Rust - Input Output

This chapter discusses how to accept values from the standard input (keyboard) and display values to the standard output (console). In this chapter, we will also discuss passing command line arguments.

Reader and Writer Types

Rust’s standard library features for input and output are organized around two traits −

• Read
• Write

Read Trait

Readers are components that your program can read bytes from. Examples include reading input from the keyboard, files, etc. The read_line() method of this trait can be used to read data, one line at a time, from a file or standard input stream.

Illustration − Reading from the Console − stdin()

Rust programs might have to accept values from the user at runtime. The following example reads values from the standard input (Keyboard) and prints it to the console.

fn main(){
   let mut line = String::new();
   println!("Enter your name :");
   let b1 = std::io::stdin().read_line(&mut line).unwrap();
   println!("Hello , {}", line);
   println!("no of bytes read , {}", b1);
}

The stdin() function returns a handle to the standard input stream of the current process, to which the read_line function can be applied. This function tries to read all the characters present in the input buffer when it encounters an end-of-line character.

Output

Enter your name :
Mohtashim
Hello , Mohtashim
no of bytes read , 10

Write Trait

Writers are components that your program can write bytes to. Examples include printing values to the console, writing to files, etc. The write() method of this trait can be used to write data to a file or standard output stream.

Illustration - Writing to the Console - stdout()

The print! or println! macros can be used to display text on the console. However, you can also use the write() standard library function to display some text to the standard output.

Let us consider an example to understand this.

use std::io::Write;
fn main() {
   let b1 = std::io::stdout().write("Tutorials ".as_bytes()).unwrap();
   let b2 = std::io::stdout().write(String::from("Point").as_bytes()).unwrap();
   std::io::stdout().write(format!("\nbytes written {}",(b1+b2)).as_bytes()).unwrap();
}

Output

Howcodex
bytes written 15

The stdout() standard library function returns a handle to the standard output stream of the current process, to which the write function can be applied. The write() method returns an enum, Result. The unwrap() is a helper method to extract the actual result from the enumeration. The unwrap method will send panic if an error occurs.

NOTE − File IO is discussed in the next chapter.

CommandLine Arguments

CommandLine arguments are passed to a program before executing it. They are like parameters passed to functions. CommandLine parameters can be used to pass values to the main() function. The std::env::args() returns the commandline arguments.

Illustration

The following example passes values as commandLine arguments to the main() function. The program is created in a file name main.rs.

//main.rs
fn main(){
   let cmd_line = std::env::args();
   println!("No of elements in arguments is :{}",cmd_line.len()); 
   //print total number of values passed
   for arg in cmd_line {
      println!("[{}]",arg); //print all values passed 
      as commandline arguments
   }
}

The program will generate a file main.exe once compiled. Multiple command line parameters should be separated by space. Execute main.exe from the terminal as main.exe hello howcodex.

NOTE − hello and howcodex are commandline arguments.

Output

No of elements in arguments is :3
[main.exe]
[hello]
[howcodex]

The output shows 3 arguments as the main.exe is the first argument.

Illustration

The following program calculates the sum of values passed as commandline arguments. A list integer values separated by space is passed to program.

fn main(){
   let cmd_line = std::env::args();
   println!("No of elements in arguments is 
   :{}",cmd_line.len()); 
   // total number of elements passed

   let mut sum = 0;
   let mut has_read_first_arg = false;

   //iterate through all the arguments and calculate their sum

   for arg in cmd_line {
      if has_read_first_arg { //skip the first argument since it is the exe file name
         sum += arg.parse::<i32>().unwrap();
      }
      has_read_first_arg = true; 
      // set the flag to true to calculate sum for the subsequent arguments.
   }
   println!("sum is {}",sum);
}

On executing the program as main.exe 1 2 3 4, the output will be −

No of elements in arguments is :5
sum is 10