Reference

Click on a function to read more.

Basics part 1

Syntax

Short description

break

break interrupts code that is repeated

Example

while True:
    print('You are nice!')
    answer = input('Should I stop saying that? Write yes in that case.')
    if answer == 'yes':
        break

Explanation

break interrupts code that is repeated. For example when while is used.

continue

continue interrupts a repetition and starts the next

Example

i = 0
while i < 100: 
    if i % 2 == 0: # continue with the next repetition if i is divisible by 2
        continue
    print(i,'is an odd number')

Explanation

continue interrupts the execution in the current repetition/iteration and continues with the next repetition. Used in while-statements and for-statements.

elif

elif is a combination of else and if

Example #1

temp = int(input('What is the temperature?'))
if temp > 20:
    print('Warm and nice!')
elif temp > 0: #if temp <= 20 and temp > 0
    print('Not very cold, but not so warm either.')

Example #2

number = int(input('Enter an integer'))
if number == 0:
    print('You entered the number 0.')
elif number > 0:
    print('You entered a positive number.')
else:
    print('You entered a negative number.')

Explanation

elif is only used after if. It is a combination of else and if, meaning ELSE (if the above is false) and IF (if the new condition is true). The condition for elif is only tested if the condition for if is false.

Syntax

elif condition:

condition

Required. Something that is True or False.

else

else can be used last in an if-statement

Example

age = int(input('How old are you?'))
if age >= 18:
    print('You are an adult!')
else:
    print('You are not an adult.')

Explanation

else is only used at the end of a block that starts with if, and the code under else is only executed if the condition for if is false.

float()

Converts to type float (decimal number)

Example

float('0.01')  #converts the string '0.01' to 0.01
float(10)  #converts the integer 10 to 10.0

Explanation

The function float() converts the argument to type float (decimal number). Usually, the argument is a string or an integer.

Syntax

float(number)

Argument

number

A number in the form of a string or an integer.

if controls the code to do different things depending on a condition

Example #1

number = 10
if number == 10:
    print('The number is equal to 10.') #this will be printed

Example #2

number = int(input('Enter an integer'))
if number == 0:
    print('You entered the number 0.')
elif number > 0:
    print('You entered a positive number.')
else:
    print('You entered a negative number.')

Explanation

if is used to control what the program does depending on a condition. If the condition is true, the code in the if-block is executed. To control what happens if the condition is not true, elif and else can be used.

Syntax

if condition:

condition

Required. Something that is True or False.

input()

Receives input from the user

Example

answer = input('What is your name?')  #The user's answer is saved in the variable answer

Explanation

The function input() allows the user to enter input into the program. It is possible to include text that describes what should be entered.

Syntax

input(prompt)

Argument

prompt

An optional text the user sees.

int()

Converts to type int (integer)

Example

int(42)  #converts the string '42' to 42
int(3.94)  #converts the decimal number 3.94 to 3

Explanation

The function int() converts the argument to type int (integer). Usually, the argument is a string or a decimal number.

Syntax

int(number)

Argument

number

A number in the form of a string or a decimal number.

print()

Prints text or the content of a variable to the screen

Example #1

print('Hello World')  #prints "Hello World"
print(1+1) #prints the number 2
print(x)  #prints the content of variable x

Example #2

print('Hello World', end='')  #prints "Hello World" without a new line

Example #3

print('Hello','to','you')  #Prints "Hello to you"
print('Hello','to','you',sep=' | ',end='!')  #prints "Hello | to | you!" without a new line at the end.

Explanation

The function print() prints a string to the screen. Automatically tries to convert what is to be printed to text.

Syntax

print(*object, sep=' ', end='\n')

Argument

*object

Required. One or more objects to be printed. Separate objects with commas.

sep

Character to separate the objects being printed. Default is space.

end

Character that ends the text being printed. Default is "\n" - which means new line.

randint()

Generates a random integer

Example

from random import randint
roll = randint(1,6)  #a random number 1-6

Explanation

The function randint() generates a random integer between two integer values a and b such that a <= N <= b. randint is in the random module.

Syntax

randint(a,b)

Argument

The smallest random number that can be generated.

The largest random number that can be generated.

round()

Rounds a number

Example #1

round(7.89)  #rounds to 8
round(1.23)  #rounds to 1

Example #2

round(1.23456,2)  #rounds 1.23
round(1.23456,4)  #rounds to 1.2346

Explanation

The function round() rounds a decimal number. By default to an integer, it is possible to set round() to round to an arbitrary number of decimals.

Syntax

round(number, decimalplaces = 0)

Argument

number

Required. The number to be rounded.

decimalplaces

Number of decimals the number should be rounded to. Default is 0.

str()

Converts to type string

Example #1

str(2.71)  #converts the decimal number 2.71 to the string '2.71'
str(10)  #converts the integer 10 to '10'

Example #2

age = 15
print('You are ' + str(age) + ' years old.')

