Loops

Python has two primitive loop commands:

• while loops

• for loops

The while Loop

With the while loop we can execute a set of statements as long as a condition is true.

// 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

With the break statement we can stop the loop even if the while condition is true:

// Example
Exit the loop when i is 3:
i = 1
while i < 6:
  print(i)
  if i == 3:
    break
  i += 1

The continue Statement

With the continue statement we can stop the current iteration, and continue with the next:

// 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

With the else statement we can run a block of code once when the condition no longer is true:

// 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

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
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

Even strings are iterable objects, they contain a sequence of characters:

// Example
// Loop through the letters in the word "banana":
for x in "banana":
  print(x)

The break Statement

With the break statement we can stop the loop before it has looped through all the items:

// Example
// Exit the loop when x is "banana":
fruits = ["apple", "banana", "cherry"]
for x in fruits:
  print(x)
  if x == "banana":
    break

// 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

With the continue statement we can stop the current iteration of the loop, and continue with the next:

// Example
Do not print banana:
fruits = ["apple", "banana", "cherry"]
for x in fruits:
  if x == "banana":
    continue
  print(x)

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
// 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
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
Increment the sequence with 3 (default is 1):
for x in range(2, 30, 3):
  print(x)

---

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
// 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
// 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

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
// 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

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
for x in [0, 1, 2]:
  pass