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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
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
a
The smallest random number that can be generated.
b
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 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.
A lexicon (English: dictionary) consists of keys and values. Each key has a value.
A key must be unique while a value can appear multiple times in the same dictionary.
Keys are used to access the value associated with that key.
Because Python uses something called hashing, it is fast to retrieve a value when we have the key.
No search is needed. In principle, even if a dictionary is very large, it does not take longer to find a value.
That is the big advantage of dictionaries.
Common types for keys are numbers and strings, but it also works to have, for example, tuples as keys.
It is possible to have all kinds of objects that are immutable. Lists therefore do not work as keys.
As a value, however, lists or any other type work well.
Create a dictionary
It is possible to create a dictionary on one line with the syntax
To then read, for example, value_1, we can write dictionary[key_1].
Example
A dictionary with the prices of different fruits. For clarity, each element is written on its own line. Note that two different fruits can have the same price.
studenter = dict()
studenter['Lund'] = 32036
studenter['Uppsala'] = 35556
studenter['Stockholm'] = 37484
print('Number of students in Lund is', studenter['Lund'])
If we write a value to a key that already exists in our dictionary, the value is overwritten.
To see how many key-value pairs there are in a dictionary, len() can be used.
Example
We see that the number of elements in a dictionary remains the same if we write to an existing key.
favorite_food = {
'Emil': 'Sushi',
'Leo': 'Hamburger',
'Tanja': 'Pizza'
}
element_1 = len(favorite_food) # 3 elements
#changes the value for an existing key
favorite_food['Tanja'] = 'Corn Flakes'
element_2 = len(favorite_food) # 3 elements
#adds a new key-value pair
favorite_food['Christine'] = 'Popcorn'
element_3 = len(favorite_food) # 4 elements
favorite_food = {
'Emil': 'Sushi',
'Leo': 'Hamburger',
'Tanja': 'Pizza'
}
element_1 = len(favorite_food) # 3 element
#changes the value for an existing key
favorite_food['Tanja'] = 'Corn Flakes'
element_2 = len(favorite_food) # 3 elements
#adds a new key-value pair
favorite_food['Christine'] = 'Popcorn'
element_3 = len(favorite_food) # 4 elements
print(element_1)
print(element_2)
print(element_3)
We can check if the key exists with if key in dictionary:.
It is generally very fast to see if a key exists or not, and it takes the same short time even if the dictionary is very large.
This can be compared to instead having a list of keys, where it takes much longer to see if the key exists in a large list compared to a small list.
(Advanced: In a list twice as large, it takes twice as long to search for a value. In a dictionary, the time is independent of the size.)
Example
If the key exists in the list, we print the value, otherwise we inform that the key does not exist.
millions_inhabitants = {
'Sweden':10.4,
'Denmark':5.8,
'Norway':5.4,
'Finland':5.5
}
key = input('Enter country.')
if key in millions_inhabitants:
print(key, millions_inhabitants[key])
else:
print(key, 'is not in the registry.')
millions_inhabitants = {
'Sweden':10.4,
'Denmark':5.8,
'Norway':5.4,
'Finland':5.5
}
key = input('Enter country.')
if key in millions_inhabitants:
print(key, millions_inhabitants[key])
else:
print(key, 'is not in the registry.')
It is easy to remove elements from a dictionary.
This is done with dictionary.pop(key).
Ant Myra lives at coordinate (0,0). Sometimes she wanders to another coordinate and then home again.
She always travels parallel to the x and y axes. She has collected which coordinates she has visited and how many times she has visited them in a dictionary.
An element like (3,2): 35 means that Myra visited coordinate (3,2) 35 times.Now Myra wants to know which coordinate she has spent the most time going back and forth to. She always walks at the same speed.To coordinate (3,2), Myra has walked the distance 35*(2+3)*2 back and forth.Print the correct coordinate in the form (x,y).