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In the description of the /tikz/shape node option on p. 216 of the TikZ & PGF manual for version 3.0.1a states:

no default, initially rectangle

It goes on to explain that this option selects

the shape either of the current node or, when this option is not given inside a node but somewhere outside, the shape of all nodes in the current scope.

What is the difference between an option that has a default value and one that has initially a certain value? If instead of initially being rectangle the /tikz/shape would be rectangle by default, what would change in its behavior and the way it can be used?

Can someone please explain and demonstrate the differences with a minimal example?

The present question is similar to this one. However that question deals with PGF (which I am unfamiliar with), whereas the current question deals with TikZ.

PGF and TikZ are two separate levels. Consider the following quote from the manual (p. 221):

Both PGF and TikZ support such multipart nodes. On the lower level, PGF provides a system for specifying that a shape consists of several parts. On the TikZ level, you specify the different node parts by using the following command:

\nodepart[<options>]{<part name>}

The point of this quote is to show that the same feature can have different manifestations in PGF vs. in TikZ and it is reasonable to ask how a certain feature manifests in one level and how it manifests in another level, and this would constitute two different questions, just as a question about TikZ's \nodepart command can coexist with a question about PGF's system for specifying that a shape consists of several parts without any of these questions being a duplicate of the other.

My question is: Is it possible to explain the difference and to give a minimal example using only TikZ concepts and syntax without resorting to low level PGF? Is the difference perceptible from a high-level user's perspective who only knows TikZ? (I am such a user.)

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Evan Aad
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    Both tikz and pgf use the same key management system, so I think that the question you marked (https://tex.stackexchange.com/questions/50856/pgf-keys-differences-between-initial-and-default) is really a duplicate. – Rmano Jul 11 '17 at 07:46
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    This is a duplicate of the previous one as tikz is layered over pgf (they have the same manual) and uses pgfkeys. – David Carlisle Jul 11 '17 at 07:50
  • @Rmano: The question is: Is it possible to explain the difference and to give a minimal example using only TikZ concepts and syntax without resorting to low level PGF tinkering? Is the difference perceptible in code that uses only TikZ? It's like asking whether a certain Java bytecode concept or construct can be expressed or distinguished using the Java language. – Evan Aad Jul 11 '17 at 07:50
  • As the shape key has not default value, you can't write \node[shape]{...}. You must provide a value (like \node[shape=circle]{...}). As shape has an initial value (rectangle), with \node{...} you get a rectangular node. – Paul Gaborit Jul 11 '17 at 07:54
  • @Evan, tried it. – Rmano Jul 11 '17 at 07:54
  • It just says the key is initialized with the value rectangle but does not have a default value. – percusse Jul 11 '17 at 08:00
  • shape doesn't use /.initial or /.default handlers. – percusse Jul 11 '17 at 08:10
  • I suspect that there are two different things here --- one of them are the /.initial and /.default keys, and another one what the manual means when they say "default whatever, initial value whatever else" --- that can be implemented with that keys or not. – Rmano Jul 11 '17 at 09:18

2 Answers2

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This answer is for explaining what the TikZ/PGF manual means for default/initial values of a keyword. It may or may not have any relation with the usage of .default and .initial keys depending on the specific implementation.

For example for shape:

  • no default means you can't say 

     \node[shape, red]{...}

    because there is no default (in this case the key is defined so that it must have an argument, so it will error out if you do; thanks, @percusse). If, for example, it had circle as default, that means that you could have used the aforementioned command to have a circle-shaped node;

  • initial means that if you say

    \node[color=red] {...}

    without mentioning shape you'll have a rectangle.

Another example could be (straight from the manual):

/pgf/tips=value (default true, initially on draw) alias /tikz/tips

This key governs in what situations arrow tips are added to a path. The following values are permissible:

  • true (the value used when no value is specified)
  • proper
  • on draw (the initial value, if the key has not yet been used at all)
  • on proper draw
  • never or false (same effect)

[...]

(although I don't understand what the comment on the last item means --- just that never and false are the same?)

Rmano
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  • Thanks. Just to make sure I understand: if a certain option x doesn't have an initial value, it means that given an object a that this option applies to, there isn't necessarily an answer to the question: "What is the value of a's option x?", because a may not have this option at all. And on the contrary, if an option has an initial value, then it is meaningful to ask the above question for every object a that this option applies to, because a will always have this option, the only unknown is the value this option has. Correct? – Evan Aad Jul 11 '17 at 08:05
  • Another question: can an option have both an initial and a default values? If so, which one "wins"? For instance, suppose a node had an option my_opt with initial value a and default b. What would be the value of the option in the following cases: (1) node [] (the option is not specified)? (2) node [my_opt] (the option is specified without an explicit value)? – Evan Aad Jul 11 '17 at 08:18
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This is the case where it sounds like the problem is related to keys but shape is a pretty legacy keyword and the way it is defined is pretty straightforward. 

\tikzoption{shape}{\edef\tikz@shape{#1}}

(\tikzoption later became \tikzset. So don't use it)

Then at the initialization of TikZ package we see 

\def\tikz@shape{rectangle}%

Note that this is the global scope so if you don't fiddle with it every TikZ picture will inherit this default value. This is not related to /.initial and /.default problem. It's the manual's classification that makes it looks like key issue.

In the manual if you see no default this means you need to supply a value. If you see no value then,... well... you don't need to provide a value. If you insist on providing a value sometimes you get an error, sometimes not.

Example, 

\node[matrix of math nodes={4}{5}] {\tan \\ \sin};

will not error. Because every style/code handler intrinsically accepts arguments. If not explicitly set by /.value forbidden handler then nothing happens.

Initially <...> means if you don't provide anything else I'm going to use <...>.

As usual, exceptions apply...

One final note about TikZ/PGF difference. All TikZ is converted to PGF code so there are no separate functionalities. It's a matter of syntax.

percusse
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