I am new to Go.
Currently, I am creating a menu in Go and I want to verify that the data type of the input from the user matches the data type of the variable defined in the code. Part of my code looks like this so far:
package main
import (
"fmt"
"reflect"
)
var option int // The variable is declared outside of the main().
func general_menu() {
fmt.Println(".......................General Menu..................................")
fmt.Println()
fmt.Println("Calculator..........................................................1")
fmt.Println("Linear algebra package..............................................2")
fmt.Println("Language change.....................................................9")
fmt.Println("Exit...............................................................10")
fmt.Println()
fmt.Println("Choose an option from the menu.")
fmt.Println()
fmt.Scan(&option)
fmt.Println()
if (option != 1 && option != 2 && option != 9 && option != 10)||reflect.TypeOf(option)!=int{
fmt.Println("Wrong option input. Please, try again.")
fmt.Println()
general_menu()
}
}
I know that this doens't work this way, and I know that "int" can not be used as part of an "if" condirion.
I would kindly appreciate any suggestions on the proper way to solve this problem.
Thanks.
Edit: I have added more of my code as kindly suggested by the contributors.
Edit: Based on the answer provided, I have tried to implement a function, but the syntax is still not correct:
func check_integers_are_not_string(x int) bool {
change := strconv.Itoa(x)
if change != nil {
return true
} else {
return false
}
} // This function returns a true boolean value if conversion from int to string was possible, meaning that the entered value is a string.
Just read the documentation of Scan - https://pkg.go.dev/fmt#Scan
It returns the number of successfully read arguments and an error. The input is mapped in your case to a variable of type int, so if a user inputs a string it will return 0 and an error. Otherwise it will return 1 and the error should be nil. You can check for that.
n, err := fmt.Scan(&option)
if n != 1 || err != nil {
// print error and go back
}
One common way to do it is to try to make the conversion and see if it succeeds.
optionInt, err := strconv.Atoi(option) // Assuming option is of type string
if err != nil {
log.Printf("String '%s' cannot be converted to type int: %v", option, err)
os.Exit(1)
}
log.Printf(`optionInt is %d.`, optionInt)
This is a good approach if you are only interested in conversion to one type. Otherwise things can quickly get more involved, utilizing constructs such as lexers and parsers, but that would warrant more information on what you are trying to accomplish.
I'm trying to implement a default value according to the option 1 of the post Golang and default values. But when I try to do go install the following error pops up in the terminal:
not enough arguments in call to test.Concat1
have ()
want (string)
Code:
package test
func Concat1(a string) string {
if a == "" {
a = "default-a"
}
return fmt.Sprintf("%s", a)
}
// other package
package main
func main() {
test.Concat1()
}
Thanks in advance.
I don't think what you are trying to do will work that way. You may want to opt for option #4 from the page you cited, which uses variadic variables. In your case looks to me like you want just a string, so it'd be something like this:
func Concat1(a ...string) string {
if len(a) == 0 {
return "a-default"
}
return a[0]
}
Go does not have optional defaults for function arguments.
You may emulate them to some extent by having a special type
to contain the set of parameters for a function.
In your toy example that would be something like
type Concat1Args struct {
a string
}
func Concat1(args Concat1Args) string {
if args.a == "" {
args.a = "default-a"
}
return fmt.Sprintf("%s", args.a)
}
The "trick" here is that in Go each type has its respective
"zero value", and when producing a value of a composite type
using the so-called literal, it's possible to initialize only some of the type's fields, so in our example that would be
s := Concat1(Concat1Args{})
vs
s := Concat1(Concat1Args{"whatever"})
I know that looks clumsy, and I have showed this mostly for
demonstration purpose. In real production code, where a function
might have a dozen of parameters or more, having them packed
in a dedicate composite type is usually the only sensible way
to go but for a case like yours it's better to just explicitly
pass "" to the function.
