Introduction to Programming with Python

26 Jan 2018

Reading time ~17 minutes

Introduction to Programming
- What does a computer do?
- What does code do?
Standalone Python Topics

Introduction to Programming

What does a computer do?

Executes commands in sequence
Each command operates on the current state of the machine

Remember: Computers are dumb

What does code do?

Code is nothing more than a stored sequence of commands carried out by the computer.

alt text

Standalone Python Topics

Functions

We will start by introducing two fundamental functions in Python:

print()
help()

Print Function

As the name implies, this function prints whatever you tell it to. For example,

print('Hello World')
print(5)

Hello World
5

print() can take multiple parameters:

print('Hello', 'World')
print(4, 2, sep='*')

Hello World
4*2

Exercise

Use the print function to print your name and age

Solution:

print('your name and age')

your name and age

Help Function

This function will tell how how to use a certain function, and what parameters it takes.

help(print)

Help on built-in function print in module builtins:

print(...)
    print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False)
    
    Prints the values to a stream, or to sys.stdout by default.
    Optional keyword arguments:
    file:  a file-like object (stream); defaults to the current sys.stdout.
    sep:   string inserted between values, default a space.
    end:   string appended after the last value, default a newline.
    flush: whether to forcibly flush the stream.

Exercise

There are many built-in functions available in Python 3.6, for a list see here.

Use the help function to check the following built in functions:

list
sum
min

Solution:

# Number 1
help(list())

# Number 2
help(sum)

# Number 3
help(min)

Help on list object:

class list(object)
 |  list(iterable=(), /)
 |  
 |  Built-in mutable sequence.
 |  
 |  If no argument is given, the constructor creates a new empty list.
 |  The argument must be an iterable if specified.
 |  
 |  Methods defined here:
 |  
 |  __add__(self, value, /)
 |      Return self+value.
 |  
 |  __contains__(self, key, /)
 |      Return key in self.
 |  
 |  __delitem__(self, key, /)
 |      Delete self[key].
 |  
 |  __eq__(self, value, /)
 |      Return self==value.
 |  
 |  __ge__(self, value, /)
 |      Return self>=value.
 |  
 |  __getattribute__(self, name, /)
 |      Return getattr(self, name).
 |  
 |  __getitem__(...)
 |      x.__getitem__(y) <==> x[y]
 |  
 |  __gt__(self, value, /)
 |      Return self>value.
 |  
 |  __iadd__(self, value, /)
 |      Implement self+=value.
 |  
 |  __imul__(self, value, /)
 |      Implement self*=value.
 |  
 |  __init__(self, /, *args, **kwargs)
 |      Initialize self.  See help(type(self)) for accurate signature.
 |  
 |  __iter__(self, /)
 |      Implement iter(self).
 |  
 |  __le__(self, value, /)
 |      Return self<=value.
 |  
 |  __len__(self, /)
 |      Return len(self).
 |  
 |  __lt__(self, value, /)
 |      Return self<value.
 |  
 |  __mul__(self, value, /)
 |      Return self*value.
 |  
 |  __ne__(self, value, /)
 |      Return self!=value.
 |  
 |  __repr__(self, /)
 |      Return repr(self).
 |  
 |  __reversed__(self, /)
 |      Return a reverse iterator over the list.
 |  
 |  __rmul__(self, value, /)
 |      Return value*self.
 |  
 |  __setitem__(self, key, value, /)
 |      Set self[key] to value.
 |  
 |  __sizeof__(self, /)
 |      Return the size of the list in memory, in bytes.
 |  
 |  append(self, object, /)
 |      Append object to the end of the list.
 |  
 |  clear(self, /)
 |      Remove all items from list.
 |  
 |  copy(self, /)
 |      Return a shallow copy of the list.
 |  
 |  count(self, value, /)
 |      Return number of occurrences of value.
 |  
 |  extend(self, iterable, /)
 |      Extend list by appending elements from the iterable.
 |  
 |  index(self, value, start=0, stop=9223372036854775807, /)
 |      Return first index of value.
 |      
 |      Raises ValueError if the value is not present.
 |  
 |  insert(self, index, object, /)
 |      Insert object before index.
 |  
 |  pop(self, index=-1, /)
 |      Remove and return item at index (default last).
 |      
 |      Raises IndexError if list is empty or index is out of range.
 |  
 |  remove(self, value, /)
 |      Remove first occurrence of value.
 |      
 |      Raises ValueError if the value is not present.
 |  
 |  reverse(self, /)
 |      Reverse *IN PLACE*.
 |  
 |  sort(self, /, *, key=None, reverse=False)
 |      Stable sort *IN PLACE*.
 |  
 |  ----------------------------------------------------------------------
 |  Static methods defined here:
 |  
 |  __new__(*args, **kwargs) from builtins.type
 |      Create and return a new object.  See help(type) for accurate signature.
 |  
 |  ----------------------------------------------------------------------
 |  Data and other attributes defined here:
 |  
 |  __hash__ = None

