Tcl Arrays

The array is a frequently occurring variable type in programming languages. In Tcl, arrays complement lists by providing additional capabilities when organizing data. Arrays are Tcl variables that have string-based indices (names). Any string value can be used as an array index.

Tcl arrays are used similarly to the way that "struct" is used in C.

set arrData(this\ is\ a\ test) "HyperWorks";

set indexvar "this is a test";
set arrData($indexvar) "HyperWorks";

set arrData(this,is,a,test) "HyperWorks";

These are all valid array definitions, but note the escape character \ used in the first version to prevent issues with spaces. Unlike brackets, parentheses do not affect how the string is interpreted and will therefore not prevent the white spaces from causing problems. The first two versions are equivalent, and will be evaluated as the same variable in the Tcl environment. The third case is different because the index is not equivalent to the string with spaces. As the index is a string, commas can be used in the indices, though they are not required. The third example illustrates using commas to delimit indices.

The array command is used in Tcl to manipulate array data. The following list contains some commonly used array commands and a summary of their usage. For more in-depth explanations and a complete listing, referred to http://www.tcl.tk/man/ or to a Tcl/Tk handbook.

array exists arrayN: Returns 1 if the array exists, 0 otherwise.
array get arrayName ?pattern?: Converts an array of format arrName(index1) containing value1 to a list of format:; index1 value1 index2 value2…; The pattern function allows a subset of indices to be selected using search capabilities as in the string match command.
array names arrayName ?mode? ?pattern?: Returns a list of indices matching pattern. If no pattern is given, returns all indices for the array. Pattern matching is the same as the string match command.
array set arrayName list: Converts a list of format:; index1 value1 index2 value2…; to arrName(index1) containing value1.
array size arrayName: Returns the number of elements in an array or 0 if the array does not exist or is empty.
array unset arrayName ?pattern?: Unsets elements of an array matching pattern, or all elements in the array if no pattern is given. Pattern matching is the same as the string match command.

Array variables are useful for related data because it is possible to search through arrays and loop through array name-value pairs.

set curve(color) 5;
set curve(name) "Curve 1";
set curve(display) "on";

To return array data by using the array name:

puts $curve(color);
5

set name "color";
puts $curve($name);
5

To find which names are available in the array:

array names curve
name display color

Arrays can also have complex names. An example is creating an array of x,y,z coordinates for a node in the model.

set xcoord 0.0;
set ycoord 1.0;
set zcoord 2.0;
set coords(1,x) $xcoord;
set coords(1,y) $ycoord;
set coords(1,z) $zcoord;

The Tcl environment comes with some default arrays. One of the more useful default arrays is the env array to obtain environment variable settings.

array names env;
Tcl returns a list of environment variables

puts "$env(Path)";
Tcl returns the Path environment variable