Golang does not support default parameters. Accordingly, variadic arguments by themselves are not analogous. However, variadic functions with the use of error handling can 'resemble' the pattern. Try the following as a simple example:
package main
import (
"errors"
"log"
)
func createSeries(p ...int) ([]int, error) {
usage := "Usage: createSeries(<length>, <optional starting value>), length should be > 0"
if len(p) == 0 {
return nil, errors.New(usage)
}
n := p[0]
if n <= 0 {
return nil, errors.New(usage)
}
var base int
if len(p) == 2 {
base = p[1]
} else if len(p) > 2 {
return nil, errors.New(usage)
}
vals := make([]int, n)
for i := 0; i < n; i++ {
vals[i] = base + i
}
return vals, nil
}
func main() {
answer, err := createSeries(4, -9)
if err != nil {
log.Fatal(err)
}
log.Println(answer)
}
Default parameters work differently in Go than they do in other languages. In a function there can be one ellipsis, always at the end, which will keep a slice of values of the same type so in your case this would be:
func Concat1(a ...string) string {
but that means that the caller may pass in any number of arguments >= 0. Also you need to check that the arguments in the slice are not empty and then assign them yourself. This means they do not get assigned a default value through any kind of special syntax in Go. This is not possible but you can do
if a[0] == "" {
a[0] = "default value"
}
If you want to make sure that the user passes either zero or one strings, just create two functions in your API, e.g.
func Concat(a string) string { // ...
func ConcatDefault() string {
Concat("default value")
}
Looking at this callback (line 412), is it possible to break out of the callback loop from inside the callback function itself? Or would I need to wrap it or extend it in some way to create that capability?
This is the code:
jsonparser.ArrayEach(data, func(value []byte, dataType jsonparser.ValueType, offset int, err error) {
if v, err := jsonparser.GetUnsafeString(value, "username"); nil == err && v == lookFor {
z = v
return // Abort in some way?
}
})
The library is still waiting for a query-like update so for now this is the cheapest way to find a value, which still outperforms any full json parser by far (doing about 9.21x to 9.38x faster on benchmarks). So I'm kind of aiming for a LIMIT 1-ish solution.
I was thinking of doing a full copy-paste and making an cloned version on the interface and making..
if t != NotExist {
cb(v, t, o, e)
}
into...
if t != NotExist {
boolVal = cb(v, t, o, e) // Extending it to have a return value
}
... return a boolean and validate as a continue indicator. But would there be a more clean way without fully duplicating this code?
Go has a common idiom that looks like this:
if val, err := func(); err != nil {
/* val and err are in scope */
...
}
/* val and err are no longer in scope */
using "short assignment". I'm certainly a fan. It feels similar to doing:
/* code not involving val */
{
int val;
if ((val = func()) == ERR_VALUE) {
/* Process the error */
}
/* Do something with val */
}
/* more code not involving val */
in C++. What trips me up is that, in the case where there is more than one variable in the first clause of the if, they have to have the same scope, i.e. you have to do either:
var err error
var val string
if val, err = func(); err != nil {
...
or
if val, err := func(); err != nil {
...
A very common use case would seem to be where you have a variable that you'd like to set in the first clause of the if, test for an error, and if there is none, continue with the rest of the program flow (and be able to use whatever values you assigned in executing the if). But, it seems to me, if you want to do that, you have to either:
Use a temporary variable, and then assign the persistent variable value inside an else:
var val
if tempval, err := func(); err != nil {
/* Process the error */
} else {
val = tempval
}
Declare the err variable with scope that extends past the if, as above.
The first option seems clunky - being forced to use an "else" clause just to make sure that the value doesn't fall out of scope - and the second throws away the advantages of limiting the scope of the variables. What idioms do more experienced Go programmers use for this (seemingly very common) situation?
The Go Programming Language Specification
If statements
"If" statements specify the conditional execution of two branches
according to the value of a boolean expression. If the expression
evaluates to true, the "if" branch is executed, otherwise, if present,
the "else" branch is executed.
IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
.
if x > max {
x = max
}
The expression may be preceded by a simple statement, which executes
before the expression is evaluated.
if x := f(); x < y {
return x
} else if x > z {
return z
} else {
return y
}
If you can't take advantage of the special form,
if val, err := fnc(); err != nil {
// ...
}
then use the regular form,
val, err := fnc()
if err != nil {
// ...
}
The regular form is the Go language necessary and usual form. The special form is a specialization, for convenience, of the regular form; it's not necessary. If the special form is more convenient to use than the regular form, use it. Otherwise, use the regular form.
