5 ITERATION (PYTHON)

PYTHON | ITERATION

ALSO IN THIS TOPIC
GETTING STARTED
VARIABLES AND DATA TYPES
OPERATORS
SELECTION
YOU ARE HERE | ITERATION
ARRAYS
FUNCTIONS AND PROCEDURES
FORMATTING DATA
ERROR CHECKING
OBJECT ORIENTED PROGRAMMING

WHAT IS ITERATION

Iteration is a process of repeating the same or similar actions until an endpoint or criteria is met. In Python we can create loops to iterate through a process, the two types of loops we will use here are while loops and for loops.

WATCH THIS VIDEO TO GUIDE YOU THROUGH THIS SECTION

SECTION 1 | FOR I IN RANGE LOOPS

A for loop will always loop at least once, it will loop for the number of times set by the programmer or until it reaches a pre-set criteria and breaks the loop. There are two types of for loops we will cover here, firstly a for i in range loop and a for i in loop.

In both types of loops the i is a variable, you can actually use any variable name instead of i but i is used as it is short for index. The i becomes a special variable when used in a for loop and the value of i changes/iterates each time the loop loops. The code below demonstrates how the value of i changes as the loop iterates.

for i in range (3):
   print (i)

By running the above for loop you will see that the output will be 0,1,2. Computer start to count at zero so remember that the count starts at 0 and loops 3 times so the count will never actually get to the value stated in the brackets (3).

You can also stipulate a start and finish number for example:

for i in range (2,5):
   print (i)

If you test the code above you will see the output will be 2,3,4.

PUTTING FOR LOOPS INTO GOOD USE

There are countless ways to use for i in range loops the below program demonstrate using the value of i in a for loop to output a given times tables in user friendly statements.

Notice in the code above that str(i + 1) was used.

Firstly, str was used because you cannot concatenate a number with a string so str tell python to treat (i + 1) as a string
Secondly, the value of (i + 1) was used because the starting value of i would have been 0 and in the last loop the value of i is 9 so by adding one it change the values used to 1 - 10. Alternatively the range could have been changed to for i in range (1,11)

Test the program and make some changes of your own.

PRINT AND COUNT

TASK
Create a program that
1: Asks the user to enter a number between 1 and 100, store the input in a variable
2: Ask the user to enter another number between 1 and 100, store the input in a variable
3: Produce a loop to count and print all of the numbers between the users first and second number
4: Count how many numbers that are between the users first and second number
5: When the loop has finish, in a user friendly statement output how many numbers there were.

EXAMPLE OUTPUT
Please enter a number between 1 and 100: 20
Please enter another number between 1 and 100: 30
Here are the numbers
21
22
23
24
25
26
27
28
29
There are 9 numbers between 20 and 30

SECTION 2 | FOR I IN LOOPS

For i in loops are used to loop through values and the i or variable name you use will be equal to each value in the loop rather than a number/count like it would be in a for i in range loop. If you want to loop through a value such as a string then you could use the following.

myString = ("King bob")
for i in (myString):
print (i)

Try the code above and you will see that the value of i becomes each letter of the string as the loop iterates. This is useful for inspecting each individual value and can be used for things such as password checks. The example below shows a for i in loop being used to check is a password contains a hash tag.

In the above code the break command is used to prevent the print statement running on every occasion that a hash tag is found in the string. Where possible using break statements should be avoided and this is further covered in later sections.

SECTION 3 | NESTED FOR LOOPS

The term nested for loop simply means putting a loop inside of a loop, this is common practice in programming so it is important to know the result of nesting. In previous tasks the value of i has been used to indicate the loop index. When nesting loops try to use different index names for each nested loop, for example use x or n on the nested loop as the index marker.

Looking at the nested loop below what do you think the outcome will be?

EXPAND THIS BOX TO SEE THE ANSWER

Tank 1
Fish 1
Fish 2
Fish 3
Fish 4
Tank 2
Fish 1
Fish 2
Fish 3
Fish 4

Each time the outer loop runs the inner loop will loop its entire range before returning to the outer loop. This is repeated until the outer loop have iterated it's entire range.

SECTION 4 | WHILE LOOPS

WATCH THIS VIDEO TO GUIDE YOU THROUGH THIS SECTION

While loops and for loops are both loops but have a clear difference in uses.

A for loop in your program will always iterate at least once.
A while loop will only iterate if a condition is met.
For loops are used when the number of iterations is known.
A while loop is used when the number of iterations is not known.

Take entering your password for example, some of the considerations could be:

If the user clicks 'sign in' the go to enter password
If the user enters the password correct then login is successful
If the use enters the password incorrect then display error message and ask the user to try again
If the user enters the password wrong 3 times then redirect user to password problems page

A for loop would not be suitable for this scenario because, only if the user clicks 'sign in' should the loop run. The loop needs to run up to 3 times if the password is incorrect but only once if the password was correct the first time, therefore we do not know how many times this loop will actually need to run, it could be zero times to three times. This is where a while loop is needed. While loops run while a condition is met.

Look at the while loop below, the value 'bob' is assigned to the variable loginName. A while loop is created and has the condition to run while the variable userName is not equal to the loginName. Try the program, you will see the loop will continue to loop until you type 'bob' when prompted for the name.

Only after the loop has finished will the program move to the next line to print the welcome message. The loop is checking if the userName is not equal to the loginName, it is checking if a condition is True of False.

Because while loops have a boolean argument to the condition of the loop it is good practice to use the boolean operators True and False within while loop creation. Using the example from above and changing to use True or False statements to start and end the loop can be seen below.

You can see that this program is much more complete than the first example and it uses False and True values to start and stop the loop as needed.

GREEN BOTTLES

The commonly known 'Green Bottle' rhyme goes:

"There are 10 green bottles hanging on the wall,
10 green bottles hanging on the wall,
and if 1 green bottle was to accidentally fall,
there would be 9 green bottles hanging on the wall"

This repeats until the are zero green bottles.

TASK
Create a program that
1: Asks the user how many bottles they want to start with
2: Output the rhyme from the starting point to zero green bottles
Note: You can use a for or while loop for this task

SECTION 5 | BREAKING A LOOP

One way to stop a while loop are to change the condition so the loop will not repeat, however the loop will not stop until it iterates back to the start of the loop. If you want to break the loop mid loop then you can use the break statement to break the loop immediately. This will work in both for and while loops as can be seen in the two example below.

The above code shows that an option for the user to exit is given, if the user chooses to exit by pressing 1, the break method is used and the loop will break.

SECTION 6 | PUTTING IT ALL TOGETHER

In this section we put all the skills together by creating two programs.

PASSWORD CHECKER
NUMBER GUESSING GAME

In these programs we use WHILE loops with IF statements and in the Number guessing Game we add a try and except method to check for input errors.

NUMBER GUESSING GAME WITH A WHILE LOOP

The video and code below show how to create a simple random number guessing game with a while loop.

NAVIGATION

GETTING STARTED | Downloading Python and creating Hello World
VARIABLES AND DATA TYPES | Storing data in variables
OPERATORS | Performing calculations and comparisons
SELECTION | Using IF statements
ITERATION | Using FOR Loops and WHILE Loops
ARRAYS | Getting started with Lists
FUNCTIONS AND PROCEDURES | Breaking your code into logical sections
FORMATTING DATA | Further data manipulation
ERROR CHECKING | Using Try and Except to catch user errors
OBJECT ORIENTED PROGRAMMING | Getting to grips with OOP