Introduction to For Loops

Much like how a while loop will repeat its own block of code, For Loop will also repeat its own block of code.

• For loops don’t depend on a conditional statement being True.

• For loops will repeat a finite amount of time dependent on the size/length of the iterating sequence.

# For Loop Format

for __iterating_variable__ in __iterating__sequence__:

    your code here

# end of for loop

• A for loop is started by using the keyword: for

• __iterating_variable__ is a label that you create to represent the individual items in the sequence

• No matter the iterating variable, the code within the for loop will execute

• The code within the for loop executes repeatedly until the iterating variable runs out of items to represent from the sequence

What is a Sequence?

Sequence is a collection of items or things.

• The sequence can be ordered(least to greatest) or unordered(shuffled)

• These sequences will have a beginning and an end; therefore, they are finite

• We can determine the size of the sequences

Examples:

• List of integers: [2,3,5,7,11,13]

• A string: "Sequence" is a sequence of 8 characters

• In Python, a data type that is classified as iterable will be able to iterate within a for loop

  • Examples: Lists, Strings, Tuples, Dictionary

Code Example 1

Code

# Using a for loop with a string
# Purpose: Output individual letters of a word.

word = 'Hello!'

for character in word:
    print('Current Character:', character)

Output

Current Character: H
Current Character: e
Current Character: l
Current Character: l
Current Character: o
Current Character: !

Explanation:

• The label character is used as a label to represent each single item from the iterating sequence for each iteration

• The variable word is a string that has the characters: Hello!

• Since character represents individual items, we were able to grab the items in the sequence in order and one-by-one

Code Example 2

Code

# Using a for loop, but ignoring the iterating variable
# Purpose: Repeat a message

word = 'Hello!'

for character in word:
    print('Goodbye!')

Output

Goodbye!
Goodbye!
Goodbye!
Goodbye!
Goodbye!
Goodbye!

Explanation:

• In Code Example 2, the iterating variable character is not used within the for loop’s code block

• The string of Goodbye! is outputted 6 times.

• There are 6 outputs because variable: word has 6 items in the sequence