Python intermediate

This tutorial is designed for people who have done some python before.

Before you continue here is a list of things you should know:

• The print command
• Variables
• Basic conditionals (if, else and elif - short for else if)
• Lists (defining them and appending to them)
• While loops
• For loops (e.g. for value in list:)

If you think you need to recap any of these there is a tutorial here that should help you. Also it might be useful to have it open in another tab so you can refer to it if you get stuck.

In python we can use various common math operators e.g:

• + is plus (you've probably used this already)
• - is minus
• * is times
• / is divide
• // is integer division (rounds down to the nearest whole number)

# Prints 12
print(10 + 2)

# Prints 8
print(10 - 2)

# Prints 20
print(10 * 2)

# Prints 5.5
print(11 / 2)

# Prints 5 (since it is 5.5 rounded down)
print(11 // 2)

Assignment operators

Very often we want to apply an operation to a variable and then store then result in the variable, for example:

a = 10
# Set a to equal a + 2
a = a + 2
# Prints 12
print(a)

In this case we can use a += 2 as a shorthand for a = a + 2.

We can also use other operators, here is the full list:

• a += b is the same as a = a + b
• a -= b is the same as a = a - b
• a *= b is the same as a = a * b
• a /= b is the same as a = a / b

Modular arithmetic (clock arithmetic)

A very useful operator is % (the modulus operator). a % b returns the remainder from dividing a by b.

For example 10 % 3 == 1 since 10 ÷ 3 = 3 remainder 1.

We can also use an assignment operator (a %= b is the same as a = a % b).

Brackets

Python follows the usual rules of operator order, for example 3 + 6 / 3 == 3 + (6 / 3) == 5, since divide (/) has a higher precedence than + (think BIDMAS).

We can use brackets just like we do in normal maths for example (3 + 6) / 3 == 3.

Exercise

We're going to write a timer in python, to do this we need to learn how to pause the program for a certain amount of time. To do this we use sleep from the time module.

Run the code below and then remove the time.sleep(1000) lines and see what changes:

import time

print("One")
# Sleep for 1 second
time.sleep(1)
print("Two")
time.sleep(1)
print("Three")

time.sleep(x) takes in the number of seconds you want to sleep for.

Your task is to create a program that asks the user for the number of seconds to run a timer, and then your program prints out minutes:seconds with the amount of time the timer has been running every second. The seconds must never be above 60.

Hint: If you have been counting for 100 seconds (this should print 1:40), then you can use integer division (//) to get the number of minutes (100 // 60 == 1) and modulus (%) to get the seconds (100 % 60 == 40).

When you're debugging this it might help to temporarily speed up your timer so you don't have to wait for a minute to see a difference.

You can find the solution that I made here.

Conclusion

Writing mathematical expressions is easy in python as long as you know the symbol to use. As a side note these are the common operators you'll find in most langauges.

Functions are an incredibly useful tool in programming.

They let us label a block of code so that we can run it by referring to its label (called the function name).

The function body is a term meaning the code that is inside the function.

Calling a function is another way of saying running the function. In python we call a function by writing the function name and then some parentheses () (brackets), e.g. function_name().

Lets look at an example:

# The name of this function is `say_hello`
def say_hello():
  print("Hello")
  # This is the french way of saying hello
  print("Bonjour")

# This code isn't indented so it isn't part of the function body
print("Hi")

# Here we call the function
say_hello()

print("Some other code")

# We can call the function as many times as we want
say_hello()
say_hello()
say_hello()

Copy and run this code and you will see that hello and bonjour is printed 4 times since we call the function four times.

Function arguments

Functions can also take in variables (called arguments or parameters).

When we call a function with arguments we must pass in values. We do this by putting them in the parentheses e.g. say_hello("Sam").

# Define a function called `say_hello` which takes in a variable called name
def say_hello(name):
  print("Hello " + name)

say_hello("Sam")
say_hello("dog")

This prints out Hello Sam and then Hello dog.

Tip: whenever you want to debug a function it is useful to imagine replacing the arguments of the function with real values such as Sam or Dog and then run through the function in your head to see what it does.

Function return

Functions can return a value, which is useful for calculating things which we can return to the caller of the function.

def add(a, b):
  return a + b

# Here we take the return value of 2+3 and assign it to the variable result
result = add(2, 3)
print(result)

# We can add strings together too
print(add("one", "two"))

# If you uncomment the next line and run it you will get an error (remember we need to convert numbers to strings to join them)
# print(add(42, " is the answer."))

Challenge 1

We're going to make a calculator using our knowledge of functions and conditionals (if statements).

1. Create a function that accepts three values operator, a, b.
2. Check if the operator is + (plus), - (minus), * (times) or / (divide).
3. For each of the four cases that the operator could be, return the result of a OPERATOR b like a calculator.

Let's say the function you create is called calculate here are some examples of what should happen:

# Should print 5 (since 2+3)
print(calculate('+', 2, 3))

# Should print 11 (since 13-2)
print(calculate('-', 13, 2))

# Should print 2 (since 5/2)
print(calculate('/', 10, 5))

# Should print 40 (since 5/2)
print(calculate('*', 2, 20))

Solution to challenge 1

If you finish then you can take a look at the solution to compare how you did it to how I did it. Here is it.

Challenge 2

Once you get challenge 1 working adapt it so that a program asks you to enter an operator, one number and then another number and it prints the result using your calculation function.

This program should keep asking and returning the result forever by using a while True: loop, but at the end of each calculation it should ask the user if they want to stop or not. If the user wants to stop use the break command to exit the loop.

Extensions:

• If the user enters an invalid operator tell them they must enter a valid operator and go back to the start of the loop (Hint: use a continue statement which skips to the next iteration of the while loop. You'll also probably want to use and between the conditions for the if statement).
• Print out the full calculation and the result in the format a OPERATOR b = RESULT. So if I enter + as the operator, 2 for a and 5 for b, the program should print 2 + 5 = 7.

Watch out: don't forget to use the int function to convert the user input into a number.

Solution to challenge 2

If you finish then you can take a look at the solution to compare how you did it to how I did it. Here is it.

Previously we learnt about using for loops with a pre-defined list, e.g:

numbers = [4, 2, 8]

for number in numbers:
  print(number)

It should print the numbers 4, 2 and 8 on individual lines.

Now we're going to learn about a really useful function called range(start, stop). What is just is generate values from the start up to, but not including, the stop.

In for loops it acts just like an list, but unlike a list which has all the values stored, generators such as range only produce values as they are needed.

This means you can't do many of the things you can normally do with a list (e.g. index - get a value at a position).

# The variable name i is commonly used as a counter
for i in range(2, 10):
  print(i)