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.

for-statement

In this section you will learn

Go through all elements in a list with for
Iterate through a string
Use range() with different numbers of arguments
About nested loops

Iterate through list with for

To iterate through a list means that the list is gone through element by element. Actually, iterate means to repeat and the expression comes from the fact that we repeat the same thing for each element in the list.

The syntax is for variable in list: where variable is an optional variable name and list is a list. Indented below is the code to be executed for each element.

Example

A simple iteration of a list where each element is printed.

colors = ['Green', 'Red', 'Blue']
for color in colors:
    print(color)

Example

Here we show how we can test each element and, as in the example, count the number of elements greater than 10.

number_list = [44, -2, 19, 5, 30, 10]
count = 0
for number in number_list:
    if number > 10:
        count = count + 1

print(count)

In previous sections, we used while to iterate through a list. It works just as well, but in the case above where we need to go through the entire list, the syntax is smoother with for. We will soon see how for is easy to use when we want to repeat something a certain number of times.

Change the code

The code contains a number of errors that need to be corrected. Change the code so that it prints the names that come before E alphabetically.

-- Output from your program will be here --

Iterate through a string

In the same way that it is possible to iterate through a list, it is possible to iterate through a string, character by character, with for.

Example

Iterate through all characters in a string.

text = 'Iterate more'
for character in text:
    print(character)

With the help of enumerate() we can also get a variable that keeps track of which iteration we are on. This variable thus functions as a counter and it starts at 0 and counts up by one for each iteration. The example below shows for strings, but it works well on all types of objects that can be iterated over.

Example

Iteration with for and enumerate.

text = 'Iterate more'
for i, character in enumerate(text):
    print('Index ' + str(i) + ': ' + str(character))

@livewire('editor-modify-correct', [ 'editorId' => 'for-make-vocals', 'code' => $code, 'rows' => 10, 'showReset' => true, 'correctAnswer' => array("N/A"), 'correctInput' => $correctInput, 'correctOutput' => $correctOutput, 'title' => 'Create', 'text' => 'Write a program that takes a string from the user and then calculates how many vowels the string contains. Print that number.', 'tip_text' => 'Receive a string from the user with input. Then go through the string's characters with a for-loop and count up a variable for each character that is a vowel.' ])

Repetitions with range()

Sometimes we want to repeat the same code a fixed number of times and then the function range() is suitable. The simplest syntax is for variable in range(n): where variable is the counter's variable name and n is the number of repetitions.

Example

Repetition of the same code three times.

for i in range(3):
    print('Python!')

Example

In this example, we see how the counter changes for each iteration. Note that it starts at 0.

for i in range(3):
    print(i)

We can choose what range() should start at by using two arguments. The first argument is what the interval starts at and the second argument is how far the counter should go (not inclusive). For example, range(1, 10) means values from 1 to 9. Keep in mind that the value in the second argument is therefore not included in the interval.

Example

Counter with variable name i that starts at 1 and ends at 9.

for i in range(1, 10):
    print(i)

It is actually possible to use three arguments as well. The third argument stands for the step length. For example, range(1,10,2) means that we start at 1 and then take two steps for each iteration, in the second iteration the value is 3 and in the last the value is 9. It is also possible to use negative step lengths for the counter to go backwards.

Example

Example of using range() with three arguments.

for i in range(10, 51, 10):
    print(i)

for i in range(5, 0, -1):
    print(i)

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?

1
2

1
4

Nested for-statements

It is common to use a for-statement inside another for-statement, so-called nested loops. It also works well with while.

Example

Two nested for-loops that print all combinations of two lists.

colors = ['blue', 'yellow', 'pink']
things = ['bag', 'car', 'house']

for color in colors:
    for thing in things:
        print(color + ' ' + thing)

Example

Two nested for-loops that print all 36 combinations of two dice rolls.

for t1 in range(1,7):
    for t2 in range(1,7):
        print(str(t1) + ', ' + str(t2))

Change the code

Change the code so that only the dice rolls that have a sum of 10 are printed.

-- Output from your program will be here --

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