The Use of the Triple Exclamation Mark

The use of the triple exclamation mark

There is no difference between !a and !!!a, since !!!a is just !!(!a) and because !a is a boolean, !!(!a) is just its double negation, therefore the same.

Triple exclamation marks on R

!!! is usually used to evaluate a list of expressions.

library(dplyr)
library(rlang)

VC_preds <- c('mpg', 'cyl')
mtcars %>% select(!!!VC_preds) %>% head

#                   mpg Cyl
#Mazda RX4         21.0   6
#Mazda RX4 Wag     21.0   6
#Datsun 710        22.8   4
#Hornet 4 Drive    21.4   6
#Hornet Sportabout 18.7   8
#Valiant           18.1   6

If VC_preds is a vector as in your example, !! should work as well.

mtcars %>% select(!!VC_preds) %>% head

Help page of ?"!!!" gives a better example to understand the difference.

vars <- syms(c("height", "mass"))
vars
#[[1]]
#height

#[[2]]
#mass

starwars %>% select(!!!vars)
# A tibble: 87 x 2
#   height  mass
#    <int> <dbl>
# 1    172    77
# 2    167    75
# 3     96    32
# 4    202   136
# 5    150    49
# 6    178   120
# 7    165    75
# 8     97    32
# 9    183    84
#10    182    77
# … with 77 more rows

What does the '!!!' syntax mean in javascript?

I assumed it was somewhat equivalent to === and !==...

No, it's just three "not" operators, a "not-not-not".

It's the same as !(!(!(x))), and is always equivalent to a single !x.

There is literally no use for this. !! is a somewhat cryptic means of converting any variable to its boolean representation, but !!! is just silly. You can chain an arbitrary number of !'s together, but it isn't useful for anything.

Is !!!foo the same as !foo?

Yup. Just for clarity:

!!x === x is not generally true, but it is true if x is already a boolean: "not (not true)" is true, and "not (not false)" is false.

!foo is always a boolean; if foo is truthy, it's false, otherwise it's true.

So if you substitute !foo in for x you get that !!(!foo) === (!foo) is always true. Removing the parentheses doesn't change the meaning, so !!!foo === !foo is always true.

Which means there's no good reason to write !!!foo in actual code. Just use !foo instead.

What is double exclamation mark in C#?

This is a null-parameter check syntax which was going to be introduced in C# 11. This propsal has since been rolled back following community feedback.

The proposal is here, and the PR doing a first roll-out to the runtime is here.

The syntax:

public void Foo(string bar!!)
{
}

Is roughly equivalent to:

public void Foo(string bar)
{
    if (bar is null)
    {
        throw new ArgumentNullException(nameof(bar));
    }
}

... although the actual implementation uses a throw helper, something like:

public void Foo(string bar)
{
    <PrivateImplementationDetails>.ThrowIfNull(bar, "bar");
}

[CompilerGenerated]
internal sealed class <PrivateImplementationDetails>
{
    internal static void Throw(string paramName)
    {
        throw new ArgumentNullException(paramName);
    }

    internal static void ThrowIfNull(object argument, string paramName)
    {
        if (argument == null)
        {
            Throw(paramName);
        }
    }
}

See on SharpLab.

Methods containing throw statements are less likely to be inlined by the JIT, so using a throw helper makes it more likely that your method can be inlined, which might remove the null-check altogether! See on SharpLab.

Note that the use of !! is an implementation detail of your method: it just causes the compiler to insert code which you could have written yourself anyway. This means that moving from throw to !! (or vice versa) is not a breaking change.

---

There are a couple of places where !! will get the compiler to generate code which you can't (easily) write by hand, however.

One place where !! is particularly useful is in records with primary constructors. For example:

public record Person(string Name!!, int Age);

In order to correctly null-check the Name parameter in previous versions of C#, you have to write this out longhand:

public record Person
{
    public string Name { get; init; }
    public int Age { get; init; }
    
    public Person(string name, int age)
    {
        if (name is null)
            throw new ArgumentNullException(nameof(name));
        (Name, Age) = (name, age);
    }
    
    public void Deconstruct(out string name, out int age) =>
        (name, age) = (Name, Age);
}

Another place where !! does something which you can't write yourself is in chained constructor calls:

public class C
{
    public C(int i) { }
    public C(string s!!) : this(s.Length) { }
}

This null-checks s before accessing s.Length, something like this (which isn't valid C#):

public C(string s)
{
    if (s is null)
        throw new ArgumentNullException(nameof(s));
    C(s.Length);
}

See on SharpLab.

Another interesting aspect is that the null-checks are inserted before field assignments in constructors. For example:

public class C
{
    private readonly ExpensiveObject e = new ExpensiveObject();
    public C(string s!!) { }
}

Is compiled as:

public class C
{
    private readonly ExpensiveObject e;
    public C(string s)
    {
        if (s is null)
            throw new ArgumentNullException(nameof(s));
        e = new ExpensiveObject();
    }
}

That is, the null-check happens before the instantiation of ExpensiveObject. See on SharpLab.

Is there a more robust rename alternative than select with triple exclamation mark?

An extension to your solution could be

library(tidyverse)

df[,setdiff(mapping, colnames(df))] <- NA

df %>% rename_all(~names(mapping))

#  x y  z
#1 1 m NA
#2 2 m NA
#3 3 m NA

---

Or another approach

map_dfc(setdiff(mapping, colnames(df)), ~df %>% mutate(!!.x := NA)) %>%
    arrange(mapping) %>%
    rename_all(~names(mapping))

What does exclamation mark mean before invoking a method in C# 8.0?

This would be the null forgiving operator.

It tells the compiler "this isn't null, trust me", so it does not issue a warning for a possible null reference.

In this particular case it tells the compiler that Entity isn't null.

Can somebody explain an exclamation mark?

! inverts the truthyness of an expression. Since chars starts out as an empty object, the first time a character is iterated over, it won't exist on a property of the object; it'll be undefined. Eg, for H:

chars[character]
// equivalent to
chars.H
// resolves to
undefined

// putting ! in front of it makes it truthy instead:
!undefined -> true

So if(!chars[character]){ is saying: if this character doesn't exist on the object yet, then execute the following block:

  chars[character] = 1;
} else {
  // The character has already been iterated over;
  // it exists on the object, and the value is a number
  // Increment that number:
  chars[character]++;
}

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