Typescript – In TypeScript how to declare an array of functions that accept a string and return a string

arraysfunctiontypescript

UPDATE – the context of this question was pre-TypeScript 1.4. Since that version, my first guess has been supported by the language. See the update to the answer.

I can declare f to be a function that accepts a string and returns a string:

var f : (string) => string

And I can declare g to be an array of string:

var g : string[]

How can I declare h to be an array of "function that accepts a string and returns a string"?

My first guess:

var h : ((string) => string)[]

That seems to be a syntax error. If I take away the extra parentheses then it's a function from string to array of string.

Best Answer

I figured it out. The problem is that the => for a function type literal is itself merely syntactic sugar and doesn't want to compose with [].

As the spec says:

A function type literal of the form

( ParamList ) => ReturnType

is exactly equivalent to the object type literal

{ ( ParamList ) : ReturnType }

So what I want is:

var h : { (s: string): string; }[]

Complete example:

var f : (string) => string

f = x => '(' + x + ')';

var h : { (s: string): string; }[]

h = [];

h.push(f);

Update:

Judging from this changeset parentheses will be allowed in type declarations in 1.4, so the "first guess" in the question will also be correct:

var h: ((string) => string)[]

Further Update It is in 1.4!

Related Solutions

C# – How to convert a byte array to a hexadecimal string, and vice versa

You can use Convert.ToHexString starting with .NET 5.
There's also a method for the reverse operation: Convert.FromHexString.

For older versions of .NET you can either use:

public static string ByteArrayToString(byte[] ba)
{
  StringBuilder hex = new StringBuilder(ba.Length * 2);
  foreach (byte b in ba)
    hex.AppendFormat("{0:x2}", b);
  return hex.ToString();
}

or:

public static string ByteArrayToString(byte[] ba)
{
  return BitConverter.ToString(ba).Replace("-","");
}

There are even more variants of doing it, for example here.

The reverse conversion would go like this:

public static byte[] StringToByteArray(String hex)
{
  int NumberChars = hex.Length;
  byte[] bytes = new byte[NumberChars / 2];
  for (int i = 0; i < NumberChars; i += 2)
    bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);
  return bytes;
}

Using Substring is the best option in combination with Convert.ToByte. See this answer for more information. If you need better performance, you must avoid Convert.ToByte before you can drop SubString.

Java – How to declare and initialize an array in Java

You can either use array declaration or array literal (but only when you declare and affect the variable right away, array literals cannot be used for re-assigning an array).

For primitive types:

int[] myIntArray = new int[3];
int[] myIntArray = {1, 2, 3};
int[] myIntArray = new int[]{1, 2, 3};

// Since Java 8. Doc of IntStream: https://docs.oracle.com/javase/8/docs/api/java/util/stream/IntStream.html

int [] myIntArray = IntStream.range(0, 100).toArray(); // From 0 to 99
int [] myIntArray = IntStream.rangeClosed(0, 100).toArray(); // From 0 to 100
int [] myIntArray = IntStream.of(12,25,36,85,28,96,47).toArray(); // The order is preserved.
int [] myIntArray = IntStream.of(12,25,36,85,28,96,47).sorted().toArray(); // Sort

For classes, for example String, it's the same:

String[] myStringArray = new String[3];
String[] myStringArray = {"a", "b", "c"};
String[] myStringArray = new String[]{"a", "b", "c"};

The third way of initializing is useful when you declare an array first and then initialize it, pass an array as a function argument, or return an array. The explicit type is required.

String[] myStringArray;
myStringArray = new String[]{"a", "b", "c"};

Best Answer

Related Solutions

C# – How to convert a byte array to a hexadecimal string, and vice versa

Java – How to declare and initialize an array in Java

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