Index out of range trying to add to slice - arrays

Question

var bar string
var i int
var a []string
for foo, _ := reader.NextWord(); foo != bar; foo, _ = reader.NextWord() {
bar = foo
fmt.Print(foo)
a[i] = foo
i++
}
Shouldn't this be creating a nil slice and then adding the value to the appropriate place? I keep getting index out of range so I assume it's not adding to a[i]...
Checking length first with
if len(a) > 0 {
a[i] = foo
}
seems to help, but not getting the results I expected. I'll keep playing around.
Update: I did end up using append... I meant to update this thread but thank you both.
package main
import (
"fmt"
"log"
"os"
"strings"
"github.com/steven-ferrer/gonsole"
)
func main() {
file, err := os.Open("test.txt")
if err != nil {
log.Println(err)
}
defer file.Close()
reader := gonsole.NewReader(file)
// cycle through
var bar string
var i int
var quality []string = make([]string, 0)
var tempName []string = make([]string, 0)
var name []string = make([]string, 0)
for foo, _ := reader.NextWord(); foo != bar; foo, _ = reader.NextWord() {
bar = foo
if strings.Contains(foo, "(normal)") {
quality = append(quality, "normal")
for state := 0; state < 1; foo, _ = reader.NextWord() {
if foo == "|" {
state = 1
}
tempName = append(tempName, foo)
}
nameString := strings.Join(tempName, "")
name = append(name, nameString)
} else if strings.Contains(foo, "(unique)") {
quality = append(quality, "unique")
for state := 0; state < 1; foo, _ = reader.NextWord() {
if foo == "|" {
state = 1
}
tempName = append(tempName, foo)
}
nameString := strings.Join(tempName, "")
name = append(name, nameString)
} else if strings.Contains(foo, "(set)") {
quality = append(quality, "set")
for state := 0; state < 1; foo, _ = reader.NextWord() {
if foo == "|" {
state = 1
}
tempName = append(tempName, foo)
}
nameString := strings.Join(tempName, "")
name = append(name, nameString)
}
if tempName != nil {
tempName = nil // clear tempName
}
i++
}

Answer 1

Your slice a needs to be allocated utilizing make.
var a []string = make([]string, n)
where n is the size of the slice.

Answer 2

Removing some of the context-specific parts of your code, you should be using the append method against a dynamic-length slice.
package main
import (
"fmt"
"strings"
)
func main() {
book := "Lorem ipsum dolor sit amet"
var words []string
for _, word := range strings.Split(book, " ") {
words = append(words, word)
}
fmt.Printf("%+v\n", words)
}
https://play.golang.org/p/LMejsrmIGb9
If you know the number of values up front, the same can be achieved for a fixed length slice by using words := make([]string, 5), but I doubt this is what you want in this case.
The reason your code is causing you errors is because your slice isn't initialized at any given length, so your indexes don't yet exist. Generally when working with a slice, append is the method you want.
Opposite to this, when working with existing slices (ie, rangeing an slice), the reason you're able to set the values using indexes is because the index has already been allocated.