Lesson 04 of 20

6 mins read

Python The Unified For Loop & Range Iteration

In Plain English

Go simplifies iteration by having only one loop keyword: for. It unifies 4 patterns: traditional 3-component index loops, condition-only while loops, infinite loops (for {}), and range loops over slices, maps, strings, and channels.

The Mental Model

"The Go for loop is a Swiss Army knife that replaces while, do-while, and foreach with a single versatile blade."

Deep Dive: How It Works

3-Component Loop: for i := 0; i < n; i++ { ... }

While Equivalent: for condition { ... }

Range Iteration: for index, value := range slice { ... }

Syntax Blueprint

for i := 0; i < 5; i++ {
    fmt.Println(i)
}

for idx, item := range items {
    fmt.Printf("%d: %s\n", idx, item)
}

Index iteration and collection range traversal using the for keyword.

Core Rules to Remember

Single Loop Keyword: No while or do-while keywords exist in Go—for handles every iteration pattern.

Range Value Copy: range returns a copy of each element; use index slice[i] to mutate in-place.

Blank Identifier: Ignore indices with for _, val := range items if index position is unneeded.

Common Beginner Traps & How to Fix Them

Mistake:Taking the memory address of the range iteration value pointer: &val.

Why it happens: In older Go versions, val was a single shared variable reused across iterations.

How to fix: Index the slice directly: &items[i].

Pro Tip: Use for key := range myMap when you only care about map keys without reading payload values.

Live Interactive Example

Hit Run Code to see it live

Range Over Slice Elements

Go 1.22 / Runtime

Output Console

Click "Run Code" to view the rendered output.

How it works: range returns 0-based index and element string copy for each item in the slice.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Calculate Sum of Squares with For Loop

Calculate the sum of squares for integers 1 through 4 (1^2 + 2^2 + 3^2 + 4^2 = 30).
Print "Sum of Squares: %d\n" with total sum.

Sandbox Output

Click "Run & Check" to test your solution.

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