What is difference between \box and \unvbox or between \box and \unhbox.
[Tex/LaTex] difference between \box and \unvbox or between \box and \unhbox
boxestex-core
Related Solutions
The difference is in the time at which the ‘right hand side’ is evaluated.
Thus \let\foo\bar defines \foo to have the value that \bar had at the point of definition. On the other hand, \def\foo{\bar} in effect defines \foo to have the value that \bar has at the point of use.
Consider:
\def\bar{hello}
\let\fooi\bar
\def\fooii{\bar}
\fooi +\fooii
\def\bar{goodbye}
\fooi +\fooii
This produces
hello+hello
hello+goodbye
This is a simple process.
However it's also a subtle one, so it might be worth highlighting a few key points:
When TeX encounters control sequences such as
\fooi, it evaluates them; if these are macros (that is, they have been defined by\def, or\letequal to something which was defined by\def), then the result is that they will expand to other tokens, which TeX will then examine in turn, and so on, recursively, until what's left is either ‘primitive’ control sequences or letters (I'm simplifying a little bit).\fooiexpands directly to the charactershello(because\barinitially did, and\fooiwas defined to have the same value).\fooii, in contrast, expands to\bar, which is then immediately reexamined and reexpanded. In the first case,\barexpands tohelloand in the second case togoodbye. The definition of\fooiihasn't changed, but\barhas been redefined in between.Getting a clear idea of the process of this recursive expansion is very helpful when learning how to develop and debug TeX macros.
\edef expands the argument, whereas \let doesn't. Here is an example to illustrate the difference:
\def\txt{a}
\def\foo{\txt}
\let\bar\foo
\bar -> a
\def\txt{b}
\bar -> b
However,
\def\txt{a}
\def\foo{\txt}
\edef\bar{\foo}
\bar -> a
\def\txt{b}
\bar -> a
There are also other differences, say, the arguments and so on. But how to expansion may be the most important(?).
This is an interesting question. May I expand the question further more?
What is the difference between
\letand\expandafter\def\expandafter\foo\expandafter? Do they always behave the same?
\def\txt{bar}
\def\foo{\txt}
\expandafter\def\expandafter\bar\expandafter{\foo}
\let\BAR\foo
{\tt \string\bar = \meaning\bar}\par
{\tt \string\BAR = \meaning\bar}
Sneaky inline answer to this rhetorical question, since I wrote the original question :)
If you are doing this on macros that take no arguments, then the difference between them is negligible. OTOH, you cannot use the
\expandafter\def...construct if you're trying to copy the definition of a macro that takes arguments.In fact, this brings to light one of the aspects that I was hoping people would discuss here.
\letcreates a literal copy of a macro at the instant that it is executed, whereas\edefwill take the contents of the macro and expand it recursively to create a new macro entirely. When you are only using the macros as places to store data (such as\def\name{Will}) then the differences are largely inconsequential, but when the macros are being used as ‘functions’ that take arguments or have contents that have various expansion restrictions applied to it (with\protected,\noexpand, and so on) then the differences can be very important indeed.
Best Answer
If you go
\setbox0\hbox{abc}then box 0 is a box register with an hbox that contains three items, the character nodes for a, b and c.So if you use
\box0it adds a single item to the current list, an hbox.However if you use
\unhbox0then you add the list that was contained in the box, not the box itself, so adds the nodes for a, b and c.See
the first paragraph is
with an hbox following the space after xxx, but the second paragraph is
with a, b, c added directly with no containing box.
adding the list directly means that any glue can take part in glue stretching and linebreaking can occur (almost) as if the contents had never been boxed.
Note that a box can always be placed in a vertical or horizontal list, it just affects how the box is stacked, but you can only unbox a box into the right kind of list.
If you change the example to
You get
as the
\box0goes directly on to the vertical list without starting a paragraph, so the followingzzzstarts a new paragraph underneath. Conversely the\unhbox0starts a horizontal list so starts a paragraph so comes after a paragraph indentation with the following zzz in the same horizontal list.