How check whether a file contains a string or not? - go

Question

I've tried to search on Google for pattern matching function between file and string but I could not find it. I've also tried to use strings.Contains(), but it gives wrong result in large input file.
Is there any function in Go for searching string in some file?
If no, is there another way to resolve this problem?
Here is my code:
package main
import (
"bufio"
"fmt"
"io/ioutil"
"os"
"strings"
)
func main() {
reader := bufio.NewReader(os.Stdin)
fmt.Print("Enter text: ")
text, _ := reader.ReadString('\n')
// read the whole file at once
b, err := ioutil.ReadFile("input.txt")
if err != nil {
panic(err)
}
s := string(b)
length := len(s)
//check whether s contains substring text
fmt.Println(strings.Contains(s, text))
}

Answer 1

If I read your question correctly you want to read from a file and determine if a string entered at the command line is in that file... And I think the problem that you are seeing has to do with the string delimiter, the reader.ReadString('\n') bit, and not string.Contains().
In my opinion it will be a little bit easier to make what you want work with fmt.Scanln; it will simplify things and will return a result that I'm pretty sure is what you want. Try this variation of your code:
package main
import (
"fmt"
"io/ioutil"
"strings"
)
func main() {
var text string
fmt.Print("Enter text: ")
// get the sub string to search from the user
fmt.Scanln(&text)
// read the whole file at once
b, err := ioutil.ReadFile("input.txt")
if err != nil {
panic(err)
}
s := string(b)
// //check whether s contains substring text
fmt.Println(strings.Contains(s, text))
}

Answer 2

I am just adding a flag to use command line arguments. If nothing is passed it will prompt you :).
package main
import (
"flag"
"fmt"
"io/ioutil"
"strings"
)
//Usage go run filename -text=dataYouAreLookingfor
//if looking for Nissan in file the command will be
// go run filename -text=Nissan
func main() {
var text string
// use it as cmdline argument
textArg := flag.String("text", "", "Text to search for")
flag.Parse()
// if cmdline arg was not passed ask
if fmt.Sprintf("%s", *textArg) == "" {
fmt.Print("Enter text: ")
// get the sub string to search from the user
fmt.Scanln(&text)
} else {
text = fmt.Sprintf("%s", *textArg)
}
// read the whole file at once
b, err := ioutil.ReadFile("input.txt")
if err != nil {
panic(err)
}
s := string(b)
// //check whether s contains substring text
fmt.Println(strings.Contains(s, text))
}