Add strings.Builder example in for's benchmark section (#838)

* Add notes about using and benchmarking strings.Builder

* Add new version of for example

* Update roman numerals tutorial with backlink

* Fix phrasing

for/v3/repeat.go

@@ -0,0 +1,14 @@
+package iteration
+
+import "strings"
+
+const repeatCount = 5
+
+// Repeat returns character repeated 5 times.
+func Repeat(character string) string {
+	var repeated strings.Builder
+	for i := 0; i < repeatCount; i++ {
+		repeated.WriteString(character)
+	}
+	return repeated.String()
+}

for/v3/repeat_test.go

@@ -0,0 +1,12 @@
+package iteration
+
+import "testing"
+
+func TestRepeat(t *testing.T) {
+	repeated := Repeat("a")
+	expected := "aaaaa"
+
+	if repeated != expected {
+		t.Errorf("expected %q but got %q", expected, repeated)
+	}
+}

iteration.md

@@ -127,6 +127,41 @@ What `136 ns/op` means is our function takes on average 136 nanoseconds to run \
 
 **Note:** Sometimes, Go can optimize your benchmarks in a way that makes them inaccurate, such as eliminating the function being benchmarked. Check your benchmarks to see if the values make sense. If they seem overly optimized, you can follow the strategies in this **[blog post](https://dave.cheney.net/2013/06/30/how-to-write-benchmarks-in-go)**.
 
+Strings in Go are immutable, meaning every concatenation, such as in our `Repeat` function, involves copying memory to accommodate the new string. This impacts performance, particularly during heavy string concatenation.
+
+The standard library provides the `strings.Builder`[stringsBuilder] type which minimizes memory copying.
+It implements a `WriteString` method which we can use to concatenate strings:
+
+```go
+const repeatCount = 5
+
+func Repeat(character string) string {
+	var repeated strings.Builder
+	for i := 0; i < repeatCount; i++ {
+		repeated.WriteString(character)
+	}
+	return repeated.String()
+}
+```
+
+**Note**: We have to call the `String` method to retrieve the final result.
+
+We can use `BenchmarkRepeat` to confirm that `strings.Builder` significantly improves performance.
+Run `go test -bench=. -benchmem`:
+
+```text
+goos: darwin
+goarch: amd64
+pkg: github.com/quii/learn-go-with-tests/for/v4
+10000000           25.70 ns/op           8 B/op           1 allocs/op
+PASS
+```
+
+The `-benchmem` flag reports information about memory allocations:
+
+* `B/op`: the number of bytes allocated per iteration
+* `allocs/op`: the number of memory allocations per iteration
+
 ## Practice exercises
 
 * Change the test so a caller can specify how many times the character is repeated and then fix the code
@@ -138,3 +173,5 @@ What `136 ns/op` means is our function takes on average 136 nanoseconds to run \
 * More TDD practice
 * Learned `for`
 * Learned how to write benchmarks
+
+[stringsBuilder]: https://pkg.go.dev/strings#Builder

roman-numerals.md

@@ -208,7 +208,8 @@ func ConvertToRoman(arabic int) string {
 }
 ```
 
-You may not have used [`strings.Builder`](https://golang.org/pkg/strings/#Builder) before
+You might remember [`strings.Builder`](https://golang.org/pkg/strings/#Builder) from our discussion
+about [benchmarking](iteration.md#benchmarking)
 
 > A Builder is used to efficiently build a string using Write methods. It minimizes memory copying.