Skip to main content

Python Loops

 

While Loops

Python 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")

 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

Print each fruit in a fruit list:

fruits = ["apple", "banana", "cherry"]
for x in fruits:
  print(x)
Try it Yourself »

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



Comments

Post a Comment

Popular posts from this blog

Python Tuples

  Python  Tuples mytuple = ( "apple" ,  "banana" ,  "cherry" ) Tuple Tuples are used to store multiple items in a single variable. Tuple is one of 4 built-in data types in Python used to store collections of data, the other 3 are List, Set, and Dictionary, all with different qualities and usage. A tuple is a collection which is ordered and  unchangeable . Tuples are written with round brackets. Example Create a Tuple: thistuple = ( "apple" ,  "banana" ,  "cherry" ) print (thistuple) Tuple Items Tuple items are ordered, unchangeable, and allow duplicate values. Tuple items are indexed, the first item has index  [0] , the second item has index  [1]  etc. Ordered When we say that tuples are ordered, it means that the items have a defined order, and that order will not change. Unchangeable Tuples are unchangeable, meaning that we cannot change, add or remove items after the tuple has been created. Allow Duplicates Since tuples...
Post a Comment
Read more

Python Strings

  Python  Strings  Strings Strings in python are surrounded by either single quotation marks, or double quotation marks. 'hello'  is the same as  "hello" . You can display a string literal with the  print()  function: Example print ( "Hello" ) print ( 'Hello' ) Assign String to a Variable Assigning a string to a variable is done with the variable name followed by an equal sign and the string: Example a =  "Hello" print (a) Multiline Strings You can assign a multiline string to a variable by using three quotes: Example You can use three double quotes: a =  "" "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua." "" print (a) Or three single quotes: Example a =  '' 'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.' '' print (a) Note:  in the res...
Post a Comment
Read more

PYTHON FILE I/O

           Python  File Open File handling is an important part of any web application. Python has several functions for creating, reading, updating, and deleting files. File Handling The key function for working with files in Python is the  open()  function. The  open()  function takes two parameters;  filename , and  mode . There are four different methods (modes) for opening a file: "r"  - Read - Default value. Opens a file for reading, error if the file does not exist "a"  - Append - Opens a file for appending, creates the file if it does not exist "w"  - Write - Opens a file for writing, creates the file if it does not exist "x"  - Create - Creates the specified file, returns an error if the file exists In addition you can specify if the file should be handled as binary or text mode "t"  - Text - Default value. Text mode "b"  - Binary - Binary mode (e.g. images) Syntax To open a file for ...
Post a Comment
Read more