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CONDITIONAL STATMENTS
Python Conditions and If statements
Python Conditions and If statements
Python supports the usual logical conditions from mathematics:
Equals: a == b
Not Equals: a != b
Less than: a < b
Less than or equal to: a <= b
Greater than: a > b
Greater than or equal to: a >= b
These conditions can be used in several ways, most commonly in "if statements" and loops.
An "if statement" is written by using the if keyword.
Example
Example
If statement:
a = 33
b = 200
if b > a:
print("b is greater than a")
In this example we use two variables, a and b, which are used as part of the if statement to test whether b is greater than a. As a is 33, and b is 200, we know that 200 is greater than 33, and so we print to screen that "b is greater than a".
Indentation
Indentation
Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Other programming languages often use curly-brackets for this purpose.
Example
Example
If statement, without indentation (will raise an error):
a = 33
b = 200
if b > a:
print("b is greater than a") # you will get an error
Elif
Elif
The elif keyword is pythons way of saying "if the previous conditions were not true, then try this condition".
Example
Example
a = 33
b = 33
if b > a:
print("b is greater than a")
elif a == b:
print("a and b are equal")
In this example a is equal to b, so the first condition is not true, but the elif condition is true, so we print to screen that "a and b are equal".
Else
Else
The else keyword catches anything which isn't caught by the preceding conditions.
Example
Example
a = 200
b = 33
if b > a:
print("b is greater than a")
elif a == b:
print("a and b are equal")
else:
print("a is greater than b")
In this example a is greater than b, so the first condition is not true, also the elif condition is not true, so we go to the else condition and print to screen that "a is greater than b".
You can also have an else without the elif:
Example
Example
a = 200
b = 33
if b > a:
print("b is greater than a")
else:
print("b is not greater than a")
Short Hand If
Short Hand If
If you have only one statement to execute, you can put it on the same line as the if statement.
Example
Example
One line if statement:
if a > b: print("a is greater than b")
Short Hand If ... Else
Short Hand If ... Else
If you have only one statement to execute, one for if, and one for else, you can put it all on the same line:
Example
Example
One line if else statement:
a = 2
b = 330
print("A") if a > b else print("B")
This technique is known as Ternary Operators, or Conditional Expressions.
You can also have multiple else statements on the same line:
Example
Example
One line if else statement, with 3 conditions:
a = 330
b = 330
print("A") if a > b else print("=") if a == b else print("B")
And
And
The and keyword is a logical operator, and is used to combine conditional statements:
Example
Example
Test if a is greater than b, AND if c is greater than a:
a = 200
b = 33
c = 500
if a > b and c > a:
print("Both conditions are True")
Or
Or
The or keyword is a logical operator, and is used to combine conditional statements:
Example
Example
Test if a is greater than b, OR if a is greater than c:
a = 200
b = 33
c = 500
if a > b or a > c:
print("At least one of the conditions is True")
Nested If
Nested If
You can have if statements inside if statements, this is called nested if statements.
Example
Example
x = 41
if x > 10:
print("Above ten,")
if x > 20:
print("and also above 20!")
else:
print("but not above 20.")
The pass Statement
The pass Statement
if statements cannot be empty, but if you for some reason have an if statement with no content, put in the pass statement to avoid getting an error.
Example
Example
a = 33
b = 200
if b > a:
pass
LOOPING STATMENTS
Python Loops
Python Loops
Python has two primitive loop commands:
while loops
for loops
The while Loop
The while Loop
With the while loop we can execute a set of statements as long as a condition is true.
Example
Example
Print i as long as i is less than 6:
i = 1
while i < 6:
print(i)
i += 1
Note: remember to increment i, or else the loop will continue forever.
The while loop requires relevant variables to be ready, in this example we need to define an indexing variable, i, which we set to 1.
The break Statement
The break Statement
With the break statement we can stop the loop even if the while condition is true:
Example
Example
Exit the loop when i is 3:
i = 1
while i < 6:
print(i)
if i == 3:
break
i += 1
The continue Statement
The continue Statement
With the continue statement we can stop the current iteration, and continue with the next:
Example
Example
Continue to the next iteration if i is 3:
i = 0
while i < 6:
i += 1
if i == 3:
continue
print(i)
The else Statement
The else Statement
With the else statement we can run a block of code once when the condition no longer is true:
Example
Example
Print a message once the condition is false:
i = 1
while i < 6:
print(i)
i += 1
else:
print("i is no longer less than 6")
Python For Loops
Python For Loops
A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).
This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.
With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.
Example
Example
Print each fruit in a fruit list:
fruits = ["apple", "banana", "cherry"]
for x in fruits:
print(x)
The for loop does not require an indexing variable to set beforehand.
Looping Through a String
Looping Through a String
Even strings are iterable objects, they contain a sequence of characters:
Example
Example
Loop through the letters in the word "banana":
for x in "banana":
print(x)
The break Statement
The break Statement
With the break statement we can stop the loop before it has looped through all the items:
Example
Example
Exit the loop when x is "banana":
fruits = ["apple", "banana", "cherry"]
for x in fruits:
print(x)
if x == "banana":
break
Example
Example
Exit the loop when x is "banana", but this time the break comes before the print:
fruits = ["apple", "banana", "cherry"]
for x in fruits:
if x == "banana":
break
print(x)
The continue Statement
The continue Statement
With the continue statement we can stop the current iteration of the loop, and continue with the next:
Example
Example
Do not print banana:
fruits = ["apple", "banana", "cherry"]
for x in fruits:
if x == "banana":
continue
print(x)
The range() Function
The range() Function
To loop through a set of code a specified number of times, we can use the range() function,
The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.
Example
Example
Using the range() function:
for x in range(6):
print(x)
Note that range(6) is not the values of 0 to 6, but the values 0 to 5.
The range() function defaults to 0 as a starting value, however it is possible to specify the starting value by adding a parameter: range(2, 6), which means values from 2 to 6 (but not including 6):
Example
Example
Using the start parameter:
for x in range(2, 6):
print(x)
The range() function defaults to increment the sequence by 1, however it is possible to specify the increment value by adding a third parameter: range(2, 30, 3):
Example
Example
Increment the sequence with 3 (default is 1):
for x in range(2, 30, 3):
print(x)
Else in For Loop
Else in For Loop
The else keyword in a for loop specifies a block of code to be executed when the loop is finished:
Example
Example
Print all numbers from 0 to 5, and print a message when the loop has ended:
for x in range(6):
print(x)
else:
print("Finally finished!")
Note: The else block will NOT be executed if the loop is stopped by a break statement.
Example
Example
Break the loop when x is 3, and see what happens with the else block:
for x in range(6):
if x == 3: break
print(x)
else:
print("Finally finished!")
Nested Loops
Nested Loops
A nested loop is a loop inside a loop.
The "inner loop" will be executed one time for each iteration of the "outer loop":
Example
Example
Print each adjective for every fruit:
adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]
for x in adj:
for y in fruits:
print(x, y)
The pass Statement
The pass Statement
for loops cannot be empty, but if you for some reason have a for loop with no content, put in the pass statement to avoid getting an error.
Example
Example
for x in [0, 1, 2]:
pass
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