For Loops

Loops, In General

Does anyone remember the two broad categories of loops? What are they, and how are they different? →

Condition Controlled Loops in Python

What programming construct (repetition structure) do we use in Python to create a condition-controlled loop? →

Count-Controlled Loops

For Loops

An Example

for i in range(5):
	print(i)

Its output:

0
1
2
3
4

For Loop Syntax

for loops repeat code by iterating over every item in some group / collection of items:

"""
for <loop variable> in <iterable object>:
	(<loop variable> can be used here)
"""

Again, the same example:

"""
 keyword    loop    an iterable object,
     for  variable  like range
       |     |      |
       |     |      |
        for i in range(5):
              print(i)
"""

Let's Break That Down a Bit

A for loop:

• consists of the keyword for (obvs!)
• followed by a loop variable (this could be named anything you like)
• followed by some iterable object (some sequence of values)
• followed by a colon
• in this case, the iterable object is returned by the range function
• the range function returns an arithmetic sequence of numbers
• in this case, that sequence is [0, 1, 2, 3, 4]

For Loops - Details, Details, Details

What Does It Do?

A for loop:

• repeats a block of code for a specific number of times
• it does this by running through all of the elements in a sequence, one at a time
• the current element can be accessed in the body of the for loop using the loop variable
• (note that the loop variable is called target variable in Starting Out with Python)
• that is… for each element in your sequence, the loop variable is set to that element

For Loops

A more technical explanation is: for loops iterate over every item in an iterable object.

An iterable object:

• is a thing (object) that contains other values / members
• is capable of returning each of its members one at a time
• examples of iterable objects include:
  • strings (we know these)
  • range objects (we'll learn these today)
  • lists (for the future!)
• so… let's see about these range objects

Range and Range Objects

A range object is another data type! It represents an arithmetic sequence, such as 0, 1, 2, 3, 4.

• a range object is created by calling the range() function.
• with one argument, it creates an arithmetic sequence from 0 up to but not including that argument
• consequently, range(5) produces 0, 1, 2, 3, 4
• to see the elements in a range, you can use the list() function
• (we'll learn more about lists later!)

Showing the Results of Range

Let's see if we can make sense of this: →

# make a range object - an arithmetic sequence from 0 through 5
numbers = range(5)

# let's look at that object
print(numbers)

# what type is it?
print(type(numbers))

# can we actually see the sequence?  yes, but we have to use list.
print(list(numbers))

range(0, 5)
<class 'range'>
[0, 1, 2, 3, 4]

So… All of That Meant…

• there's a type called range
• the string representation of a range shows you the original arguments that you passed in
• you can call the built-in list function (used to convert value into a list) to show the exact ints in the list

Quick Summary So Far…

We've learned two new built in functions

• range returns a range object, a representation of an arithmetic sequence
• list returns a list object
  • for now, we use list in combination with print to show each number in a range object
  • we'll learn more about lists later

Range Life…

Range

range() returns a range object, an arithmetic sequence of numbers.

• one argument: range(stop)
  • returns a series of numbers starting with 0, up to, but not including stop by ones
  • range(3) → 0, 1, 2
• two arguments: range(start, stop)
  • returns a series of numbers starting with start , up to, but not including stop by ones
  • range(5, 10) → 5, 6, 7, 8, 9
• three arguments: so many arguments, you'll have to go to the next slide →

Range (Continued)

(continued from previous slide)

• three arguments: range(start, stop, step)
  • returns a series of numbers starting with start , up to, but not including stop by step
  • range(4, 9, 2) → 4, 6, 8

Some Other Things You Should Know About Range

• negative step goes backwards
  • range(8, 3, -2) → 8, 6, 4
• printing the result of a range object just gives you the original arguments that you used to call range
  • print(range(8, 3, -2)) → range(8, 3, -2)
• use the list function to show the elements in a range
  • print(list(range(8, 3, -2)))

Guess That Series of Numbers

Given the following calls to the range function, what is the start, stop, and step? What is the resulting arithmetic sequence? →

range(3)

# start:0, end:3, step:1
0, 1, 2

range(10, 16)

# start:10, end:16, step:1
10, 11, 12, 13, 14, 15

range(-2, 3)

