Array data types

An array is an ordered collection of elements that all share the same base type. Instead of declaring ten variables for ten temperature sensors, you define a single array type and declare one variable that holds all ten readings.

The array editor

Creating a type with Array as the derivation opens this editor:

Array editor for the TempReadings type: a Name field set to "TempReadings", a Base Type dropdown showing BOOL, an Initial Value text field, and a Dimensions section with one row showing "0..9"

Three things to fill in:

FieldNotes
NameThe type's identifier (set when you created it; editable here).
Base TypeThe type of each element. Pick from base IEC types or any other user-defined type (yes, you can have an array of structures, or an array of arrays).
Initial ValueOptional. Cold-start value for the whole array. For simple cases, leave it blank; the elements default to their base type's zero (FALSE / 0 / 0.0).

Below that, the Dimensions section is where the actual size lives.

Dimensions

A dimension is a range, written LOW..HIGH:

DimensionElement countIndices
0..9100, 1, 2, 3, 4, 5, 6, 7, 8, 9
1..551, 2, 3, 4, 5
0..991000–99

Click the + in the Dimensions toolbar to add a new dimension. For a multi-dimensional array, add more rows. The array's full shape is the cross-product of every dimension you list.

DimensionsResulting arrayTotal elements
0..91-D array of 10 elements10
0..2, 0..32-D array (3 rows × 4 columns)12
0..9, 0..9, 0..93-D array (10×10×10)1000

The IEC declaration syntax for the type, shown in code mode, looks like this:

iec
TYPE
    TempReadings : ARRAY[0..9] OF REAL;
END_TYPE

Or, for a 2-D array:

iec
TYPE
    RecipeTable : ARRAY[0..2, 0..3] OF INT;
END_TYPE

Picking a base type

Click Base Type to open the picker. Same categories as the variables editor:

  • Base types: any IEC scalar (BOOL, INT, REAL, TIME, STRING, …).
  • User data types: any other custom type you've defined, including other arrays or structures.

ARRAY[0..9] OF SensorReading is a perfectly legal type, ten copies of a structure. The same goes for ARRAY[0..9] OF SomeEnum, a fixed-size column of enum values.

Using an array in your code

Declare a variable of the array type just like any other:

iec
VAR
    temperatures : TempReadings;
END_VAR

Then index into it. Both ST text and graphical block calls accept array variables; element access uses square brackets:

iec
(* Read element at index 3 *)
current := temperatures[3];

(* Write element at index 0 *)
temperatures[0] := 22.5;

(* Loop over the whole array *)
FOR i := 0 TO 9 DO
    temperatures[i] := 0.0;
END_FOR;

For multi-dimensional arrays, separate indices with commas:

iec
recipe : RecipeTable;       (* 3 rows × 4 columns *)
recipe[1, 2] := 100;        (* row 1, column 2 *)

Initial values

To pre-fill the array, type a comma-separated list into the Initial Value field. The syntax is [v0, v1, v2, ...] with one value per element, in index order:

iec
TempReadings : ARRAY[0..4] OF REAL := [20.0, 21.0, 22.0, 23.0, 24.0];

Use the repeat-count form n(v) for runs of the same value:

iec
ZerosAndOnes : ARRAY[0..7] OF INT := [4(0), 4(1)];   (* 0,0,0,0,1,1,1,1 *)

For multi-dimensional arrays, the initial value is a comma-separated list of all elements in row-major order:

iec
TwoByTwo : ARRAY[0..1, 0..1] OF INT := [1, 2, 3, 4];   (* [0,0]=1, [0,1]=2, [1,0]=3, [1,1]=4 *)

Bounds-checking

In this build, array accesses are not bounds-checked at runtime. Indexing past the declared range is undefined behaviour (typically reads/writes adjacent memory, sometimes wraps around, depending on the compiler). Use ST IF guards or careful loop bounds to keep accesses inside the declared range.

What's next