Go is a block-structured programming language tracing it's ancestry back to Algol 60, C, Pascal, Modula 2, and Oberon.
The Go Programming Language Specification
Blocks
Declarations and scope
Therefore, you can write
x := false
{
x := true
if x {
fmt.Println(x)
}
}
fmt.Println(x)
or, equivalently, as a convenience,
x := false
if x := true; x {
fmt.Println(x)
}
fmt.Println(x)
The output in both cases is
true
false
This question already has answers here:
What is the idiomatic Go equivalent of C's ternary operator?
(14 answers)
Closed 1 year ago.
Please see https://golangdocs.com/ternary-operator-in-golang as pointed by #accdias (see comments)
Can I write a simple if-else statement with variable assignment in go (golang) as I would do in php? For example:
$var = ( $a > $b )? $a: $b;
Currently I have to use the following:
var c int
if a > b {
c = a
} else {
c = b
}
Sorry I cannot remember the name if this control statement and I couldn't find the info in-site or through google search. :/
As the comments mentioned, Go doesn't support ternary one liners. The shortest form I can think of is this:
var c int
if c = b; a > b {
c = a
}
But please don't do that, it's not worth it and will only confuse people who read your code.
As the others mentioned, Go does not support ternary one-liners. However, I wrote a utility function that could help you achieve what you want.
// IfThenElse evaluates a condition, if true returns the first parameter otherwise the second
func IfThenElse(condition bool, a interface{}, b interface{}) interface{} {
if condition {
return a
}
return b
}
Here are some test cases to show how you can use it
func TestIfThenElse(t *testing.T) {
assert.Equal(t, IfThenElse(1 == 1, "Yes", false), "Yes")
assert.Equal(t, IfThenElse(1 != 1, nil, 1), 1)
assert.Equal(t, IfThenElse(1 < 2, nil, "No"), nil)
}
For fun, I wrote more useful utility functions such as:
IfThen(1 == 1, "Yes") // "Yes"
IfThen(1 != 1, "Woo") // nil
IfThen(1 < 2, "Less") // "Less"
IfThenElse(1 == 1, "Yes", false) // "Yes"
IfThenElse(1 != 1, nil, 1) // 1
IfThenElse(1 < 2, nil, "No") // nil
DefaultIfNil(nil, nil) // nil
DefaultIfNil(nil, "") // ""
DefaultIfNil("A", "B") // "A"
DefaultIfNil(true, "B") // true
DefaultIfNil(1, false) // 1
FirstNonNil(nil, nil) // nil
FirstNonNil(nil, "") // ""
FirstNonNil("A", "B") // "A"
FirstNonNil(true, "B") // true
FirstNonNil(1, false) // 1
FirstNonNil(nil, nil, nil, 10) // 10
FirstNonNil(nil, nil, nil, nil, nil) // nil
FirstNonNil() // nil
If you would like to use any of these, you can find them here https://github.com/shomali11/util
I often use the following:
c := b
if a > b {
c = a
}
basically the same as #Not_a_Golfer's but using type inference.
Thanks for pointing toward the correct answer.
I have just checked the Golang FAQ (duh) and it clearly states, this is not available in the language:
Does Go have the ?: operator?
There is no ternary form in Go. You may use the following to achieve the same result:
if expr {
n = trueVal
} else {
n = falseVal
}
additional info found that might be of interest on the subject:
Rosetta Code for Conditional Structures in Go
Ternary Operator in Go experiment from this guy
One possible way to do this in just one line by using a map, simple I am checking whether a > b if it is true I am assigning c to a otherwise b
c := map[bool]int{true: a, false: b}[a > b]
However, this looks amazing but in some cases it might NOT be the perfect solution because of evaluation order. For example, if I am checking whether an object is not nil get some property out of it, look at the following code snippet which will panic in case of myObj equals nil
type MyStruct struct {
field1 string
field2 string
}
var myObj *MyStruct
myObj = nil
myField := map[bool]string{true: myObj.field1, false: "empty!"}[myObj != nil}
Because map will be created and built first before evaluating the condition so in case of myObj = nil this will simply panic.