# start:-2, end:3, step:1
-2, -1, 0, 1, 2

Guess That Series of Numbers

What is the start, stop, and step? What is the resulting arithmetic sequence? →

range(200, 501, 100)

# start:200, end:501, step:100
200, 300, 400, 500

range(0, 10, 5)

# start:0, end:10, step: 5
0, 5

range(5, -11, -5)

# start:5, end:-11, step:-5
5, 0, -5, -10

For Loop Examples

And Now, Back to For Loops

for whatevs in range(1, 4):
		print(str(whatevs) + " Mississippi")

• notice the different loop variable name, whatevs?
• What does this snippet of code output?

1 Mississippi
2 Mississippi
3 Mississippi

Let's Take a Closer Look

for whatevs in range(1, 4):
		print(str(whatevs) + " Mississippi")

Going over this step-by-step, what is the value of whatevs, and what is printed? →

Another For Loop Example

for num in range(6, 13, 3):
	result = num * num
	print(str(num) + " squared is " + str(result))

And… step-by-step, that's: →

How About Another?

What is the output of this code? Let's read through it line-by-line to figure it out. →

for whatevs in range(1, 4):
	if whatevs == 2:
		print("let's skip this one")
	else:
		print(str(whatevs) + " Mississippi")

1 Mississippi
let's skip this one
3 Mississippi

For Loop Exercises

Some Quick Excercises

Let's do these together →

• count to 100 starting from 1
• count to 100 by twos from 0
• count backwards starting from 100 down to and including 0

Fizz Buzz

• fizz buzz
• print out 1 to 100 …with the following exceptions:
• for multiples of three, print out "Fizz" instead of the number
• for multiples of five, print out "Buzz" instead of the number
• for multiples of both three and five print “FizzBuzz”
• example output, next slide, plz!

FizzBuzz Output

1
2
Fizz
4
Buzz
Fizz
.
.
14
FizzBuzz
16
.
.
98
Fizz
Buzz

FizzBuzz Solution

for i in range(1, 101):
	if i % 3 == 0 and i % 5 == 0:
		print("FizzBuzz")
	elif i % 3 == 0:
		print("Fizz")
	elif i % 5 == 0:
		print("Buzz")
	else:
		print(i)

Using an Accumulator Variable

Accumulator Variable

An accumulator variable is a variable used to keep the running total of a repeated calculation or operation that's within a loop:

• a variable is declared outside of the loop
• it is added to within the loop

Some examples include:

• counting the number of times a while loop runs (count += 1)
• incrementally building a string within a loop (s += word)

Summing Numbers

• sum the first 1-100 numbers, print out the resulting the sum
• (which, of course, doesn't need a loop)
• (and there's this story)
• (and the list of things named after him!?)

total = 0
for i in range(1, 101):
	total = total + i
print(total)

Counting Dice For Loop

Roll a die 1000 times; count how many times a one is rolled! Print out the result. Use a for loop.→

import random
ones = 0
for count in range(1, 1000):
	roll = random.randint(1, 6)
	if roll == 1:
		ones = ones + 1
print(str(ones) + " of 1000 rolls were ones")

Counting Dice While Loop

Roll a die 1000 times; count how many times a one is rolled! Print out the result. Use a while loop.→

import random
ones = 0
count = 0
while count < 1000:
	roll = random.randint(1, 6)
	if roll == 1:
		ones = ones + 1
	count = count + 1
print(str(ones) + " of 1000 rolls were ones")

Using User Input to Influence Number of Repetitions

Controlling a For Loop With User Input

We can base the number of repetitions that a for loop goes through by asking the user to enter a value.

Let's check out a programming problem where this idea might come in handy… →

A Ladder

Make me a ladder! →

• ask for a height, make a ladder with that many rungs
• can you do this using a for loop?
• or how about just string multiplication?

How tall do you want this ladder to be?
> 3

 ========
 |      |
 ========
 |      |
 ========
 |      |

A Ladder Implementation

(Or… two implementations, really)

height = int(input('How tall do you want this ladder to be?'))

for i in range(height):
	print('========\n|      |')

# or... just multiply instead of using the for loop
# print(height * '========\n|      |')