Python Quick Guide - Step 2 Control Flow (3) - Loops (1): for Loops

This is a continuation of “Step 2 Control Flow.”

2.2. Loops

Loops are control flow structures that allow you to execute the same process multiple times.
Python has two main types of loops:

1. for loop: Repeats a process for each element in a list or other iterable
2. while loop: Repeats a process as long as a condition is True

2.2.1. for Loops

Basics of for Loops

The for loop sequentially executes a process for each element in an iterable object, such as a list.

• In the for fruit in fruits line, each element in the fruits list is assigned to the fruit variable in sequence
• The indented block below is executed repeatedly for each element in fruits

flowchart TD
    START[Start] --> INIT["fruits = ['apple', 'banana', 'orange']"]
    INIT --> FOR{"for fruit in fruits"}
    FOR -->|No more elements| END["Loop ended"]
    FOR -->|Next element exists| LOOP_BODY["Loop process\n(print(fruit))"]
    LOOP_BODY --> FOR

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What are Iterables?

Objects that can be used in looping constructs like for loops are called iterables.
In Python, the following objects are treated as iterables:

• Lists: ["apple", "banana", "orange"]
• Tuples: (1, 2, 3)
• Sets: {1, 2, 3}
• Dictionaries: {"name": "Alice", "age": 25}
• Strings: "Python"
• Return values of the range function: range(5)

Lists, Tuples, and Sets

Dictionaries

Dictionaries are also iterables. When used in a for loop, they return the keys.
The following dictionary methods also return iterable objects:

• .keys() method: Returns the list of keys
• .values() method: Returns the list of values
• .items() method: Returns the list of key-value pairs

• In the .items() method example above, unpacking is used to break down the key-value pair into separate key and value variables.

Strings

Strings are also a type of iterable object.
When used in a for loop, they allow processing one character at a time.

The range Function

The range function generates a sequence of numbers in a specified range.
The range function has multiple usage patterns similar to list slicing:

• range(stop): Generate integers from 0 to (stop - 1) in sequence

• range(start, stop): Generate integers from start to (stop - 1) in sequence

• range(start, stop, step): Generate integers from start to (stop - 1) with the specified step interval

Like slicing, specifying a negative number for step generates numbers in reverse order.

1 x 1 = 1
1 x 2 = 2
...
9 x 9 = 81

Loop Control Statements

break: Terminating a Loop

The break statement allows you to terminate a loop early and exit from it.

In this example, the fruits list is processed sequentially, but when “banana” is found, the loop is terminated by the break statement. Therefore, “orange” and subsequent elements are not processed.

flowchart TD
    START[Start] --> INIT["fruits = ['apple', 'banana', 'orange', 'grape', 'melon']"]
    INIT --> FOR{"for fruit in fruits"}

    FOR -->|No more elements| END["Loop ended"]
    FOR -->|Next element exists| ELEMENT_PROCESS

    subgraph ELEMENT_PROCESS[Loop Process]
        LOOP_START["Loop start"] --> CHECK{"if fruit == 'banana'"}
        CHECK -->|No| NEXT["Proceed to next process"]
        CHECK -->|Yes| BREAK["break\n(terminate loop)"]
    end

    NEXT --> FOR
    BREAK --> END

    style BREAK fill:#ffeb3b,stroke:#333,stroke-width:2px

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    linkStyle 3 stroke:green,stroke-width:2px;
    linkStyle 5 stroke:red,stroke-width:2px;
    linkStyle 6 stroke:green,stroke-width:2px;

continue: Skipping Loop Iterations

The continue statement is a control flow statement used to skip the remaining part of the current loop iteration and move to the next iteration.
Unlike break which terminates the entire loop, continue only skips the current iteration.

In this example, each number in the list is checked, and if it’s even, the continue statement skips further processing. As a result, only odd numbers are displayed.

flowchart TD
    START[Start] --> INIT["numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]"]
    INIT --> FOR{"for num in numbers"}

    FOR -->|No more elements| END["Loop ended"]
    FOR -->|Next element exists| ELEMENT_PROCESS

    subgraph ELEMENT_PROCESS[Loop Process]
        LOOP_START["Loop start"] --> CHECK{"if num % 2 == 0"}
        CHECK -->|No| NEXT["Proceed to next process\n(print(f'Number: {num}'))"]
        CHECK -->|Yes| CONTINUE["continue\n(to next iteration)"]
    end

    NEXT --> FOR
    CONTINUE --> FOR

    style CONTINUE fill:#80deea,stroke:#333,stroke-width:2px

    linkStyle 2 stroke:red,stroke-width:2px;
    linkStyle 3 stroke:green,stroke-width:2px;
    linkStyle 5 stroke:red,stroke-width:2px;
    linkStyle 6 stroke:green,stroke-width:2px;

Difference between break and continue

break and continue in Nested Loops

When loops are nested, break and continue statements behave as follows:

• A break statement terminates only the innermost loop containing it
• A continue statement skips only the current iteration of the innermost loop containing it

Advanced for Loops

enumerate Function: Loops with Indices

The enumerate function provides both the index (position) and value for each element in an iterable.

You can also change the starting value of the index:

zip Function: Processing Multiple Iterables Simultaneously

The zip function is useful for processing multiple iterables simultaneously.
(Unlike nested for loops, it’s used when you only need pairs at the same position)

Note that zip stops when the shortest iterable is exhausted.

for-else Statement

You can add an else clause to a for loop. The else block is executed when the loop completes normally (i.e., was not terminated by a break statement).

As shown above, the for-else statement allows you to concisely write code to search for an element in a list and handle the case when it’s not found.

Summary of for Loops