Using .tcl to automate assigning nodes/elements to sets without directly specifying IDs

The command you are looking for is hm_createmark.  There are all kinds of options for selecting entities with this option including by displayed.

The help file for hm_createmark is below but to short circuit what you are looking for here is an example.

hm_createmark elements 1 "advanced" "displayed"

set lst_elem_id [hm_getmark elements 1]

In HW the mark is a "container" for selecting entities and there are 2 for each entity type 1 and 2.  So the first command is adding all the displayed elements to mark 1 and the second command retrieves the element ids in mark 1 and sets them to a variable named lst_elem_id.

https://2021.help.altair.com/2021.2/hwdesktop/hwd/topics/reference/hm/hm_createmark.htm?zoom_highlight=hm_createmark

usually what i do is to use the commands mentioned by Ben, above and also

*setvalue outputblocks id=4 ids={elems 9313-30840}

Would be something like:

hm_createmark elements 1 "by comp name" "my_comp"; #select elements by component name

set lst_elem_id [hm_getmark elements 1]; #store the IDs of the elements selected in mark #1 into a list

set outputblock_name "my_output_block"; #define the name of the output block to be edited (using name it's easier than using Ids)

eval "*setvalue outputblocks name=$outputblock_name ids=\{elems $lst_elem_id\}" ; #add the elements inside the list into your output block

The eval command will first take this string and substitute the variables into it, and then run the *setvalue command.

Thank you for all your help!

I made use of some options within hm_createmark (the link to the reference page provided above was especially useful) and decided to go the route of being more specific than "advanced" "displayed" so as to avoid needing to ensure the correct visibility of components.

I also found 'set', 'eval', and 'hm_markremove' to be useful and my solution to my two issues in the original question is as follows for anyone coming across this question in the future:

1. For output blocks, where I want all the solid elements in a specific component "LatticeMesh" to be assigned to "Solid Elems":

hm_createmark elements 1 "by comp name" "LatticeMesh"
*setvalue outputblocks name="Solid Elems" ids={elems [hm_getmark elements 1]}

2. For node sets, where I wanted to select by plane:

hm_createmark nodes 1 "on plane" "0 0 0 0 1 0 0.00001 1 1"
*setvalue sets name="BOTTOM" ids={nodes [hm_getmark nodes 1]}
set nodes_rem1 [hm_getmark nodes 1]
hm_createmark nodes 1 "on plane" "0 0.01 0 0 1 0 0.00001 1 1"
*setvalue sets name="TOP" ids={nodes [hm_getmark nodes 1]}
set nodes_rem2 [hm_getmark nodes 1]

I then wanted an additional node set called SIDES to contain nodes on the sides of my cube, but to not contain nodes already contained within the TOP and BOTTOM node sets (if they were, an error with BCs would result). My solution was to follow up with:

hm_createmark nodes 2 "on plane" "0.005 0.005 0 1 0 0 0.00001 1 1"
eval hm_markremove nodes 2 $nodes_rem1, $nodes_rem2
set side_list_1 [hm_getmark nodes 2]
hm_createmark nodes 2 "on plane" "-0.005 0.005 0 1 0 0 0.00001 1 1"
eval hm_markremove nodes 2 $nodes_rem1, $nodes_rem2
set side_list_2 [hm_getmark nodes 2]
hm_createmark nodes 2 "on plane" "0 0.005 0.005 0 0 1 0.00001 1 1"
eval hm_markremove nodes 2 $nodes_rem1, $nodes_rem2
set side_list_3 [hm_getmark nodes 2]
hm_createmark nodes 2 "on plane" "0 0.005 -0.005 0 0 1 0.00001 1 1"
eval hm_markremove nodes 2 $nodes_rem1, $nodes_rem2
set side_list_4 [hm_getmark nodes 2]
set sets_name "SIDES"
eval "*setvalue sets name=$sets_name ids=\{nodes $side_list_1, $side_list_2, $side_list_3, $side_list_4\}"

which was a (perhaps tedious) method of selecting some nodes on the side of the cube based on a plane, then removing any nodes that were also contained within TOP and BOTTOM (which I assigned to nodes_rem1 and nodes_rem2) and finally adding everything to one set called SIDES.