Lesson 2. A glorified calculator

After reading lesson 2, you’ll be able to

• Teach a computer to do your math
• Declare variables and constants
• See how declaration and assignment differ
• Use the standard library to generate pseudorandom numbers

Computer programs are capable of a great many things. In this lesson you’ll write programs to solve mathematical problems.

2.1. Performing calculations

There are days when we think it would be nice to be younger and weigh a little less. In this regard, Mars has a lot to offer. Mars takes 687 Earth days to travel around the sun, and its weaker gravitational force means everything weighs approximately 38% of what it does on Earth.

To calculate how young and light Nathan would be on Mars, we wrote a small program, shown in listing 2.1. Go provides the same arithmetic operators as other programming languages: +, -, *, /, and % for addition, subtraction, multiplication, division, and modulus respectively.

Tip

The modulus operator (%) obtains the remainder of dividing two whole numbers (for example, 42 % 10 is 2).

Listing 2.1. Hello Mars: mars.go

// My weight loss program.                                 1
package main

import "fmt"

// main is the function where it all begins.               1
func main() {
    fmt.Print("My weight on the surface of Mars is ")
    fmt.Print(149.0 * 0.3783)                              2
    fmt.Print(" lbs, and I would be ")
    fmt.Print(41 * 365 / 687)                              3
    fmt.Print(" years old.")
}

• 1 A comment for human readers
• 2 Prints 56.3667
• 3 Prints 21

Note

Though listing 2.1 displays weight in pounds, the chosen unit of measurement doesn’t impact the weight calculation. Whichever unit you choose, the weight on Mars is 37.83% of the weight on Earth.

The code in the preceding listing begins with a comment. When Go sees a double slash //, it ignores everything until the end of the line. Computer programming is all about communication. Code communicates your instructions to a computer, and when written well, it communicates your intentions to other people. Comments are just for us. They don’t affect how a program runs.

The preceding listing calls the Print function several times to display a sentence on a single line. Alternatively, you can pass a list of arguments separated by commas. An argument to Println can be text, a number, or a mathematical expression:

fmt.Println("My weight on the surface of Mars is", 149.0*0.3783, "lbs, and
I would be", 41*365.2425/687, "years old.")                             1

• 1 Prints My weight on the surface of Mars is 56.3667 lbs, and I would be 21.79758733624454 years old.

Tip

After modifying your code, click the Format button in the Go Playground. It will automatically reformat the indentation and spacing of your code without changing what it does.

2.2. Formatted print

The Print and Println functions have a sibling that gives more control over output. By using Printf, shown in the following listing, you can insert values anywhere in the text.

Listing 2.2. Printf: fmt.go

fmt.Printf("My weight on the surface of Mars is %v lbs,", 149.0*0.3783)  1
fmt.Printf(" and I would be %v years old.
", 41*365/687)                2

• 1 Prints My weight on the surface of Mars is 56.3667 lbs,
• 2 Prints and I would be 21 years old.

Unlike Print and Println, the first argument to Printf is always text. The text contains the format verb %v, which is substituted for the value of the expression provided by the second argument.

Note

We’ll introduce more format verbs (other than %v) as needed in upcoming lessons. For a complete reference, see the online documentation at golang.org/pkg/fmt/.

The Println function automatically moves to the next line, but Printf and Print don’t. Whenever you want to move to a new line, place in the text.

If multiple format verbs are specified, the Printf function will substitute multiple values in order:

fmt.Printf("My weight on the surface of %v is %v lbs.
", "Earth", 149.0) 1

• 1 Prints My weight on the surface of Earth is 149 lbs.

In addition to substituting values anywhere in a sentence, Printf can help you align text. Specify a width as part of the format verb, such as %4v to pad a value to a width of 4 characters. A positive number pads with spaces to the left, and a negative number pads with spaces to the right:

fmt.Printf("%-15v $%4v
", "SpaceX", 94)
fmt.Printf("%-15v $%4v
", "Virgin Galactic", 100)

The preceding code displays the following output:

SpaceX          $  94
Virgin Galactic $ 100

2.3. Constants and variables

The calculations in listing 2.1 are performed on literal numbers. It isn’t clear what the numbers mean, particularly values like 0.3783. Programmers sometimes refer to unclear literal numbers as magic numbers. Constants and variables can help by providing descriptive names.

After seeing the benefits of living on Mars, our next question is how long the trip will take. Traveling at the speed of light would be ideal. Light travels at a constant speed in the vacuum of space, which makes the math easy. On the other hand, the distance between Earth and Mars varies significantly, depending on where the planets are in their orbits around the Sun.

The following listing introduces two new keywords, const and var, for declaring constants and variables respectively.