Explanation

The function str() converts the argument to a string. Often the argument is an integer or a decimal number. It can also be significantly more complex data types that can be converted to strings.

Syntax

str(object)

Argument

object

An object that can be converted to a string.

while

while is used to repeat code

Example

n = int(input('How many numbers do you want to print?'))
i = 1
while i <= n:
    print(i)
    i = i + 1

Explanation

while repeats the code in the block as long as the condition is true.

Syntax

while condition:

condition

Required. Something that is True or False. Must become false at some point for the while-block to be exited.

Basics part 2

Syntax

Short description

abs

Absolute value

Example

a = abs(5) # a = 5
b = abs(-5.2) # b = 5.2

Explanation

abs takes the absolute value of a number and thus always returns a positive number.

Syntax

abs(number)

Argument

number

Required. A valid number, integer, float, or complex number.

append

Adds an element to a list

Example #1

my_list = [1,2,3]
my_list.append(4) # now [1,2,3,4]

Example #2

my_list = ["one",2,3.0] # in Python, a list can consist of different types
my_list.append([1,2,3,4]) # now ["one",2,3.0,[1,2,3,4]]

Explanation

append adds an element to the end of a list. The list must already exist. It can be empty.

Syntax

my_list.append(element)

Argument

element

Required. The element to be added to the end of the list.

def

Defines a function

Example #1

def print_hello():
    print('Hello!')

Example #2

def hello_name(name):
    print('Hello ' + name)

hello_name('Tanja') #Prints "Hello Tanja"
hello_name('Emil') #Prints "Hello Emil"

Example #3

def perimeter_rectangle(width, height):
    perimeter = 2*width + 2*height
    return perimeter

Explanation

def defines a new function. A function can have arguments that you can use to pass data to the function. The function can also return data using return.

for

for is used to repeat code

Example #1

for i in range(10):  # i goes from 0 to 9
    print(i) # runs ten times

Example #2

for character in 'Pythonlab.dev':
    print(character)

Example #3

for element in [1,3,3,7]:
    print(element)

Explanation

for repeats the code in the block for all elements in an object. The object is for example a range, list or string.

Syntax

for variable in object:

variable

Required. The variable changes content to the current element for each iteration.

object

Required. The object to iterate over. For example, a list.

len

Returns the number of elements in an object

Example #1

my_list = [1,2,3]
length = len(my_list) # length = 3

Example #2

country = 'Sweden'
length = len(country) # length = 7

Example #3

tuple = ('Sweden', 22472, 'Lund')
length = len(tuple) # length = 3

Explanation

len returns the length or number of elements in an object such as a string, list, tuple, dictionary or a range.

Syntax

len(object)

Argument

object

Required. The object whose length is to be returned.

max

Returns the largest element

Example #1

lista = [1,2,3,4]
largest = max(lista) # largest = 4

Example #2

max(a,0) # 0 if a is negative, otherwise a

Explanation

max finds the largest element.

Syntax

max(object)

Argument

object

Required. An iterable object.

min

Returns the smallest element

Example #1

lista = [1,2,3,4]
smallest = min(lista) # smallest = 1

Example #2

min(a,0) # 0 if a is positive, otherwise a

Explanation

min finds the smallest element.

Syntax

min(object)

Argument

object

Required. An iterable object.

pop

Removes an element from a list

Example #1

lista = [1,2,3]
lista.pop() # now [1,2]

Example #2

lista = [1,2,3]
lista.pop(0) # now [2,3]

Explanation

pop removes an element at a given position from a list.

Syntax

lista.pop(element)

Argument

element

Optional. Index of the element to be removed from the list. Default value is -1 (last element).

return

Returns data from a function

Example

def square_and_add(x,y):
    return x**2 + y**2

result = square_and_add(5,7)
print('Squaring and adding 5 and 7 gives', result)

Explanation

return sends data back from a function to the place where it was called.

sum

Sums the elements in an object

Example #1

lista = [1,2,3,4]
sum_val = sum(lista) # sum_val = 10

Example #2

tupel = (1,2,3)
sum_val = sum(tupel) # sum_val = 6

Explanation

sum adds all elements in an object to a sum.

Syntax

sum(object)

Argument

object

Required. An iterable object.

Lists

In this section, you will learn

How to create a list
How to read the value at a specific position in the list
How to add and remove elements in a list
Some useful functions associated with lists

List Basics

When programming, it is very common to use a structure that can store a number of values in a specific order. This structure has many names, such as array, list, or vector. In Python, the word list is used.

