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— title: Defining macros within macros category: programming tags: macros permalink: /FAQ-hash date: 2014-06-10 —

The way to think of this is that `##` gets replaced by `#` in just the same way that `#1` gets replaced by “whatever is the first argument”.

So if you define a macro: ```latex \newcommand\a[1]{+#1+#1+#1+} ``` or (using the TeX primitive `\def`): ```latex \def\a#1{+#1+#1+#1+} ``` and use it as `\a{b}`, the macro expansion produces “+b+b+b+”, as most people would expect.

However, if we now replace part of the macro: ```latex \newcommand\a[1]{+#1+\newcommand\x[1]{xxx#1}} ``` then `\a{b}` will give us the rather odd

+b+`\newcommand{x}[1]{xxxb}`

so that the new `\x` ignores its argument.

If we use the TeX primitive: ```latex \def\a#1{+#1+\def\x #1{xxx#1}} ``` `\a{b}` will expand to “+b+`\def\x b{xxxb}`”. This defines `\x` to be a macro _delimited_ by `b`, and taking no arguments, which is surely not what was intended!

Actually, to define `\x` to take an argument, we need ```latex \newcommand\a[1]{+#1+\newcommand\x[1]{xxx##1}} ``` or, using the TeX primitive definition: ```latex \def\a#1{+#1+\def\x ##1{xxx##1}} ``` and `\a{b}` will expand to

+b+`\def\x #1{xxx#1}`

because `#1` gets replaced by “b” and `##` gets replaced by `#`.

To nest a definition inside a definition inside a definition then you need `####1`, doubling the number of `#` signs; and at the next level you need 8 `#`s each time, and so on.