completed check digit algorithm and file writing

error.go

@@ -0,0 +1,10 @@
+package main
+
+import "log"
+
+func Error(err error) {
+	if err != nil {
+		log.Fatal(err)
+	}
+	return
+}

main.go

@@ -1,5 +1,89 @@
 package main
 
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
 func main() {
 
+	directory, err := os.Getwd()
+	Error(err)
+
+	fmt.Printf("What is the name of the file: ")
+	reader := bufio.NewReader(os.Stdin)
+	input, err := reader.ReadString('\n')
+	Error(err)
+
+	input = strings.TrimSpace(input)
+	file, err := os.Open(input)
+	Error(err)
+	var upcs []string
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		upcs = append(upcs, scanner.Text())
+	}
+	err = file.Close()
+	Error(err)
+	if scanner.Err() != nil {
+		fmt.Println(scanner.Err())
+	}
+
+	var gtins []string
+
+	for _, upc := range upcs {
+
+		upcLength := len(upc)
+
+		// Check Digit Formula
+		// Take all the digits in odd positions, add them and multiply by three
+		// Take all the digits in even positions (except for the last one) and add to the number you got above
+		// Divide that number by 10 and take the remainder
+		// If the remainder is not 0, subtract it from 10
+
+		oddNumber := 0
+		evenNumber := 0
+		var remainder int
+
+		for i, num := range upc {
+			if i%2 == 0 {
+				oddString, err := strconv.Atoi(string(num))
+				Error(err)
+				oddNumber += oddString
+			} else {
+				if upcLength-1 > i {
+					evenString, err := strconv.Atoi(string(num))
+					Error(err)
+					evenNumber += evenString
+				}
+			}
+		}
+
+		oddNumber = oddNumber * 3
+		remainder = (oddNumber + evenNumber) % 10
+		if remainder != 0 {
+			remainder = 10 - remainder
+		}
+		remainderString := strconv.Itoa(remainder)
+
+		gtins = append(gtins, upc+remainderString)
+	}
+
+	if _, err := os.Stat(directory + "/answer.txt"); err == nil {
+		e := os.Remove(directory + "/answer.txt")
+		Error(e)
+	}
+
+	answerFile, err := os.Create(directory + "/answer.txt")
+	Error(err)
+
+	defer answerFile.Close()
+
+	for _, gtin := range gtins {
+		_, err := answerFile.WriteString(gtin + "\n")
+		Error(err)
+	}
 }