for Loops & range

Go is refreshingly minimal: there is exactly one loop keyword, for , and it covers every job — counted loops, while-style loops, infinite loops, and (with range ) walking over slices, maps and strings. Master these forms and you can iterate over anything.

Learn for Loops & range in our free Go course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick recall.

Part of the free Go course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn in This Lesson

1️⃣ One Keyword, Three Shapes

Every for in Go is a variation on the same keyword. The full form has three parts separated by semicolons: an init statement, a condition , and a post statement. Drop the init and post and you have a while loop. Drop everything and you have an infinite loop you exit with break .

2️⃣ break, continue & Nesting

break leaves the loop immediately; continue skips the rest of the current iteration and jumps to the next. Loops nest freely — an outer loop for rows, an inner loop for columns. (Go also supports labelled breaks to exit an outer loop from inside an inner one, which you'll meet later.)

3️⃣ range — Walk Any Collection

range is how you iterate collections idiomatically. Over a slice it yields the index and the value ; over a map it yields the key and value (in random order, by design — never rely on map order); over a string it yields the byte index and the rune . When you don't need the index or key, discard it with the blank identifier _ .

Your turn — make a countdown with a three-part loop that decrements.

Now find the best score with a range loop. You don't need the index here.

🧩 Reorder Challenge

These lines sum a slice with range , but they're scrambled. Put them in order so the program prints 6 (assume a func main wraps them).

• nums := []int{1, 2, 3}
• for _, n := range nums {
• fmt.Println(total)

Why: you must create the slice and the accumulator ( total := 0 ) before the loop can use them. The for header opens the block, the body total += n runs once per element, the brace closes the loop, and only then can you print the final total: 1+2+3 = 6.

Pro Tips

• 💡 Prefer range over manual indexing when walking a whole collection — it's clearer and avoids off-by-one bugs.
• 💡 Never depend on map iteration order. Go randomizes it on purpose; sort the keys first if you need a stable order.
• 💡 Range over a string for characters , not s[i] — indexing gives raw bytes, which breaks on non-ASCII text.
• 💡 Use a labelled break (e.g. break outer ) to escape nested loops cleanly instead of a flag variable.

Common Errors (and the fix)

• Infinite loop that never ends — you forgot to advance the counter (the i++ post-statement) or your condition can never become false. Double-check the exit condition.
• "i declared and not used" in a range loop — you took the index but never used it. Replace it with _ .
• "cannot range over int" — older Go can't range over a plain number. Range over a slice/map/string, or use a three-part for . (Go 1.22+ does allow for i := range 5 .)
• Off-by-one — i <= len(s) reads one past the end. Use i < len(s) , or better, range.

📋 Quick Reference

Goal

Syntax

Count 0..n-1

for i := 0; i < n; i++ { }

While

for cond { }

Infinite

for { ... break }

Slice items

for i, v := range s { }

Values only

for _, v := range s { }

Map entries

for k, v := range m { }

🧠 Quick Recall

Predict the output before you reveal the answer.

012 — i runs 0, 1, 2 and stops when i reaches 3 because the condition i < 3 is then false.

s := []string{"a","b"}; for i, v := range s { fmt.Print(i, v) }

0 a1 b — range yields index 0 with "a", then index 1 with "b". (Println-style spacing: each pair prints with a space between i and v.)

for i := 1; i <= 4; i++ { if i == 2 { continue }; fmt.Print(i) }

134 — when i is 2, continue skips the print and moves to the next iteration, so 2 is omitted.

Frequently Asked Questions

Go doesn't have one — a for with only a condition is a while loop: for x < 10 { } . One keyword, fewer surprises.

Q: Why does my map print in a different order each run?

Go deliberately randomizes map iteration order so you never accidentally rely on it. If you need a fixed order, collect the keys into a slice and sort them first.

Q: Can I loop a fixed number of times without a counter variable?

In Go 1.22+ yes: for range 5 { } runs five times. On older versions use a three-part loop and ignore the counter.

Mini-Challenge: Count Vowels

Range over the string, inspect each rune, and tally the vowels. Write it yourself and match the example output.

🎉 Lesson Complete!

• ✅ for is Go's only loop — counted, while, and infinite forms
• ✅ break exits; continue skips to the next iteration
• ✅ range yields index+value (slices), key+value (maps), index+rune (strings)
• ✅ Discard what you don't need with the blank identifier _
• ✅ Never rely on map iteration order — it's randomized
• ✅ Next lesson: Arrays — fixed-size sequences and the foundation of slices

Practice quiz

Continue this course

• Previous: Control Flow (if, switch)
• Next: Arrays