# Why doesn't Dynamic update if it only contains a Set statement? [strict]

Consider the 3 following manually entered evaluations where a Set statement is brought to the Front-End by Dynamic:

Obviously the Set statement in Dynamic is evaluated only once, at the beginning. Despite remaining on the screen at all times and despite manual future alteration of t, t=0 does not seem to be evaluated again. According to documentation I would expect the Set statement to be evaluated each time t changes.

Why is this so? I failed to see the explanation in the neat answers Using Refresh[..] with TrackedSymbols! and advanced Dynamic documentation.

This is further surprising, considering that adding t; in the Dynamic expression, recovers systematic evaluation of t=0 when t is altered:

Later increment: following the suggestion by Kuba in the comments about using t[], here is a third case where t[0] apparently does not depend on t (like the first case) but actually updates this time.

Thank you for you help and keep on the good work.

• The statement t = 0 does not depend on the value of t. The statement t; t = 0 does, since t is evaluated. – Michael E2 Dec 8 '18 at 20:59
• Ok, it is not sufficient that t formally (lexically) appears in the expression to be updated by Dynamic. I better understand the logics now, somehow clearer than the multipage documentation, thank you. – SteppingStone Dec 8 '18 at 21:24
• Comments are not for extended discussion; this conversation has been moved to chat. – Kuba Dec 8 '18 at 22:14
• +1 -- If @Kuba & I can have an argument about the correct answer to a question, without the OP having to clarify the question at all, then the question deserves an upvote from me. :) – Michael E2 Dec 12 '18 at 20:00

I think the following two criteria explain why Dynamic[t = 0] does not update and Dynamic[t; t = 0] updates.

Suppose we have Dynamic[code] where code is some relatively simple code depending only on a symbol t. Then code will be update (executed) dynamic when

1. t is evaluated when code is evaluated and
2. t has changed its value.

There are several abstruse examples in the chat conversation, but I believe they are all explained by these criteria.

Dynamic[t = 0]: Set does not evaluated the LHS t, although it might change its value. Since criterion 1 is not satisfied, there is no update even if condition 2 is satisfied (when t = 1 is executed and changes the value of t).

Dynamic[t; t = 0]: Now t is evaluated before Set. Since criterion 1 is satisfied, there is an update when condition 2 is satisfied (when t = 1 is executed and changes the value of t).

Dynamic[t[] = 0]: Set does not evaluated the LHS t[] but the head t is evaluated. Since criterion 1 is satisfied, there is an update when condition 2 is satisfied (when t[] = 1 is executed and changes the value of t).

Dynamic[t[[1]] = 0]: Assuming t has been appropriately initialized (e.g. t = {1}), Set does not evaluated the LHS t[[1]] nor does it evaluate t. The FullForm of the LHS is Part[t, 1], and presumably the head Part is evaluated. In order to alter t, Set cannot evaluate t in this case. Since criterion 1 is not satisfied, there is no update even if condition 2 is satisfied (when t[[1]] = 1 is executed and changes the value of t).