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— title: Defining macros within macros category: programming tags: macros permalink: /FAQ-hash date: 2014-06-10 —
# Defining macros within macros
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+<code class=“verb”>\def\x bb;xxxb}</code>. 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+<code class="verb">\def\x #1{xxx#1}</code>
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.