Listing 2.3. Traveling at the speed of light: lightspeed.go

// How long does it take to get to Mars?
package main

import "fmt"

func main() {
    const lightSpeed = 299792 // km/s
    var distance = 56000000   // km

    fmt.Println(distance/lightSpeed, "seconds")         1

    distance = 401000000
    fmt.Println(distance/lightSpeed, "seconds")         2
 }

• 1 Prints 186 seconds
• 2 Prints 1337 seconds

Type listing 2.3 into the Go Playground and click Run. Light speed is pretty convenient; you probably wouldn’t hear anyone asking, “Are we there yet?”

The first calculation is based on Mars and Earth being nearby, with distance declared and assigned an initial value of 56,000,000 km. Then the distance variable is assigned a new value of 401,000,000 km, with the planets on opposite sides of the Sun, though plotting a course directly through the Sun could be problematic.

Note

The lightSpeed constant can’t be changed. If you try to assign it a new value, the Go compiler will report the error “cannot assign to lightSpeed.”

Note

Variables must be declared before you can use them. Go will report an error if you assign a value to a variable that hasn’t been declared with var—for example, speed = 16. This restriction can help catch mistakes, such as accidentally assigning a value to distence when you intended to type distance.

2.4. Taking a shortcut

There may not be any shortcuts to visit Mars, but Go provides a few keystroke-saving shortcuts.

2.4.1. Declare multiple variables at once

When you declare variables or constants, you can declare each one on its own line like this:

var distance = 56000000
var speed = 100800

Or you can declare them as a group:

var (
    distance = 56000000
    speed = 100800
)

Yet another option is to declare multiple variables on a single line:

var distance, speed = 56000000, 100800

Before you declare multiple variables as a group or on a single line, consider whether or not the variables are related. Always keep in mind the readability of your code.

2.4.2. Increment and assignment operators

There are a few shortcuts to perform assignment with other operations. The last two lines of the following listing are equivalent.

Listing 2.4. Assignment operators: shortcut.go

var weight = 149.0
weight = weight * 0.3783
weight *= 0.3783

Incrementing by one has an additional shortcut, as shown in the following listing.

Listing 2.5. Increment operator

var age = 41
age = age + 1             1
age += 1
age++

• 1 happy birthday!

You can decrement with count-- or shorten other operations like price /= 2 in the same way.

Note

In case you’re wondering, Go does not support the prefix increment ++count like C and Java.

2.5. Think of a number

Think of a number between 1 and 10.

Got it? Okay.

Now have your computer “think” of a number between 1 and 10. Your computer can generate pseudorandom numbers using the rand package. They’re called pseudorandom because they’re more or less random, but not truly random.

The code in listing 2.6 will display two numbers between 1–10. Passing 10 to Intn returns a number from 0–9, to which you add 1 and assign the result to num. The num variable can’t be a Go constant because it’s the result of a function call.

Note

If you forget to add 1, you’ll get a number between 0–9. Because we want a number between 1–10, that’s an example of an off-by-one error, a classic programming mistake.

Listing 2.6. Random numbers: rand.go

package main

import (
    "fmt"
    "math/rand"
)

func main() {
    var num = rand.Intn(10) + 1
    fmt.Println(num)

    num = rand.Intn(10) + 1
    fmt.Println(num)
}

The import path for the rand package is math/rand. The Intn function is prefixed with the package name rand, but the import path is longer.

Tip

To use a new package, it must be listed as an import. The Go Playground can add import paths for you. First ensure the Imports checkbox is checked and then click the Format button. The Go Playground will determine which packages are being used and update your import paths.

Note

Every time you run listing 2.6, the same two pseudorandom numbers are displayed. It’s rigged! In the Go Playground, time stands still and results are cached, but these numbers are good enough for our purposes.

Summary

• The Print, Println, and Printf functions display text and numbers on the screen.
• With Printf and the %v format verb, values can be placed anywhere in the displayed text.
• Constants are declared with the const keyword and can’t be changed.
• Variables are declared with var and can be assigned new values while a program is running.
• The math/rand import path refers to the rand package.
• The Intn function in the rand package generates pseudorandom numbers.
• You used 5 of the 25 Go keywords: package, import, func, const, and var.

Let’s see if you got this...

Experiment: malacandra.go

Malacandra is much nearer than that: we shall make it in about twenty-eight days.

C.S. Lewis, Out of the Silent Planet

Malacandra is another name for Mars in The Space Trilogy by C. S. Lewis. Write a program to determine how fast a ship would need to travel (in km/h) in order to reach Malacandra in 28 days. Assume a distance of 56,000,000 km.

Compare your solution to the code listing in the appendix.