Note!
The window is too narrow to use Pythonlab.dev. Use a device with a keyboard!
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.
Simple Calculations
In this section you will learn
Use + on strings
Perform simple calculations
About order of operations
Using the + operator
In this section, we will use the most common operators. Typical mathematical operators are +−* and /.
Of these, only + can be used between strings. Adding numbers with + works exactly as you imagine.
Example
The operator + between two numbers and between two strings.
As you can see in the example above with 'hey ', it is fine to use spaces as a termination of a string to create a space to the next word. It does not work to combine a string with a number, for example,
the program does not work if you write 'A string' + 10.
Change the code so that an integer division is performed instead of regular division.
-- Output from your program will be here --
Order of Operations
As you have probably learned, multiplication and division have higher priority than addition and subtraction. This means that for an expression like
$$4+7\cdot2=18$$
the multiplication \(7\cdot2\) is performed first and then that result is added to 4. This is exactly how it works in Python as well. Of the operators we have looked at in this
section, you can see the order of precedence in the table below.
Priority
Operators
High
**
|
* / // %
Low
+ −
Just like in mathematics, you can use parentheses to change the order. To perform the addition first in the example above, we can write
$$(4+7)\cdot2=22$$
Now you know more about how you can use Python as a calculator.
Print what \(2^{137} + 543\cdot263\cdot1000\) becomes
-- Output from your program will be here --
The number you calculated above is very large with its 42 digits. It is actually quite unusual for programming languages to handle such large numbers.
In Python, there is no upper limit to how large a number can be. The disadvantage of allowing arbitrarily large numbers is that the performance of calculations becomes worse.