fb_reaction_name()

name: fb_reaction_name(o, n, i, default)

• name the name the student gave the reaction force at object i, or default
• o the meclib object list (stackjson_parse(objects))
• n the names array (content of the names input field)
• i index of the support or contact object that is replaced by the reaction
• default value returned if no single reaction force can be found at i

Source

fb_reaction_name() returns the name of the reaction force that meclib matched to object i. It returns default if

• n[i] is not a list of matched loads (the object is not deactivated or has no reaction yet),
• more than one load is matched to object i, or
• the matched load is not a force (e.g. a moment).

It never produces feedback text and has no [text, OK] result. It is meant for cosmetic, dynamic labels, e.g. to show the student's own name of the reaction next to a numeric input field, falling back to a fixed placeholder as long as the reaction is not yet resolved. Check the reaction itself with Feedback for "contact" (fb_contact()), Feedback for unidirectional supports (fb_unidir()) or fb_bar on the same index, and its name with fb_name().

Example: label in front of an input field

Taken from demo_questions/contact_demo_01.xml. The label is the feedback of a separate PRT FA_label, placed in front of the input field. The PRT gives no score; it only shows the name.

Question text:

[[feedback:FA_label]] [[input:S_FA]] [[validation:S_FA]] [[feedback:FA]]

PRT FA_label, feedback style Formative (no score, no symbols):

Feedback variables:

obj: stackjson_parse(objects);
nameA: fb_reaction_name(obj, names, i_A, "A");

Node 1: SAns true, TAns true, answer test AlgEquiv, quiet, score 0; feedback in both branches:

\({@nameA@} = \)

Here the feedback is a dynamic part of the question text, so its format may stay Markdown (the yellow feedback background is not wanted here).

Situation at object i_A	label
contact A shown, or no force on it yet	A = (the default)
one force named F_A on the deactivated contact	F_A =
two forces on the deactivated contact	A = (the default)
a moment on the deactivated contact	A = (the default)