Convert byte slice to io.Reader in Golang [SOLVED]

In this post, we’ll examine how to translate a byte slice into an io.Reader in Golang. In Go, a byte slice is a dynamic array (slice) of bytes, which can represent any type of data in memory, including file contents, data sent over the network, etc. It might be advantageous to convert a byte slice into an io.Reader because it has the Read method so it enables us to combine it with other elements of our application and the Go standard library.

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Example 1: Convert a byte slice to io.Reader

Let’s look at an example of how to convert a byte slice to an io. Read in Golang. We can use the NewReader() function to convert a byte slice to bytes.Reader which implements the io.Reader interface. To learn a little bit more about interfaces in Go, readEffective Go: Interfaces and Types.

func NewReader(b []byte) *Reader: NewReader returns a new Reader reading from b.

package main

import (
    "bytes"
    "fmt"
    "log"
)

func main() {
    testData := []byte("this is some random data in GoLinuxCloud example!")

    // byte slice to bytes.Reader, which implements the io.Reader interface
    reader := bytes.NewReader(testData)

    // read only 10 byte from our io.Reader
    buf := make([]byte, 10)

    if _, err := reader.Read(buf); err != nil {
        log.Fatal(err)
    }
    fmt.Println(string(buf))
}

Output:

this is so

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Example 2: Convert an io.Reader to a byte slice

Let’s see an example of how we can convert an io.Reader to a byte slice in Go using the Buffer type:

type Buffer: A Buffer is a variable-sized buffer of bytes with Read and Write methods. The zero value for Buffer is an empty buffer ready to use.

package main

import (
    "bytes"
    "io"
    "log"
    "strings"
)

func main() {
    reader := strings.NewReader("this is some random data in GoLinuxCloud example!")

    // a bytes.Buffer and read from io.Reader
    buf := new(bytes.Buffer)
    buf.ReadFrom(reader)

    // get a byte slice from bytes.Buffer
    data := buf.Bytes()

    // only read the first 10 bytes
    log.Println(string(data[:10]))
}

Output:

2022/12/22 20:13:06 this is so

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Example 3: Convert an io.Reader to a string

Method-1: Using bytes.Buffer

We also can convert an io.Reader() to a string as the code shown below:

package main

import (
    "bytes"
    "io"
    "log"
    "strings"
)

func main() {
    reader := strings.NewReader("this is some random data in GoLinuxCloud example!")

    // a bytes.Buffer and read from io.Reader
    buf := new(bytes.Buffer)
    buf.ReadFrom(reader)

    // return a string
    data := buf.String()

    // print the string
    log.Println(data)
}

Output:

2022/12/22 20:18:26 this is some random data in GoLinuxCloud example!

Method-2: Using strings.Builder

Another way to convert to string is using the strings.Builder type:

type Builder: A Builder is used to efficiently build a string using Write methods. It minimizes memory copying. The zero value is ready to use. Do not copy a non-zero Builder.

package main

import (
    "fmt"
    "io"
    "strings"
)

func main() {
    reader := strings.NewReader("this is some random data in GoLinuxCloud example!")

    // byte slice to bytes.Reader, which implements the io.Reader interface
    buf := new(strings.Builder)
    _, err := io.Copy(buf, reader)
    if err != nil {
        fmt.Println(err)
    }
    // check errors
    fmt.Println(buf.String())
}

Output:

this is some random data in GoLinuxCloud example!

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Summary

In this tutorial, we can come to the conclusion that: To get a type that implementsio.Readerfrom a[]byte slice, we can use bytes.NewReaderin thebytes package. We can use the Buffer type to convert from an io.Reader() to a byte slice or a string.

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References

https://pkg.go.dev/bytes#NewReader
Convert byte slice to io.Reader - Stack Overflow
Is there a way in go to convert a []byte slice to an io.Reader?