10. Reading error messages — Homework solutions

The .go solution files are in exercises/10/homework/solutions/. These problems practise the loop: run, read the first error, fix one thing, run again.

Problem 1 — Name the error

Problem. Identify the error, then fix it.

How to think about it. Run the file, read the message, and match it to one of the four errors: newline in string, undefined, not used, or the brace error.

Worked solution. The starter is:

package main

import "fmt"

func main() {
    fmt.Println("Hello,", Keiko)
}

Running it prints:

# command-line-arguments
exercises\10\homework\01-name-the-error.go:10:24: undefined: Keiko

Without quotes, Keiko is a name, and nothing by that name exists. It was meant to be text. A fix:

package main

import "fmt"

func main() {
    fmt.Println("Hello,", "Keiko")
}

// error: undefined: Keiko

Problem 2 — Fix the quote

Problem. Repair an unfinished string. Go reports:

# command-line-arguments
exercises\10\homework\02-fix-the-quote.go:8:39: newline in string
exercises\10\homework\02-fix-the-quote.go:8:39: syntax error: unexpected newline in argument list; possibly missing comma or )

Worked solution.

package main

import "fmt"

func main() {
    fmt.Println("A wizard is never late.")
}

The only change is the missing closing " before the ).

Common mistakes.

• Adding the quote after the ). The closing quote goes at the end of the text, before the closing parenthesis. Otherwise the ) becomes part of the string, and Go reports syntax error: unexpected newline in argument list; possibly missing comma or ).

Problem 3 — Fix the typo

Problem. A misspelled function name causes an undefined: error.

Worked solution. The starter has fmt.Prinln("Fixed!"). The message names the culprit:

# command-line-arguments
exercises\10\homework\03-fix-the-typo.go:8:6: undefined: fmt.Prinln

Fix the spelling:

package main

import "fmt"

func main() {
    fmt.Println("Fixed!")
}

Common mistakes.

• "Fixing" it by inventing a different name. fmt only has the names it has. When the message says fmt.Prinln, the fix is almost always the real function spelled correctly — fmt.Println.

Challenge — Three in a row

Problem. Three mistakes, one of each kind, fixed one at a time.

How to think about it. Run, fix the first error, and run again. The starter is:

package main

import "fmt"

func main()
{
    fmt.Println("Starting up)
    fmt.Prinln("Working")
    fmt.Println("done")
}

The first run shows two of the three mistakes:

# command-line-arguments
exercises\10\homework\04-three-in-a-row.go:10:1: syntax error: unexpected semicolon or newline before {
exercises\10\homework\04-three-in-a-row.go:11:27: newline in string

Move the { up and run again. The file is one line shorter now, so the line numbers change:

# command-line-arguments
exercises\10\homework\04-three-in-a-row.go:10:27: newline in string
exercises\10\homework\04-three-in-a-row.go:10:27: syntax error: unexpected newline in argument list; possibly missing comma or )

Add the closing " and run again. Only now can Go check the names:

# command-line-arguments
exercises\10\homework\04-three-in-a-row.go:11:6: undefined: fmt.Prinln

Fix the spelling for the last pass. The loop, working as intended.

Worked solution.

package main

import "fmt"

func main() {
    fmt.Println("Starting up")
    fmt.Println("Working")
    fmt.Println("done")
}

It prints:

Starting up
Working
done

Common mistakes.

• Fixing all three at once and losing track of which change did what. One error, one fix, run again — even when you think you see them all.

Done?

That is the end of Part 2. You can install Go and run a program, print exactly what you want, comment your code, and read Go's errors. The Part 2 mini-project — the ASCII Name Banner — puts your fmt.Println skills to work building a picture out of text. Then Part 3 digs into variables, strings, numbers, and input.