A list allows duplicates. It is also possible to have lists inside a list to create a matrix or a two-dimensional list. In Python, different elements can be of different types, which is not the case in many other languages.

We start by creating a list consisting of three fruits. To create the list, square brackets [ ] are used. Values in the list are separated by commas. Using print, we print the contents of the list.

Example

We create a list named fruits containing three strings of fruit names and print it.

fruits = ['Apple', 'Banana', 'Mango']
print(fruits)

In a list, we always start counting from zero. The first element has index 0. In the example above, we can read the first value in the list by writing fruits[0]. To read the second value in the list, we write fruits[1], and so on.

In Python, we can use negative indices to go backwards from the end. The last element is read with fruits[-1], and the second to last element is read with fruits[-2].

The fact that the first element in a list is element 0 is usually confusing for beginners. What's worse, there are programming languages where counting starts from one, although this is uncommon.

Example

How we can read values. We use the notation fruits[i] where i is the index of the element in the list.

fruits = ['Apple', 'Banana', 'Mango']
first_element = fruits[0]
second_element = fruits[1]
third_element = fruits[2]
last_element = fruits[-1]

What does the code do?

Read the code below and try to figure out what the program prints. Run the program after you answer and see if you were correct.

-- Output from your program will be here --

Question: What will the program print?

Java

Python

PHP

Adding and Removing Elements

We can write fruits = [] to create an empty list. For each list created, a number of functions are available that can be used on the list, such as append() and pop(). We write the list's name, followed by a dot, to access its functions. For example, we can write fruits.append('Pear') to add another fruit to the list. The new element is added to the end of the list.

The function pop(index) can be used to remove the element at position index from a list. For example, if we want to remove the first element in the list, we write fruits.pop(0). If we want to remove the last element, it is enough to write fruits.pop(); without an argument, -1 is used as the default index. The pop() function also returns the element that was removed, in case we want to use it.

Example

We start with an empty list, add a couple of elements, and then remove the first and last. Click run to see what the list contains at the different steps.

fruits = []
fruits.append('Apple')
fruits.append('Banana')
fruits.append('Mango') #fruits = ['Apple', 'Banana', 'Mango']

fruits.pop(0) #remove the first element, 'Apple'.
fruits.pop(-1) #remove the last element, 'Mango'.

Example

A list can contain any data type, for example, integers. Here, a list of three integers is first created. The integer 20 is added to the list, and then the second integer in the list is removed.

numbers = [10, 5, 35]
numbers.append(20)
removed = numbers.pop(1)
print('We removed the number ' + str(removed))

What does the code do?

Read the code below and try to figure out what the program prints. Run the program after you answer and see if you were correct.

-- Output from your program will be here --

Question: What will the program print?

Volvo

Tesla

Audi

It is good to know that if the list is large, it is quite inefficient to, for example, remove and add elements to the list (except at the end of the list). If this needs to be done often, there are better data structures that we will learn more about later.

List Length

To see how many elements are in a list, we can use the helper function len() with the list as an argument. This is useful, for example, if we want to loop through the entire list using while.

Example

Prints each element in the list on its own line. Also takes the opportunity to show how a list can be created with one element per line.

fun_english_words = [
    'Shenanigans', 
    'Bamboozle', 
    'Bodacious', 
    'Brouhaha', 
    'Canoodle', 
    'Malarkey'
]

count = len(fun_english_words)
index = 0
while index < count:
  print(fun_english_words[index])
  index = index + 1

In the example above, the condition to continue the loop is index < count. It is important that index never becomes as large as count. The list fun_english_words contains six elements. If we try to read outside the list when index = 6 with fun_english_words[6], the program will stop and we will get an error message.

Example

Example of an error message if we try to read outside the list. The last element is read with my_list[5]. my_list[6] results in an error message of type IndexError.

my_list = [1, 2, 3, 4, 5, 6]
print( my_list[5] ) #prints 6
print( my_list[6] ) #causes an error

Does the element exist in the list?

Using in, we can check if an element exists in the list. For example, 5 in my_list is True if 5 is in the list. We can combine this with not to check if the element is not in the list.

Example

We create a list of names and then test if a name is in the list or not. Feel free to test with other names.

the_team = ['Anton', 'Beatrice', 'Cecilia', 'David']

if 'Cecilia' in the_team:
  print('Cecilia is on the team.')

if 'Zlatan' not in the_team:
  print('Zlatan is not on the team.')

Test your knowledge

Below are two exercises to learn how to use lists.

Modify the code

The program should receive a number of integers from the user, put them in a list, and then print the sorted list. To sort the list, my_list.sort() is used.

-- Output from your program will be here --

Filter out positive numbers

Write a function named filter that takes a list as its only argument. Return a new list that contains only the positive integers from the original list and in the same order.

-- Output from your program will be here --

Status

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