The frequency of occurrence can be counted as follows.
The rest is sort, but after the postscript I tried to think about by myself.

https://play.golang.org/p/sjDV88hRXPr

package main
import (
    "bufio"
    "bytes"
    "fmt"
    "strings"
)
var source = `
I have a pen.
I have an apple.
Apple pen!
I have a pen.
I have a pineapple.
Pineapple pen!
`
func main () {
    count: = map [string] int {}
    scanner: = bufio.NewScanner (bytes.NewBuffer ([] byte (source)))
    scanner.Split (bufio.ScanWords)
    for scanner.Scan () {
        word: = strings.TrimRight (scanner.Text (), ".!")
        count [word] + = 1
    }
    fmt.Println (count)
}

Output

map [Apple: 1 I: 4 Pineapple: 1 a: 3 an: 1 apple: 1 have: 4 pen: 4 pineapple: 1]