Read File in Golang

Introduction

Go, often known as Golang, is an open-source, compiled, and statically typed programming language developed by Google. It is designed to be simple, high-performing, readable, and efficient. Golang provides a large built-in library that can be used to conduct file read and write operations. With these built-in libraries, we can read the complete file at once, line by line, word by word, or in chunks, depending on the needs and convenience of the programmer.

Read File as String

In Go, the ReadFile() function from the ioutil/os package is used to read a complete file's content/text. This function reads the entire content of the text file into a slice of bytes. The ioutil package should not be used to read huge files, so the ReadFile() function is adequate for tiny or small files. The following is the implementation of the ReadFile() function.

Program

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Explanation

content, error := ioutil.ReadFile("test.txt") This ioutil package includes a ReadFile() function that can read the entire file content and store the results in a content variable. If the reading of the file content was unsuccessful, we have an error check that prints the type of error.

Now, we simply convert the content of the content variable into the string and store it in the str variable. Finally, we print the string.

Read File Line by Line

We may use a handy bufio.Scanner structure to read a file line by line. Its constructor, NewScanner(), accepts an open file (remember to close the file after the procedure is complete) and allows you to read the following lines using the Scan() and Text() methods. You can inspect errors found during file reading using the Err() function.

Let us see the implementation of the program, which reads a file line by line.

Program

Output

Explanation

We first read the file test.txt using the os.Open() utility and store it in the file variable. Then we use the file to create a new scanner. The Scan() function reads the next line of the file, which is accessible via the Text() function. The Err() function will return any errors that occurred during scanning after Scan returns false. Err() will return nil if the error is EOF (End of File).

Read File by Words

In Go, we can also read a file word by word. For this, we use the ScanWords from the bufio package.

Let's see a program that can read a file test.txt word by word.

Program

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Explanation

This program is very similar to the process followed in the above program. The only difference is the use of buffalo.ScanWords to scan the line word by word.

Read File in Chunks

When you have a huge file or don't want to keep the entire file in memory, you can read it in fixed-size chunks. In this situation, you must generate a byte slice of the required size (chunkSize in the example) to serve as a buffer for the subsequent read bytes. We can load the next chunk of the file's data using the Read() function of the File type. The reading loop ends when an io.EOF error occurs, indicating that the file has reached its end.

Let's implement this logic to write a program that read a file in chunks.

Program

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Explanation

The open() function in the os package is used to open a file for simple access and reading of the information, allowing us to utilize the defer keyword to close the file to avoid memory leaks.

b:= make ([]byte, chunkSize) this line creates a buffer to keep bytes slices of specified length and capacity into which the file's bytes will be read.

We use the Read() function of the file type, which reads up to a specified length (i.e., in this case, its chunkSize) and returns the number of bytes read by it. We save the returned bytes in a variable. The slice is then read from index 0 to n-1 and printed up to the number of bytes returned by the Read() function. The reading loop ends when an io.EOF error occurs, indicating that the file has reached its end.

Read Binary File

In Go, we can also read the binary file (like jpeg, jpg, png, etc.). To do so, we use the hex package, which implements hexadecimal encoding and decoding.

Program

Output