Your First Go Program

The package main Declaration

Every Go source file starts with a package declaration. To produce a standalone, runnable program, the file must belong to 'package main' -- this tells the Go tool that this package is an entry point rather than a reusable library.

If you name it anything else, 'go build' will produce a library archive instead of an executable.

Example: The package main Declaration

// Every Go file belongs to a package; main builds an executable
package main

// Import the fmt package
import "fmt"

// main is where the program starts running
func main() {
	// Print the values followed by a newline
	fmt.Println("package main marks this as a runnable program")
}

The main Function

Inside package main, Go looks for a function literally named main with no parameters and no return value -- this is where execution begins.

Everything that happens when you run the program is triggered from inside, or reachable from, this single function.

Example: The main Function

// Every Go file belongs to a package; main builds an executable
package main

// Import the fmt package
import "fmt"

// main is where the program starts running
func main() {
	// Print the values followed by a newline
	fmt.Println("Execution starts here, inside func main()")
}

Importing and Using fmt

The fmt package (short for format) is part of Go's standard library and provides functions for formatted input/output, most commonly fmt.Println for printing a line and fmt.Printf for printing with format verbs like %s or %d. You bring it into scope with an import statement at the top of the file.

Example: Importing and Using fmt

// Every Go file belongs to a package; main builds an executable
package main

// Import the fmt package
import "fmt"

// main is where the program starts running
func main() {
	// Short declaration: create name and infer the type
	name := "Gopher"
	// Print using a format string
	fmt.Printf("Hello, %s! Welcome to Go.\n", name)
}

Running Your Program

The 'go run' command compiles your source file into a temporary binary and immediately executes it, printing the output to your terminal. This is the fastest way to try out small programs while learning, without manually managing build artifacts.

Example: Running Your Program

go run hello.go