Parse number string digits - go

Question

I am trying to calculate the multiplication result of a few digits which are part of a long digits string. Here is my code:
package main
import (
"fmt"
"strconv"
)
func main() {
inputNum := "73167176531330624919225119"
mult := getMult(3, inputNum)
fmt.Printf("Mult = %d", mult)
}
func getMult(startIndex int, inputNum string) int {
mult := 0
for i := 0; i < 10; i++ {
digit, err := strconv.Atoi(string(inputNum[startIndex+i]))
if err != nil {
mult *= int(digit)
} else {
fmt.Printf("Error converting %s to int : %s\n", string(inputNum[startIndex+i]), err.Error())
}
}
return mult
}
The result I want to get is 6*7*1*7*6*5*3*1*3*3 = 238140
But I an getting a runtime error:
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x20 pc=0x40130e]
goroutine 1 [running]:
main.getMult(0x3, 0x534d40, 0x1a, 0x4d2701)
test.go:25 +0x17e
main.main()
test.go:10 +0x55
exit status 2

Answer 1

There are a couple problems...
First, you need to start mult at 1, otherwise you will just continually multiply by 0 and always get 0.
Secondly you have the logic for your err check flipped. It should be if err == nil
This seems to do what you want:
func getMult(startIndex int, inputNum string) int {
mult := 1
for i := 0; i < 10; i++ {
digit, err := strconv.Atoi(string(inputNum[startIndex+i]))
if err == nil {
mult *= int(digit)
} else {
fmt.Println(err)
}
}
return mult
}
The error you were getting was because you were trying to print err.Error() when err itself was nil (due to the logical flip of != and ==)

Answer 2

your code will work with fixing these two typos:
change mult := 0 to mult := 1
and change err != nil to err == nil like this:
package main
import (
"fmt"
"strconv"
)
func main() {
inputNum := "73167176531330624919225119"
mult := getMult(3, inputNum)
fmt.Printf("Mult = %d", mult)
}
func getMult(startIndex int, inputNum string) int {
mult := 1
for i := 0; i < 10; i++ {
digit, err := strconv.Atoi(string(inputNum[startIndex+i]))
if err == nil {
mult *= int(digit)
} else {
fmt.Printf("Error converting %s to int : %s\n", string(inputNum[startIndex+i]), err.Error())
}
}
return mult
}
also you may use inputNum[3:13] to get new string from index 3 with length 10,
and you may use int(v - '0') to convert one character to integer number,
then use for range like this:
package main
import "fmt"
func main() {
inputNum := "73167176531330624919225119"
mult := getMult(inputNum[3:13])
fmt.Printf("Mult = %d \n", mult) // Mult = 238140
}
func getMult(str string) int {
m := 1
for _, v := range str {
if v >= '0' && v <= '9' {
m *= int(v - '0')
} else {
fmt.Printf("Error converting %q to int\n", v)
break
}
}
return m
}
output:
Mult = 238140