2>
<bool expr1> or <bool expr2>
not <bool expr>

  • and is True only when <exp1> and <expr2 are True. "If you are over 16 and have a license, you can drive"

  • or is True if either <expr1> and <expr2> is True. "Either cats or dogs are allowed on the train."

  • not flips the boolean of the expression (e.g. if <expr> is True, not <expr> is False) "I am the tallest! Not!"

Example: happy.py

wk03 happy

Exercise: testcolor.py

Here are several ways to write this program!

wk03 testcolor1
wk03 testcolor2
wk03 testcolor3

Exercise: Bug 1

The following code doesn’t execute correctly. Why?

wk03 testcolor bug3

Exercise: Bug 2

The following code doesn’t execute correctly. Why?

wk03 testcolor bug1

Exercise: Bug 3

The following code doesn’t execute correctly. Why?

wk03 testcolor bug2

Importing modules

Modules extend the built-in functionality in Python. Without modules, we’d need to implement a lot of useful functionalities ourselves. Modules allow us to leverage code built by other people. When we learn about functions, we will take a step towards making our own re-useable code! Two useful modules are

  • math

  • random

link::https://docs.python.org/3/py-modindex.html[Index of all Python modules]

math

To use a module, we must import it into our program. How we import the module affects how we reference the module’s symbols in our code.

Syntax:

import <module_name> # method 1
from <module_name> import <symbols> # method 2

Demo:

If we say import math, we must prepend the module name to use symbols (e.g. functions and variables) defined in it.

pimento[~]$ 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.
>>> import math
>>> r = 2
>>> print(math.pi * r * r)
12.566370614359172

If we say from math import pi, we can use the variable pi directly.

pimento[~]$ 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.
>>> from math import pi
>>> print(pi * r * r)
12.566370614359172

We can import all symbols from a module with *, e.g. from math import *

pimento[~]$ 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.
>>> from math import *
>>> sin(pi)
1.2246467991473532e-16

random

We will use three useful functions from the random module

  • randrange(<start>, <end>) which generates random integers between <start> and <end>-1 (like range)

  • choice(<sequence>) which generate a random member from <sequence>

  • random() which generates a random float between 0.0 and 1.0

pimento[~]$ 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.
>>> import random
>>> random.random()
0.9016097746776163
>>> random.random()
0.0003006488342669478
>>> random.random()
0.8610908502580705
>>> random.random()
0.060160433647061695
>>> random.random()
0.9268107119044854
>>> random.random()
0.2297457902726423
>>> random.randrange(0, 7)
1
>>> random.randrange(0, 7)
2
>>> random.randrange(0, 7)
1
>>> random.randrange(0, 7)
3
>>> random.randrange(0, 7)
1
>>> random.randrange(0, 7)
3
>>> random.randrange(0, 7)
2
>>> random.randrange(0, 7)
6
>>> random.choice("hello friends")
' '
>>> random.choice("hello friends")
'e'
>>> random.choice("hello friends")
'e'
>>> random.choice("hello friends")
'd'
>>> random.choice("ABCDEF")
'A'
>>> random.choice("ABCDEF")
'C'
>>> random.choice("ABCDEF")
'C'
>>> random.choice("ABCDEF")
'D'
>>> random.choice([0,3,5,-6])
-6
>>> random.choice([0,3,5,-6])
3
>>> random.choice([0,3,5,-6])
3
>>> random.choice([0,3,5,-6])
5
>>> random.choice(range(4))
1
>>> random.choice(range(4))
0
>>> random.choice(range(4))
3
>>> random.choice(range(4))
1
>>> random.choice([0.4, 5.6])
0.4
>>> random.choice([0.4, 5.6])
5.6
>>> random.choice([0.4, 5.6])
0.4
>>> random.choice([0.4, 5.6])
0.4

Exercise: guess.py

We can nest if statements into for loops (and vice versa).

wk03 guess