gap.cbmd

---
title: "Codebraid with GAP (Groups, Algorithms, Programming - a System for Computational Discrete Algebra)"
---


## Inline code


### Run

Inline code with `.cb-run` gives raw stdout.

`Print(1 + 2);`{.gap .cb-run session=firsttest example=true}


### Expression and inline notebook

Inline code with `.cb-expr` evaluates an expression and then inserts the raw
output into the document, where it is interpreted as Markdown.  Inline code
with `.cb-nb` (`nb` is short for `notebook`) is similar, except output is
shown verbatim.  Notice that since these are expressions, a trailing semicolon
must not be used. 

`StringFormatted("*{}*", Sum([1..10]))`{.gap .cb-expr example=true}

`StringFormatted("*{}*", Sum([1..10]))`{.gap .cb-nb example=true}


### Stderr

In the event of an error, inline code automatically shows stderr by default.
This code is executed in its own session, `inline_error`, so that it does
not impact other examples.

`1 + "a"`{.gap .cb-run session=inline_error example=true}


### Source errors

A message is also displayed for errors in the Markdown source.  This usually
includes the name of the document source and the approximate line number.

`Print(1 + 2);`{.gp .cb-run session=inline_source_error example=true}


### Code

Inline code with `.cb-code` simply displays the code.  Nothing is executed.

`Print("Hello from GAP!");`{.gap .cb-code example=true}

The output is identical to that of
```
`Print("Hello from GAP!");`{.gap}
```
so `.cb-code` is only really useful when it is combined with other Codebraid
features.  For example, it is possible to give code with `.cb-code` a `name`,
and then copy it by `name` into a separate location where it is executed.  See
[Copying code and output](#copying).



## Block code


### Run

Code blocks with `.cb-run` give raw stdout.  There is continuity between code
blocks so long as they are in the same session; variables persist.

```{.gap .cb-run session=hello example=true}
x := "Hello from *GAP!*";
```

```{.gap .cb-run session=hello example=true}
Print(x);
```


### Stderr

Code blocks show stderr automatically by default.

```{.gap .cb-nb session=block_error example=true}
x := 12;
y := x + "x";
```


### Source errors

A message is also displayed for errors in the Markdown source.  This usually
includes the name of the document source and the approximate line number.

```{.gap .cb-ruuun session=block_source_error example=true}
print(1 + 2);
```


### Code

Code blocks with `.cb-code` simply display the code.  Nothing is executed.

```{.gap .cb-code example=true}
Print("Hello from GAP!");
```

The output is essentially identical to that of a normal code block; the only
differences are some default display options, like line numbering.  Thus
`.cb-code` is primarily useful when it is combined with other Codebraid
features.  For example, it is possible to give code with `.cb-code` a `name`,
and then copy it by `name` into a separate location where it is executed.  See
[Copying code and output](#copying).



## Other options

By default, stdout and stderr are only shown if they are non-empty.  In some
situations, it may be useful to represent empty output visually as
confirmation that there indeed was none.

```{.gap .cb-run show=code+stdout+stderr:verbatim_or_empty example=true}
x := 1 + 2;;
```

It is also possible to selectively hide output from a code chunk.

```{.gap .cb-nb hide=stdout example=true}
Print(x);
```

`hide` takes any combination of `code`, `stderr`, and `stdout`, or simply
`all`.



## Advanced features {#advanced}

### Copying code and output {#copying}

Code chunks can be named using the `name` keyword, which takes an
identifier-style name.  Then the `copy` keyword can be used in other code
chunks to copy a named chunk or a combination of named chunks.  When `copy` is
used with `cb-run`, `cb-expr`, or `cb-nb`, the code is copied and then
executed as if it had been entered directly.  When `copy` is used with
`cb-code`, the code is copied and displayed, but nothing is executed.  When
`copy` is used with the special command `cb-paste`, both the code and output
are copied but nothing is executed.  This is useful in executing code and then
showing snippets of the code and/or output in different parts of a document.

`copy` works with named code chunks anywhere in a document; a named code chunk
does not have to appear in a document before the location where it is copied.
A code chunk that copies another code chunk can itself have a `name`, and then
itself be copied.

The next two code chunks are named.

```{.gap .cb-run name=part1 session=copy_source example=true}
rnums := List([1..10], x -> Random([1..x]));;
```

```{.gap .cb-run name=part2 session=copy_source example=true}
PrintFormatted("Random numbers: {}", rnums);
```

Now the code and output of the previous two code chunks are copied and
combined.  Because the content for this code block is copied from other code
chunks, the code block itself should be empty, or may alternately contain a
space or underscore as a placeholder.

```{.gap .cb-paste copy=part1+part2 show=code+stdout example=true}
```

It would also be possible to copy and re-execute the code.

```{.gap .cb-run copy=part1+part2 session=copied show=code+stdout:raw example=true}
```

Another option is to display code, and then copy and execute it later.

```{.gap .cb-code name=hello example=true}
Print("Hello from GAP!");
```

```{.gap .cb-nb copy=hello example=true}
```


### Including external files

External files can be included for display or execution using `include_file`.
The default encoding is UTF-8; other encodings can be selected with
`include_encoding`.  Instead of including the entire file, it is possible to
include only a selected range of lines with `include_lines`.  It is also
possible to include part of a file that matches a regular expression with
`include_regex`, as shown in the example.  Other options for controlling what
is included based on starting or ending literal strings or regular expressions
can be found in the documentation.

```{.html .cb-code include_file=gap.html include_regex="<header.*?/header>" example=true}
```

When `include_file` is used with `cb-code`, the included code is simply
displayed.  When `include_file` is used with `cb-run` or another command that
executes code, the included code is executed just as if it had been entered
directly.