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Dfns and Assignment


A dfn (pronounced "dee-fun" with a very short "u" sound) is a way of writing functions in APL. It starts and ends with curly braces {}, has a right argument (omega) and an optional left argument (alpha).

      3{⍺}5      ⍝ ⍺ is the (optional) left argument
      {⍵}'apl'   ⍝ ⍵ is the right argument
      {⍺}5       ⍝ Calling a dyadic function monadically results in an error
      3{⍵}       ⍝ Calling a function without a right argument results in an error
SYNTAX ERROR: Missing right argument

From here, when functions are first introduced, F⍵ ("eff omega") denotes a monadic function F and ⍺F⍵ ("alpha eff omega") denotes a dyadic function.


Names are assigned with the left arrow name ← expression. We say "name gets [function or array]".

      four equals one plus three   ⍝ 1 means true, 0 means false

We can use a name in the same line in which it is defined. In production code it is best to avoid this unless an expression is very short.

Read the following as "squared numbers divided by the sum of squares":

      squared÷+/squared←¯1 0 1 2*2
0.1666666667 0 0.1666666667 0.6666666667`

Syntactic name class

You may come across the following error:

      count ← {+/⍵}
      count ← {+/⍵} 1 0 0 1 0 1 0
SYNTAX ERROR: Invalid modified assignment, or an attempt was made to change nameclass on assignment
      count←{+/⍵}1 0 0 1 0 1 0

Things in APL have both a word and a number which identifies what type of thing it is. This is called its name class. So far we have met variables (nameclass 2) and functions (nameclass 3). There are more than these, but they will be introduced in relevant chapters.

In Dyalog APL, if a name already has a function assigned, that same name cannot then be assigned an array value. Nor vice versa. If this happens, erase the name and try again.

      )ERASE count
      count←{+/⍵}1 0 0 1 0 1 0

What is this )ERASE thing?

We have just used a system command. They are available while using the Dyalog interpreter or TryAPL interactively. However, they cannot be used inside functions and they are not standard APL syntax. In the section on system functions and system commands we will learn about things like showing a list of the currently defined names and how to erase names programmatically (there is a system function ⎕EX). In the meantime, we will introduce system functions and commands as needed.

Multiline functions and the editor

You can do quite a lot in a single line of APL. However, it is not long before you want to keep sequences of multiple statements available for re-use. Of course we can write functions which consist of multiple statements.

The statement separator, (diamond), allows us to write multiple APL statements in a single line. Some people think that it is more readable to spread multiple statements across multiple lines of a function. However, it is worth being aware that APL diamonds are equivalent to newline characters in terms of execution. The following two definitions of the Mean function are equivalent.

 Mean ← {
    sum ← +/⍵
    count ← ≢⍵
    sum ÷ count

 Mean ← { sum ← +/⍵ ⋄ count ← ≢⍵ ⋄ sum÷count }

Separate statements are executed from left to right and top to bottom.

To edit multiline functions in the IDE for Microsoft Windows and the RIDE, invoke the editor with the system command )ED. You can find a step-by-step example of creating a multiline function in the Dyalog editor in chapter 5 of Mastering Dyalog APL.

On TryAPL, the current execution block can be continued on to a new line using Alt+Enter. The continuation line begins with a tab character. To execute the block, simply press Enter after your final line is typed. Here is an example defining a multiline dfn:

  1. Type Sum ← { and press Alt+Enter
  2. Type ⍺+⍵ and press Alt+Enter
  3. Type } and press just Enter
  4. The function Sum is now defined in your workspace. Try the expression 3 Sum 4.

Problem set 2

The following problems can be solved with single-line dfns.

  1. Eggs

    A recipe serving 4 people uses 3 eggs. Write the function Eggs which computes the number of eggs which need cracking to serve people. Using a fraction of an egg requires that a whole egg be cracked.

          Eggs 4

          Eggs 100

          Eggs ⍳12
    1 2 3 3 4 5 6 6 7 8 9 9

    Eggs ← {⌈⍵×3÷4}
  2. Write a function To which returns integers from to inclusive.

          3 To 3
          3 To 4
    3 4
          1 To 7
    1 2 3 4 5 6 7
          ¯3 To 5
    ¯3 ¯2 ¯1 0 1 2 3 4 5

    BONUS: What if ⍺>⍵?

          3 To 5
    3 4 5
          5 To 3
    5 4 3
          5 To ¯2
    5 4 3 2 1 0 ¯1 ¯2


    In the simple case, make sure to generate enough numbers and use as an offset:

    To ← {⍺+¯1+⍳1+⍵-⍺}
    In general we take into account whether the difference is positive or negative:
    To ← {⍺+(×d)ׯ1+⍳1+|d←⍵-⍺}

  3. The formula to convert temperature from Celsius (\(T_C\)) to Fahrenheit (\(T_F\)) in traditional mathematical notation is as follows:

    \[T_F = {32 + {{9}\over{5}}\times {T_C}}\]

    Write the function CtoF to convert temperatures from Celcius to Farenheit.

          CtoF 11.3 23 0 16 ¯10 38
    52.34 73.4 32 60.8 14 100.4

    CtoF ← {32+⍵×9÷5}
  4. Prime Time

    A prime number is a positive whole number greater than \(1\) which can be divided only by itself and \(1\) with no remainder.

    Write a dfn which returns 1 if its argument is prime and 0 otherwise.

              IsPrime 21
              IsPrime 17

    There are several ways to code this, but the basic method is to count the number of divisors.

    IsPrime ← {2=+/d=⌊d←⍵÷⍳⍵}
    IsPrime ← {2=+/0=(⍳⍵)|⍵}