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Multiple Return Values

Name: Multiple Return Values
Uploaded: 2017-06-27T03:18:18-04:00
Duration: 326 s
Description: It's possible to return multiple values from a Go function. This is usually used to communicate errors.

5:26 with Jay McGavren

It's possible to return multiple values from a Go function. This is usually used to communicate errors.

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Can return multiple values from a function. Usually used to communicate errors.

package main

import (
    "fmt"
    "log"
    "math"
)

func main() {
    squareRoot, err := squareRoot(9)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(squareRoot)
}

func squareRoot(x float64) (float64, error) {
    if x < 0 {
        return 0, fmt.Errorf("can't take square root of negative number")
    }
    return math.Sqrt(x), nil
}

Can ignore a return value by assigning to blank identifier, _.
- This code gives a compile error:

package main

import (
    "fmt"
    "os"
)

func main() {
    fileInfo := os.Stat("existent.txt")
    fmt.Println(fileInfo.Size())
    fileInfo = os.Stat("nonexistent.txt")
    fmt.Println(fileInfo.Size())
}

We can get it to compile by adding blank identifiers.
- But this isn't always a good idea. In this case, checking the size of "existent.txt" works, because the file exists. But our users get a panic, a runtime error, when we try to get the size of "nonexistent.txt", because the file doesn't exist.
We can fix this by checking to see if the error value is anything other than nil, and handling the error if it is.

package main

import (
    "fmt"
    "os"
)

func main() {
    fileInfo, error := os.Stat("existent.txt")
    if error != nil {
        fmt.Println(error)
    } else {
        fmt.Println(fileInfo.Size())
    }
    fileInfo, error = os.Stat("nonexistent.txt")
    if error != nil {
        fmt.Println(error)
    } else {
        fmt.Println(fileInfo.Size())
    }
}

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It's possible to return multiple values from a single function. 0:00

This is usually used to communicate errors. 0:03

You set this up by declaring multiple return values within parentheses, 0:06

following the parentheses that accept parameters for the function. 0:10

So here, we specify that this squareRoot function is going to return one value 0:14

that's a float64 type, and then a second value that's an error type. 0:19

Now, as you might be aware, 0:24

it's not possible to take a square root of a negative number. 0:26

So, we've set our square root function up to check for this. 0:30

It looks at the parameter that got passed in, x, and 0:32

if it's less than 0 then we return the value 0, which we're going to ignore. 0:36

As well as the new error value, which we get by calling the format packages Errorf 0:41

function, and passing it a string, can't take the square root of a negative number. 0:47

If x is greater than 0, or equal to 0, then we'll proceed as normal. 0:53

We call the math packages square root function and 0:58

pass it the parameter that we got. 1:01

We then return the value nil, which represents that we have no error. 1:05

We then handle those multiple return values up here in the main function. 1:09

We call square root, up here, and 1:14

then we assign two variables at once based on its return value. 1:17

The square root variable will hopefully contain the square root of whatever 1:21

argument we pass the square root. 1:25

And the error value will hopefully be nil. 1:27

But in the event that there was a problem, say if we were to call square 1:30

root with a negative number, the error value will not be nil. 1:34

We'll have passed the return value of this error f function, 1:38

which will be an error value, which will then logout to the console. 1:45

Calling the fatal function from the log packages cause 1:50

your program to print an error message and then exit immediately. 1:55

So this line down here would never be reached if there was an error. 1:59

So, we handled the error here and 2:03

in the event that there's not an error we proceed to print out the square root. 2:04

So let's try running this. 2:10

Okay, and because we called square root with a negative number we see 2:13

our error message printed out here, can't take square root of a negative number. 2:17

Where as if we were to change this to a valid value say get the square root of 9, 2:21

if we try running this again it runs successfully and 2:27

it prints out our square root, 3. 2:31

These function calls also return multiple values. 2:34

But in this case we're getting a compile error because we aren't doing anything 2:37

with the second value We're getting the multiple value os.Stat() in single value 2:41

context. 2:46

We can get this code to compile by adding the blank identifier. 2:48

This will assign the second value, the error value, 2:53

from os.Stat to a throwaway variable, this underscore that you see here. 2:56

So if we have this down here as well our program will then compile and run. 3:04

So as you can see up here that we get a file 3:07

size for the existent text file. 3:13

However down here we get a run time panic, this is a special type of 3:18

error that occurs after your program compiles and while it's running. 3:23

And that's something that you want to avoid wherever possible. 3:29

And this is why that assigning error values to the blank identifier isn't 3:32

always a good idea. 3:36

In this case, checking the size of the existent.txt file worked 3:38

because that file was on the disk, but, nonexistent.txt wasn't. 3:42

And therefore we got an error which we ignored and when we tried to print 3:48

out the size on this invalid file info object, our program panicked. 3:53

We can fix this by checking to see if the error return value from os.stat 3:59

is anything other than nil. 4:03

And if it is, handling the error. 4:05

So instead of the blank identifier here, 4:07

let's assign this error return value to a variable called error. 4:09

And here let's do a test. 4:14

Let's say if the error Is not equal to nil. 4:16

Then we will print out the error. 4:24

And if the error is nil, it means that we got our file info back successfully. 4:31

So we can move this format print line file info. 4:35

Size line up into the else clause. 4:40

And we'll be able to print the file info out successfully. 4:47

We'll do the same down here following getting os.stat for the second file. 4:50

We need to be sure to assign not to the blank Identifier, 4:56

but to the error variable. 5:00

We'll test if it's nil. 5:03

If it's not nil, we'll print out the error. 5:04

Otherwise, we'll print out the file info. 5:06

Now let's try running this again. 5:09

And you see for our first file we got a file size of 0, 5:12

which, because it does exist but its size is 0. 5:15

And for the second file it prints out our error. 5:19

It tried to run stats on nonexistant.txt, but there was no such file. 5:22

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Go Language Overview

Multiple Return Values

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