Adding a count method to the package, so you can find out how many items in a slice satisfy and given condition

count.go

@@ -0,0 +1,13 @@
+package underscore
+
+// Count returns the number of elements in the slice that satisfy the predicate.
+// example: Count([]int{1,2,3,4,5}, func(n int) bool { return n%2 == 0 }) // 2
+func Count[T comparable](slice []T, predicate func(T) bool) int {
+	count := 0
+	for _, item := range slice {
+		if predicate(item) {
+			count++
+		}
+	}
+	return count
+}

count_test.go

@@ -0,0 +1,64 @@
+package underscore
+
+import (
+	"github.com/stretchr/testify/assert"
+	"strings"
+	"testing"
+)
+
+func Test_Count_Can_Count_Numbers(t *testing.T) {
+	numbers := Range(1, 100)
+	count := Count(numbers, func(n int) bool {
+		return n%2 == 0
+	})
+
+	assert.Equal(t, 50, count)
+}
+
+type People struct {
+	Name   string
+	Age    int
+	Gender string
+}
+
+func Test_Count_Can_Count__People(t *testing.T) {
+	people := []People{
+		{Name: "Andy", Age: 43, Gender: "M"},
+		{Name: "Fred", Age: 33, Gender: "M"},
+		{Name: "Jack", Age: 23, Gender: "M"},
+		{Name: "Jill", Age: 43, Gender: "F"},
+		{Name: "Anna", Age: 33, Gender: "F"},
+		{Name: "Arya", Age: 23, Gender: "F"},
+		{Name: "Jane", Age: 13, Gender: "F"},
+	}
+
+	a := Count(people, func(p People) bool {
+		return strings.HasPrefix(p.Name, "A")
+	})
+	assert.Equal(t, 3, a)
+
+	females := Count(people, func(p People) bool {
+		return p.Gender == "F"
+	})
+	assert.Equal(t, 4, females)
+
+	males := Count(people, func(p People) bool {
+		return p.Gender == "M"
+	})
+	assert.Equal(t, 3, males)
+
+	over30 := Count(people, func(p People) bool {
+		return p.Age > 30
+	})
+	assert.Equal(t, 4, over30)
+
+	under30 := Count(people, func(p People) bool {
+		return p.Age < 30
+	})
+	assert.Equal(t, 3, under30)
+
+	under20 := Count(people, func(p People) bool {
+		return p.Age < 20
+	})
+	assert.Equal(t, 1, under20)
+}