An off-by-one error is a boundary mistake in which a loop or index begins, ends, or counts one position incorrectly. In Go, table-driven tests expose these errors quickly by comparing generated output with an exact expected result. section 6.3.1
Check both boundaries when reviewing a loop:
- Verify the initial index matches the domain being modeled; starting at 0 produces a different sequence from one beginning at 1. section 6.3.1
- Verify whether the target value is included; a condition such as i < n excludes n. section 6.3.1
- Add representative boundary cases to a table-driven test and rerun go test after correcting the implementation. section 6.3.1
Fizz Buzz boundary failure
A Fizz Buzz loop using `for i := int64(0); i < n; i++` starts with 0 and stops before n. For an expected sequence from 1 through 37, the test fails immediately; correcting the start, endpoint, and formatting produces a passing `go test` result. section 6.3.1
Listing 6.12 Basic Fizz Buzz
package main
import (
"fmt"
"strings"
"strconv"
)
func main() {
var input string
fmt.Println("Enter a number for fizzbuzz")
fmt.Scanln(&input) #1
numInput, err := strconv.ParseInt(input, 10, 16)
if err != nil {
panic("that's not a number!")
}
result := fizzbuzz(numInput) #2
fmt.Println("result:", result)
}
func fizzbuzz(n int64) string {
var fizzbuzzes []string
for i := int64(0); i < n; i++ { #3
v := ""
isThree := i % 3 == 0
isFive := i % 5 == 0
if isThree && isFive {
v = "Fizz Buzz"
} else if isThree {
v = "Fizz"
} else if isFive {
v = "Buzz "
} else {
v = fmt.Sprintf("%d", i)
}
fizzbuzzes = append(fizzbuzzes, v)
}
return strings.Join(fizzbuzzes, " ")
}
Inspect the complete Fizz Buzz implementation to see the loop boundaries that create the error. section 6.3.1
