Go predicate
last modified April 11, 2024
In this article we show how to work with predicates in Golang.
$ go version go version go1.22.2 linux/amd64
We use Go version 1.22.2.
A predicate in general meaning is a statement about something that is either true or false. In programming, predicates represent single argument functions that return a boolean value.
Go simple predicate example
In the next example, we have a simple predicate.
package main
import (
"fmt"
)
func isPositive(val int) bool {
if val > 0 {
return true
} else {
return false
}
}
func main() {
vals := []int{-2, 0, 4, 3, 1, 9, 7, -3, -5, 6}
vals2 := []int{}
for _, val := range vals {
if isPositive(val) {
vals2 = append(vals2, val)
}
}
fmt.Println(vals)
fmt.Println(vals2)
}
The example takes all positive numbers from a slice into another slice.
func isPositive(val int) bool {
if val > 0 {
return true
} else {
return false
}
}
The isPositive function is a predicate. It returns a boolean value
indicating whether a value is positive.
for _, val := range vals {
if isPositive(val) {
vals2 = append(vals2, val)
}
}
We go through the slice of values and apply the predicat on each of the elements.
$ go run main.go [-2 0 4 3 1 9 7 -3 -5 6] [4 3 1 9 7 6]
Go any function
The any (also called exists) function iterates over elements of a collection and returns true if the predicate is valid for at least one element.
package main
import (
"fmt"
"strings"
)
func any(data []string, f func(string) bool) bool {
for _, v := range data {
if f(v) {
return true
}
}
return false
}
func main() {
words := []string{"falcon", "war", "sun", "cup", "rock"}
w := "e"
r := any(words, func(s string) bool {
return strings.Contains(s, w)
})
if r {
fmt.Printf("The slice contains an element with %s\n", w)
} else {
fmt.Printf("The slice does not contain an element with %s\n", w)
}
}
In the example we use the any function to check if at least
one word of the words slice has letter 'w'.
func any(data []string, f func(string) bool) bool {
for _, v := range data {
if f(v) {
return true
}
}
return false
}
The any is a function that takes a slice and a predicate as
parameters.
r := any(words, func(s string) bool {
return strings.Contains(s, w)
})
The predicate is an anonymous function which utilizes the
strings.Contains function.
$ go run main.go The slice does not contain an element with e
Go all function
The all function iterates over elements of a collection and returns true if the predicate is valid for all elements.
package main
import "fmt"
func all(data []string, f func(string) bool) bool {
for _, e := range data {
if !f(e) {
return false
}
}
return true
}
func main() {
words := []string{"war", "water", "cup", "tree", "storm"}
n := 3
res := all(words, func(s string) bool {
return len(s) >= n
})
if res {
fmt.Printf("All words have %d+ characters", n)
} else {
fmt.Printf("It is not true that all words have %d+ characters", n)
}
}
In the example we use the all function to check if all words have
n or more letters.
$ go run main.go All words have 3+ characters
Go filter function
The filter function returns a new collection containing only the elements of the collection for which the given predicate returns true.
package main
import (
"fmt"
"strings"
)
func filter(data []string, f func(string) bool) []string {
fltd := make([]string, 0)
for _, e := range data {
if f(e) {
fltd = append(fltd, e)
}
}
return fltd
}
func main() {
words := []string{"war", "water", "cup", "tree", "storm"}
p := "w"
res := filter(words, func(s string) bool {
return strings.HasPrefix(s, p)
})
fmt.Println(res)
}
The example uses the filter function to find all words that start
with 'w'.
func filter(data []string, f func(string) bool) []string {
fltd := make([]string, 0)
for _, e := range data {
if f(e) {
fltd = append(fltd, e)
}
}
return fltd
}
The filter function takes the collection and the predicate as
parameters. It creates a new fltd slice into which we append all
the elements that satisfy the passed predicate function.
$ go run main.go [war water]
Source
The Go Programming Language Specification
In this article we have covered predicates in Golang.
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