*strings.Reader leaving target buffer blank

Question

I am attempting to pass an *strings.Reader from one function to another. The content of the reader is "Foo". This is the code that creates/passes the reader:

_, err = funcNum2(strings.NewReader("Foo"))

... and the code that reads from it...

buf := []byte{}
_, err := reader.Read(buf)
if err != nil {
    fmt.Printf("Error reading from reader: %v\n", err)
}

When I run the debugger and look at the value of reader in method2, I find that reader still contains "Foo". However, can you please help me understand why, when I try to read into buf, buf is empty?

Raed does not consume any bytes because the size of `buf` is zero. See https://go.dev/play/p/TXFj1Ns4tTX. — Charlie Tumahai, Aug 11 '22 at 03:35
Reading the [`io.Reader`](https://pkg.go.dev/io#Reader) documentation will illuminate what is going on here. — Hymns For Disco, Aug 11 '22 at 06:28

score 0 · Accepted Answer · answered Aug 11 '22 at 03:35

when assigning buf:=[]byte{}, a byte slice of size 0 is assigned. In order to read from buffer you can take size of the reader's buffer, reader.Size(), create a byte slice of that size. Write to it.

following code works

package main

import (
    "fmt"
    "strings"
)

func print_reader(read *strings.Reader) {
    buf := make([]byte, 4) //give read.Size() here
    n, err := read.Read(buf)
    fmt.Println(n, err, string(buf), read.Size())
}

func main() {
    rd := strings.NewReader("test")
    print_reader(rd)
}

*strings.Reader leaving target buffer blank

1 Answers1