Python: For Loops

A loop is programming statement that cause a statement or set of statements to execute repeatedly. Most loops should have condition logic built into them that causes the loop to end. A loop that does not end is known as an "infinite loop". Looping statements are also sometimes referred to as repetition or iteration statements.

The two main types of loops are:

• Count-controlled loop, i.e. definite loop
  • Repeats instructions a specific number of times
  • Often performed with a for loop
• Condition-controlled loop, i.e. indefinite loop
  • Repeats instructions while a condition is true
  • Often performed with a while loop, do-while loop, or do-until loop

Python has both for and whie looop.

Another ways loops can be categorized is as a pre-test loop or post-test loop. Python only has pre-test loops, i.e. it doe snot haev a do ... while loop.

Example: For loop in Python

	for x in range(1, 5):
	   print ("x=", x)
	

Output:

  x= 1
  x= 2
  x= 3
  x= 4

Example: For loop with accumulator in Python

	total = 0
	for x in range(4):
		total = total + x
		print("x=", x, "Total=", total)
	

Output:

  x= 0 Total= 0
  x= 1 Total= 1
  x= 2 Total= 3
  x= 3 Total= 6

Example: For loop in Python

	max = int(input("Enter maximum number:"))
	for i in range(1,max):
		print("i =", i)
	

Output:

  Enter maximum number: 4 {user input)
  i = 1
  i = 2
  i = 3

Example: For loop in Python

	for i in range(3,0,-1):
		print (i)
	

Output:

  3
  2
  1

Python has three loop control structures:

• break statement - terminates loop and transfers execution to statement immediately following loop.
• continue statement - causes loop to skip remainder of its body and retest condition prior to reiterating.
• pass statement - used when statement is required syntactically but you do not want any command or code to execute.

Example: For loop in Python

	# Prints odd numbers between 0 and 7
	for x in range(1, 8):
		if x % 2 == 0:
			continue
		print (x)
	

Output:

  1
  3
  5
  7

Example: For loop in Python based on user input (n)

	n = int(input('Enter maximum value: '))
	for i in range(n):
		if i % 2 == 0:
			continue
		print (i)
	

Example: Nested For loop in Python

	for x in range(1, 4):
		for y in range(1, 3):
			print (x, '*', y, '=', x*y )
	

Output:

  1 * 1 = 1
  1 * 2 = 2
  2 * 1 = 2
  2 * 2 = 4
  3 * 1 = 3
  3 * 2 = 6

Example: For loop with list in Python

	list = [5,10,20,30]
	sum = 0
	for num in list:
		sum = sum + num
	print ("The sum is: ",  sum)
	

Output:

  The sum is 65

Example: For loop with list in Python

	fruits = ['oranges', 'pears', 'grapes', 'apples']
	for fruit in fruits:
		print ("I like %s" % fruit)
	

Output:

  I like oranges
  I like pears
  I like grapes
  I like apples

Example: For loop in Python

	string = "Hello"
	for x in string:
		print (x)
	

Output:

  H
  e
  l
  l
  o

Example: For loop in Python

	# Loop will call function three times and display quote three times
	def displayQuote():
		print("The computer is incredibly fast, accurate, and stupid. \n"
			  "Man is unbelievably slow, inaccurate, and brilliant. \n"
			  "The marriage of the two is a force beyond calculation. \n"
			  "-Leo Cherne, attributed, The World of Work: Careers and the Future.\n")

	for x in range(0,3):
		displayQuote()
	

Example: For loop inside function in Python

	def calcRaise(salary, perc):
		total = 0
		print("Current salary is: ", salary)
		for x in range(1, 6):
			salary = salary * (1 + perc)
			print("Salary in the next year is $%.2f" % salary)
			total = total +  salary
		return total


	salary = float(input("Enter current salary: "))
	perc = float(input("Enter annual raise amount: "))
	total = calcRaise(salary, perc)
	print("You will make a total of $%.2f over the next five years" % total)
	

Output:

  Enter current salary: 50000
  Enter annual raise amount: .04
  Current salary is: 50000.0
  Salary in the next year is $52000.00
  Salary in the next year is $54080.00
  Salary in the next year is $56243.20
  Salary in the next year is $58492.93
  Salary in the next year is $60832.65
  You will make a total of $281648.77 over the next five years

Example: For loop with list and nested if statement in Python

	at_home = ["oranges", "olives","bananas","pickles"]
	for food in at_home:
		if food == "anchovies":
			print("No more anchovies please!")
			break
		print("Great. I really like " + food)
	else:
		print("Fantastic - no anchovies!")
	print("I'm full now.")
	

Output:

  Great. I really like oranges
  Great. I really like olives
  Great. I really like bananas
  Great. I really like pickles
  Fantastic - no anchovies!
  I'm full now.

Example: For loop in Python - 99 bottles of water using decrement

	for i in range(99, 0, -1):
		if i == 1:
		   print('1 bottle of water on the wall, 1 bottle of water!')
		   print('Take it down, pass it around, no more bottles of water on the wall!')
		else:
		   print('{0} bottles of water on the wall, {0} bottles of water!'.format(i))
		   print('Take one down, pass it around, {0} more bottles of water on the wall!'.format(i - 1))
	

Example: For loop inside function in Python with accumulator (total) and returning two variables

	def loopFunction(total):		# function returns two values
		num = int(input("How many times do you want to loop? "))
		for x in range(num):
			total = total + x
			print("x=", x)
		return num, total
	total = 0
	print("Before loop: x=", x, "and total =", total)
	x, total = loopFunction(total)
	print("After loop: x=", x, "and total =", total)
	

Output: