Skip to content

Your First Session

This page walks you through a first interactive session with MARPLE. Launch the REPL (marple) and follow along.

MARPLE as a calculator

APL works right to left, like mathematics. Try some arithmetic:

      2 + 3
5
      10 - 4
6
      3 × 5
15
      20 ÷ 4
5

Note

APL uses × for multiplication and ÷ for division, not * and /. The * symbol means power (exponentiation), and / means reduce.

Right-to-left evaluation means there's no precedence — every function takes everything to its right as its argument:

      2 × 3 + 4
14

This is 2 × (3 + 4), not (2 × 3) + 4. There's no precedence to memorise: just read right to left.

Vectors

Type several numbers separated by spaces to create a vector:

      1 2 3 4 5
1 2 3 4 5

Arithmetic works element-wise on vectors:

      1 2 3 + 10 20 30
11 22 33
      2 × 1 2 3 4 5
2 4 6 8 10

When one argument is a scalar and the other is a vector, the scalar is paired with every element. This is called scalar extension:

      100 + 1 2 3
101 102 103

Generating arrays with Iota

The ⍳ function (iota) generates a sequence of integers:

      ⍳5
1 2 3 4 5
      ⍳10
1 2 3 4 5 6 7 8 9 10

Reduce

The / operator inserts a function between every element of a vector. +/ sums a vector:

      +/ 1 2 3 4 5
15
      ×/ 1 2 3 4 5
120

×/ computes a product — so ×/⍳5 gives you 5 factorial.

Matrices

Use ⍴ (rho) to reshape a vector into a matrix. The left argument is the shape:

      3 4 ⍴ ⍳12
 1  2  3  4
 5  6  7  8
 9 10 11 12

This creates a 3-row, 4-column matrix filled with the numbers 1 to 12.

You can ask for an array's shape with monadic ⍴:

      M ← 3 4 ⍴ ⍳12
      ⍴M
3 4

Assigning names

The ← arrow assigns a value to a name:

      prices ← 9.99 14.50 3.25 21.00
      prices
9.99 14.5 3.25 21
      +/prices
48.74
      total ← +/prices
      total
48.74

Names are case-sensitive. M, m, and Matrix are three different names.

Defining functions

Curly braces {} define a dfn (direct function). Inside, ⍵ is the right argument and ⍺ is the optional left argument:

      double ← {2×⍵}
      double 5
10
      double 1 2 3
2 4 6

A dyadic dfn:

      avg ← {(+/⍵)÷⍴⍵}
      avg 3 5 7 9
6

Saving your work

Give your workspace a name and save it:

      )wsid myfirst
      )save

Next time, load it back:

      )load myfirst

What next?