[Tex/LaTex] the difference between \dots and \ldots

macrospunctuation

Is there a difference between \dots and \ldots, the ellipsis?

If yes: Which? When to use \dots and when to use \ldots?

If no: Why are there both at all?

Best Answer

in latex.ltx, \ldots is equated (\let) to \dots. so in text, it's the same, and you can use either.

with amsmath, the situation is more complicated; \dots tries to determine by context (only in math mode) whether to use \ldots or \cdots, with several more options for the author if amsmath doesn't get it right.

so in general, it's easier to just use \dots and not have to remember the details. but if the alternative chosen by amsmath isn't appropriate, the original commands can still be used explicitly.

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[Tex/LaTex] the difference between \let and \def

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 \let equal 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).
\fooi expands directly to the characters hello (because \bar initially did, and \fooi was defined to have the same value).
\fooii, in contrast, expands to \bar, which is then immediately reexamined and reexpanded. In the first case, \bar expands to hello and in the second case to goodbye. The definition of \fooii hasn't changed, but \bar has 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.

[Tex/LaTex] the difference between \def and \newcommand

\def is a TeX primitive, \newcommand is a LaTeX overlay on top of \def. The most obvious benefits of \newcommand over \def are:

\newcommand checks whether or not the command already exists
\newcommand allows you to define an optional argument

In general, anything that \newcommand does can be done by \def, but it usually involves a little trickery and unless you want something that \newcommand can't do, there's no point in reinventing the wheel.

In answer to your particular points:

Yes and No. A command defined using \def has to know exactly what its options are, and how they will be presented to it. However, a TeX command can be a bit clever and examine things other than its arguments. The way that optional commands are handled is to look at the next character in the stream and if it is [ then one version of the command is called, whereas if it isn't then another is called. So if you're prepared to do some hackery, then optional arguments are possible via \def, but it isn't half so easy as \newcommand.
No. Since \def doesn't check for a command already being defined, you can redefine stuff simply using \def again. To get the checking, you need to use the \@ifundefined command (note the ampersat (@) sign!).
No!

There's probably lots more to be said on the differences, but that's enough from me.

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[Tex/LaTex] the difference between \let and \def

[Tex/LaTex] the difference between \def and \newcommand

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