Help on built-in function sum in module builtins:

sum(iterable, start=0, /)
    Return the sum of a 'start' value (default: 0) plus an iterable of numbers
    
    When the iterable is empty, return the start value.
    This function is intended specifically for use with numeric values and may
    reject non-numeric types.

Help on built-in function min in module builtins:

min(...)
    min(iterable, *[, default=obj, key=func]) -> value
    min(arg1, arg2, *args, *[, key=func]) -> value
    
    With a single iterable argument, return its smallest item. The
    default keyword-only argument specifies an object to return if
    the provided iterable is empty.
    With two or more arguments, return the smallest argument.

Data Types Part 1

There are various data types, a few common examples are:

int : integer
float : floating point (decimal number)
str : string (text)
bool : boolean (i.e. True/False)

For example:

myInteger = 2
myFloat = 3.1415
myString = 'apple'
myBoolean = True

We can also check the type of variables:

print(type(myInteger))
print(type(myFloat))
print(type(myString))
print(type(myBoolean))

<class 'int'>
<class 'float'>
<class 'str'>
<class 'float'>

Another useful feature is type casting

print(float(myInteger))
print(int(myFloat))
print(bool())

2.0
3
False

Exercise

Does int() correctly round floats?
What are the numerical equivalents of booleans?
What strings can you successfully cast into ints, floats, and booleans?

Solution:

# Number 1
print(int(3.1415))
print(int(2.99))

# Number 2
print(float(True))
print(float(False))

# Number 3
print(float('5'))
print(float('inf'))
print(bool(6))

3
2
1.0
0.0
5.0
inf
True

Note on Naming Schemes

May consist of letters, numbers, and underscores
Cannot start with a number
Is case sensitive

For example, the following line returns an error, why?

5_five = 5
print(five_5)

SyntaxError: invalid token

It is also a good idea to chose variable names that are useful, for example, blah = 3.14, isn’t particularly useful. Perhaps pi = 3.14 is a better choice.

Basic Math

Python is a very valuable tool for evaluating mathematical expressions. Here are some basic mathmatical operations you can peform in Python.

a = 2
b = 4.5
c = "apple"

print(a+b)
print(a-b)
print(a*b)
print(a/b)
print(c+str(a))

6.5
-2.5
9.0
0.4444444444444444
apple2

Exercise

What do the following operators do?
- **
- //
- %
What combinations of data types work and don’t work?
Did you find anything you didn’t expect?

Solution:

# Number 1
print(3**3)
print(10//3)
print(10%3)

# Number 2
print(3**"apple")

# Number 3
print(5*"apple")

27
3
1

TypeError: unsupported operand type(s) for ** or pow(): 'int' and 'str'

'appleappleappleappleapple'

Conditionals

Conditionals with Math

print(3 == 3)
print(3 > 5)
print(3 < 5)
print(1 <= 2)
print(2 >= 1)
print(2 != 1)

True
False
True
True
True
True

a = 5/3
b = 1 + 2/3
print(a == b)
print(a)
print(b)

False
1.6666666666666667
1.6666666666666665

print("apple" == "banana")
print(len("apple") == 5)
print("b" in "banana")
print("q" in "apple")

False
True
True
False

Functions

def my_first_function():
    print('I made my first function!')
    
my_first_function()

I made my first function!