Not to forget to mention that you can still do the conditions in just one simple line, check the following:
var c int
...
if a > b { c = a } else { c = b}
A very similar construction is available in the language
**if <statement>; <evaluation> {
[statements ...]
} else {
[statements ...]
}*
*
i.e.
if path,err := os.Executable(); err != nil {
log.Println(err)
} else {
log.Println(path)
}
Use lambda function instead of ternary operator
Example 1
to give the max int
package main
func main() {
println( func(a,b int) int {if a>b {return a} else {return b} }(1,2) )
}
Example 2
Suppose you have this must(err error) function to handle errors and you want to use it when a condition isn't fulfilled.
(enjoy at https://play.golang.com/p/COXyo0qIslP)
package main
import (
"errors"
"log"
"os"
)
// must is a little helper to handle errors. If passed error != nil, it simply panics.
func must(err error) {
if err != nil {
log.Println(err)
panic(err)
}
}
func main() {
tmpDir := os.TempDir()
// Make sure os.TempDir didn't return empty string
// reusing my favourite `must` helper
// Isn't that kinda creepy now though?
must(func() error {
var err error
if len(tmpDir) > 0 {
err = nil
} else {
err = errors.New("os.TempDir is empty")
}
return err
}()) // Don't forget that empty parentheses to invoke the lambda.
println("We happy with", tmpDir)
}
Sometimes, I try to use anonymous function to achieve defining and assigning happen at the same line. like below:
a, b = 4, 8
c := func() int {
if a >b {
return a
}
return b
} ()
https://play.golang.org/p/rMjqytMYeQ0
Like user2680100 said, in Golang you can have the structure:
if <statement>; <evaluation> {
[statements ...]
} else {
[statements ...]
}
This is useful to shortcut some expressions that need error checking, or another kind of boolean checking, like:
var number int64
if v := os.Getenv("NUMBER"); v != "" {
if number, err = strconv.ParseInt(v, 10, 64); err != nil {
os.Exit(42)
}
} else {
os.Exit(1)
}
With this you can achieve something like (in C):
Sprite *buffer = get_sprite("foo.png");
Sprite *foo_sprite = (buffer != 0) ? buffer : donut_sprite
But is evident that this sugar in Golang have to be used with moderation, for me, personally, I like to use this sugar with max of one level of nesting, like:
var number int64
if v := os.Getenv("NUMBER"); v != "" {
number, err = strconv.ParseInt(v, 10, 64)
if err != nil {
os.Exit(42)
}
} else {
os.Exit(1)
}
You can also implement ternary expressions with functions like func Ternary(b bool, a interface{}, b interface{}) { ... } but i don't like this approach, looks like a creation of a exception case in syntax, and creation of this "features", in my personal opinion, reduce the focus on that matters, that is algorithm and readability, but, the most important thing that makes me don't go for this way is that fact that this can bring a kind of overhead, and bring more cycles to in your program execution.
You can use a closure for this:
func doif(b bool, f1, f2 func()) {
switch{
case b:
f1()
case !b:
f2()
}
}
func dothis() { fmt.Println("Condition is true") }
func dothat() { fmt.Println("Condition is false") }
func main () {
condition := true
doif(condition, func() { dothis() }, func() { dothat() })
}
The only gripe I have with the closure syntax in Go is there is no alias for the default zero parameter zero return function, then it would be much nicer (think like how you declare map, array and slice literals with just a type name).
Or even the shorter version, as a commenter just suggested:
func doif(b bool, f1, f2 func()) {
switch{
case b:
f1()
case !b:
f2()
}
}
func dothis() { fmt.Println("Condition is true") }
func dothat() { fmt.Println("Condition is false") }
func main () {
condition := true
doif(condition, dothis, dothat)
}
You would still need to use a closure if you needed to give parameters to the functions. This could be obviated in the case of passing methods rather than just functions I think, where the parameters are the struct associated with the methods.
As everyone else pointed out, there's no ternary operator in Go.
For your particular example though, if you want to use a single liner, you could use Max.
import "math"
...
c := math.Max(a, b)
Ternary ? operator alternatives | golang if else one line
You can’t write a short one-line conditional in Go language ; there is no ternary conditional operator.
Read more about if..else of Golang