Loops are control statements which allows a block of code to be executed multiple times.

This repetition of a code can be fixed number of times (for loop) or while some condition is fulfilled (while loop).

Each execution of the code, during the loop, is called an iteration!

Syntax

			while condition :
				block
		

The block will be executed while the condition is True!

Inside the block we have to change the variable used in condition to prevent an endless loop. Or we can use break statement (discussed further)

Flow

Simple example

			i = 1
			while i<=5 :
				print(i)
				i += 1
		

			1
			2
			3
			4
			5
		

This is just a simple example. For fixed numbers of times loops, it is better to user for with range loop, which will be discussed next

Example: endless loop (find the problem)

If you run next code, your Python will run an endless loop. Use CTRL+C or CTRL+Z to stop it

			# print the numbers from 10 to 1:
			i = 10
			while i>=1 :
				print(i)
				i = 1
		

Example - proper use case of while

While loop is suitable, when we do not know in advance the number of iterations needed

			# ask user to enter a name (string), until it contains at least 3 symbols
			# the len function on string returns the number of symbols in a string
			user_name = input("Enter a name, please: ")
			user_name_length = len(user_name)

			while user_name_length < 3:
				user_name = input("Enter a name (at least 3 symbols): ")
				user_name_length = len(user_name)

			print("Thank you, {}!".format(user_name))
		

Example: sum all numbers in [1..100]

			i = 1
			sum = 0
			while i <= 100:
				sum += i
				i += 1
			print("sum = ", sum)
		

			sum =  5050
		

Same task is better to be implemented with for loop!

Task: sum even numbers in [1..100]

Modify the previous example, but calculate the sum only of the even numbers in the given interval [1..100]

			sum =  2550
		

Syntax

			for item in sequence :
				#do something with item
		

Python for statement is different than the "C-based" for loops in other popular languages (C#, Java, PHP, JavaScript)

In Python, for statement iterates over the items of any sequence.

This is common to foreach loop concept in above-mentioned languages

Flow

Simple example 1

Iterate over symbols in string:

			for s in "ada":
				print(s.capitalize())
		

			A
			D
			A
		

Simple example 2

Iterate over list of numbers:

			for num in [1,2,3,4]:
				print(num)
		

			1
			2
			3
			4
		

Nested for loops

			for i in [1,2,3]:
				for j in "abv":
					print(j)
				print("\n") #prints new line
		

More examples and real-world usage of the for statement will be shown in Sequence data types theme!

Syntax in while loop

			while condition:
				block 1
				if break_cond:
					break  # loop is terminated, block 2 is skipped
				block 2
		

Syntax in for loop

			for item in sequence :
				block 1
				if break_cond:
					break  # loop is terminated, block 2 is skipped
				block 2
		

Flow

Example - Output letters in a string, until 'i' letter is reached

			str = "alibaba"
			for s in str:
			    if s == "i": break
			    print(s)
		

			a
			l
		

do-while emulations

Python did not have do-while loop, as in other languages. But it can be easily emulated.

Reason: The Zen of Python

Example - do-while emulation with break

			# ask user to enter a name (string), until it contains at least 3 symbols
			while True:
				user_name = input("Enter a name (at least 3 symbols): ")
				user_name_length = len(user_name)
				if user_name_length > 3:
					break

			print("Thank you, {}!".format(user_name))
		

Example - do-while emulation without break

			# ask user to enter a name (string), until it contains at least 3 symbols
			user_name = input("Enter a name, please: ")
			user_name_length = len(user_name)

			while user_name_length < 3:
				user_name = input("Enter a name (at least 3 symbols): ")
				user_name_length = len(user_name)

			print("Thank you, {}!".format(user_name))
		

Can you imagine how the code would look like, if the "do" block was more than 1 line long?

Returns the control to the beginning of the loop.

code after continue will be skipped.

Usually, continue statement is dependent on some condition.

Syntax in while loop

			while condition:
				block 1
				if continue_cond:
					continue  # go to while condition
				block 2
		

Flow

Example - print all numbers in [1..5], but skip 3

			for i in [1,2,3,4,5]:
			  if i == 3:
			     continue
			  print(i)
		

			1
			2
			4
			5
		

Example - print symbols in a string, excluding vowels:

			str = "alabala"
			for s in str:
			  if s in ["a", "e", "i", "o", "u", "y"]:
			      continue
			  print(s)
		

			l
			b
			l
		

"Guess the number" - full version

Write the full version of the "Guess the number" game, implementing the same rules as given in Guess the number game - the beginning, but giving the user the chance to try more than once.

The user now will have 5 tries to guess.

If he/she could not manage to guess the number for 5 tries, the game stops, with a message:
"You lost! My number was X"

where X is the machine number

"Guess the number" - solutions

a solution with while loop

guess_the_number_while_loop.py

a solution with for loop

guess_the_number_for_loop.py