Adding program to check if the number is an Armstrong number (#442)

* Adding program to check if the number is an armstrong number

* Updated Documentation in README.md

* Update loop in isarmstrong

Co-authored-by: github-action <${GITHUB_ACTOR}@users.noreply.github.com>

Author: kavithajm

README.md

@@ -46,6 +46,16 @@ Read our [Contribution Guidelines](CONTRIBUTING.md) before you contribute.
 1. [`Result`](./strings/ahocorasick/ahocorasick.go#L9): No description provided.
 
 
+---
+</details><details>
+	<summary> <strong> armstrong </strong> </summary>	
+
+---
+
+##### Functions:
+
+1. [`IsArmstrong`](./math/armstrong/isarmstrong.go#L14): No description provided.
+
 ---
 </details><details>
 	<summary> <strong> avl </strong> </summary>	

math/armstrong/isarmstrong.go

@@ -0,0 +1,32 @@
+// isarmstrong.go
+// description: Checks if the given number is armstrong number or not
+// details: An Armstrong number is a n-digit number that is equal to the sum of each of its digits taken to the nth power.
+// ref: https://mathlair.allfunandgames.ca/armstrong.php
+// author: Kavitha J
+
+package armstrong
+
+import (
+	"math"
+	"strconv"
+)
+
+func IsArmstrong(number int) bool {
+	var rightMost int
+	var sum int = 0
+	var tempNum int = number
+
+	// to get the number of digits in the number
+	length := float64(len(strconv.Itoa(number)))
+
+	// get the right most digit and break the loop once all digits are iterated
+	for tempNum > 0 {
+		rightMost = tempNum % 10
+		sum += int(math.Pow(float64(rightMost), length))
+
+		// update the input digit minus the processed rightMost
+		tempNum /= 10
+	}
+
+	return number == sum
+}

math/armstrong/isarmstrong_test.go

@@ -0,0 +1,53 @@
+// isarmstrong_test.go
+
+package armstrong
+
+import "testing"
+
+var testCases = []struct {
+	name     string // test description
+	input    int    // user input
+	expected bool   // expected return
+}{
+	{
+		"negative number: Not an armstrong number",
+		-140,
+		false,
+	},
+	{
+		"positive number: Not an armstrong number",
+		23,
+		false,
+	},
+	{
+		"smallest armstrong number",
+		0,
+		true,
+	},
+	{
+		"smallest armstrong number with more than one digit",
+		153,
+		true,
+	},
+	{
+		"random armstrong number",
+		407,
+		true,
+	},
+	{
+		"random armstrong number",
+		9474,
+		true,
+	},
+}
+
+func TestIsArmstrong(t *testing.T) {
+	for _, test := range testCases {
+		t.Run(test.name, func(t *testing.T) {
+			funcResult := IsArmstrong(test.input)
+			if test.expected != funcResult {
+				t.Errorf("Expected answer '%t' for the number '%d' but answer given was %t", test.expected, test.input, funcResult)
+			}
+		})
+	}
+}