We can also introduce parameters into our functions.

def newton_force(mass, accel=9.81):
    return mass * accel

force = newton_force(mass=100)

print(force, '[N]')

force2 = newton_force(accel=100, mass=100)

print(force2, '[N]')

981.0 [N]
10000 [N]

Exercise

Create a function which returns the hypotenuse of a right triangle. Recall: \(c^2 = a^2 + b^2 \Rightarrow c = \sqrt{a^2 + b^2} \equiv (a^2 + b^2)^{0.5}\)

Solution:

def hypot(a, b):
    """Put Your Code Here"""
    return (a**2 + b**2)**0.5

hypot(3, 4)

5.0

Flow Control

If Statements

def do_it(a, b):
    if a == b:
        print("Yes")
    elif b > a:
        print("No")
    elif b < a:
        print("This other thing!")
    else:
        print("I give up")

do_it(3, 3)
do_it(10, 100)
do_it(0.1, 0)

Yes
No
This other thing!

While Loops

a = 5
b = 1

while a > b:
    print('{} is greater than {}'.format(a,b))
    print(f"{a} is greater than {b}")
    b = b+1
    # b += 1

is greater than 1
is greater than 1
is greater than 2
is greater than 2
is greater than 3
is greater than 3
is greater than 4
is greater than 4

Data Types Part 2 - Electric Boogaloo

Lists

my_list = [
    "apple", 
    "banana", 
    "shark", 
    15
]
print(my_list)

['apple', 'banana', 'shark', 15]

Adding and Removing Elements

my_list.append("ballistic missile")
print(my_list)

['apple', 'banana', 'shark', 15, 'ballistic missile']

my_list.remove("banana")
print(my_list)

['apple', 'shark', 15, 'ballistic missile']

List Indexing and Slicing

print(my_list[0])
print(my_list[3])
print(my_list[1:3])
print(my_list[-1])

apple
ballistic missile
['shark', 15]
shark

Exercise

Create a function which takes a list as a parameter and an integer, n. Return the n-th entry of the list.

Solution:

def nth_entry(a_list, n):
    """Your Code Here"""
    return a_list[n]
    
nth_entry(my_list, -1)

Multi-Dimensional Arrays

array_2d = [[0,1,2,3], [2,5,3,2], [6,3,1,7]]
print(array_2d)

[[0, 1, 2, 3], [2, 5, 3, 2], [6, 3, 1, 7]]

print(array_2d[0])
print(array_2d[1][0:2])
print(array_2d[0:2])
print(array_2d[-1][-1])

[0, 1, 2, 3]
[2, 5]
[[0, 1, 2, 3], [2, 5, 3, 2]]
7

Flow Control Part 2 - Back 2 Tha Hood

for item in my_list:
    print(item)

apple
banana
shark
15

my_list_2 = [1,2,3,4,5,6]

for item in my_list_2:
    print(item**2 + 3.8*item + 5.1)

899999999999999
7
5
3
1
9

for item in my_list_2:
    if item > 3:
        print(item, "True")
    else:
        print(item, "False")

False
False
False
True
True
True

range() function

my_range = range(len(myList))
for i in my_range:
    print('{}: {}'.format(i, myList[i]))

apple
banana
shark
15

List Comprehension

my_list_3 = [i**2 for i in my_list_2]
print(my_list_3)

[1, 4, 9, 16, 25, 36]

Exercise

Define a function that uses a for loop to draw the following pattern (out to the n-th line with n asterisks).
*
**
***
****
*****
…

Solution:

list(range(5))

[0, 1, 2, 3, 4]

def star_tree(n):
    """Your Code Here"""
    for i in range(n):
        print('*'*(i+1))

star_tree(n=5)

*
**
***
****
*****

Exercise

Classic Challenge : Fizz Buzz

Given a list of integers from 1 to 50.

Print “Fizz” if the integer is divisible by 3.
- i.e. 3, 6, 9
Print “Buzz” if the integer is divisible by 5.
- i.e. 5, 10, 15
Print “FizzBuzz” if the integer is divisible by 3 AND 5.
- i.e. 15, 30, 45

Challenge: Do it in as few lines as possible!

Solution:

for num in range(1, 50): 
    print(num, "Fizz"*(num%3 == 0) + "Buzz"*(num%5 == 0))