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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('Hej','på','dig') #Prints "Hello to you"
print('Hej','på','dig',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.
Logical Expressions
In this section you will learn
What a logical expression is
A new boolean data type that can store the value true or false
How and and or are used in logical expressions
Logical Expressions
A logical expression is something that is either true (English: true) or false (English: false).
Let's say we ask for the user's age. Is the user's age greater than 18? It is either true or false.
We can write age > 18 and this is a logical expression.
If we use print on a logical expression in Python, we will get either True or False as output.
Example
Two examples of logical expressions that are true. In both cases, a variable is compared with a number.
x = 5
print(x == 5) #x is equal to 5? (true)
print(x != 10) #x is NOT equal to 10? (true)
x = 5
print(x == 5) #x is equal to 5? (true)
print(x != 10) #x is NOT equal to 10? (true)
Note that two equals signs == are used to compare if two things are equal.
It is a very common mistake to accidentally write only one equals sign when comparing.
Example
Two examples of logical expressions that are false. In this example, two variables are compared with each other.
x = 5
y = 10
print(x > y) #x is greater than y? (false)
print(y <= x) #y is less than or equal to x? (false)
x = 5
y = 10
print(x > y) #x is greater than y? (false)
print(y <= x) #y is less than or equal to x? (false)
In the examples above, different types of comparisons were used, and the most common comparisons are summarized in the table below.
Comparison
Means
==
Equal to
!=
Not equal to
<
Less than
>
Greater than
<=
Less than or equal to
>=
Greater than or equal to
It is fine to compare integers (int) with decimal numbers (float), but for natural reasons it is not possible to compare numbers with strings.
Between two strings, it is excellent to use all the comparisons above. The strings are compared alphabetically, for example, 'sun' < 'star' is true.
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?
0.3
False
0.3
True
If you want to read more about what actually happened in the exercise above, see here on python.org.
Data type for true or false
It is excellent to store the value of a logical expression (True or False) in a variable.
It is called a boolean variable. You will not hear the word "boolesk" often, usually the English word "boolean" or just "bool" is used. A boolean is the data type that takes up the least space in memory; it only needs one bit.
0 for False and 1 for True.
Example
True is stored in variable x because 3 > 2 is true.
x = 3 > 2
x = 3 > 2
print(x)
Combine logical expressions
We will now use two so-called logical operators, and (English: and) and or (English: or).
With and, the expressions on both sides of and must be true for the combined expression to be true.
If any of the expressions around and are false, the entire expression becomes false.
A common use is to check if a variable is within a range, see the example below.
Example of and
Check if variable x is between 10 and 20. Try changing x.
x = 15
print(x > 10 and x < 20)
x = 15
print(x > 10 and x < 20)
With or, at least one of the expressions around or must be true for the combined expression to be true.
For the combined expression to be false, both logical expressions around or must be false.
A common use is to check if a variable is outside a range, see the example.
Example of or
Check if variable x is outside the range [10, 20]. Try changing x.