What is the golang Fscanf function?

The Fscanf function in the Go programming language is used to read data from a source of your choosing, format it into a string, and store the resultant strings into the destinations specified by a list of additional arguments you provide. It differs from the simple Fscan function as it supports custom format specifiers and uses a format string to format the output string. In contrast, in Fscan, only default formats are used to format the string.

To use this function, you must import the fmt package in your file and access the Fscanf function within, using the . notation: fmt.Fscanf. Here Fscanf is the actual function, while fmt is the Go package that stores the definition of this function.

Function definition

The definition of the Fscanf function inside the fmt package is as follows:

Parameters

The fmt.Fscanf function takes 3 parameters as described below:

  • src: The source from where the input is to be taken. This should be an object of type io.Reader

io.Reader objects are, in simple terms, objects that have a built-in read method.

  • format: This argument is of type string and represents the string containing custom specifiers that the Fscanf uses to format the final output string.

  • a ...interface{}: The list of all arguments that you want to store data in. After taking in the input, the input string is automatically split on space characters. The components are stored sequentially into the given arguments. If there are fewer arguments than the different splits of the input string, then the extra pieces are discarded.

Common specifiers

The following is a table of the most commonly used format specifiers in Go and their descriptions:

Specifiers Description
%s To print a string
%d To print an integer
%v To print values of all elements in a structure
%+v To print the names and values of all elements in a structure

Return values

The fmt.Fscanf function can return two things:

  • count: The number of arguments the function writes to.

  • err: Any error thrown during the execution of the function.

Example

The following example is a simple program where we first initialize a buffer variable of type bytes.Buffer with a string using the Fprint function. Then we read data from the buffer, store it into our desired variables, and use the Printf function to print a new string to the standard output.

Fscanf reads from the input source sequentially. Hence, we must give the list of arguments in the order specified in the format string.

Here, we require an input with the specific format matching the sequence of arguments we have given the Fscanf function.

For example, with the first word being a string, the second can be anything since it is stored in temp and is unused. The third can be an int, and another string after that.

Here, we are using: “Faraz owns 500 acres of land”. We then use the Fscanf function to read this input and store parts of the string corresponding to the amount of land, units of measurement, and the owner’s name. We then use Printf to print a new string.

The bytes data type has a built-in Read function which makes it a compatible argument to send to the Fscanf function. To use the Buffer data type, you need to import the bytes package.

package main
 
import (
    "fmt"
    "bytes"
)
 
func main() {
 
  var src_buff bytes.Buffer
  //assigning our buffer a message
  message := "Faraz owns 500 acres of land"
  fmt.Fprint(&src_buff, message)
  var name string
  var unit string
  var amount int
  var temp string
  // taking input and storing in variable using the buffer string
  fmt.Fscanf(&src_buff,"%s %s %d %s", &name, &temp, &amount, &unit)
 
  // print out new string using the extracted values 
  fmt.Printf ("%d %s of land is owned by %s\n",amount, unit, name);
 
}

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