This chapter describes the GAP help system. The help system lets you read the manual interactively.
?
section
The help command ?
displays the section with the name section on the
screen. For example ?Help
will display this section on the screen.
You should not type in the single quotes. They are only used in help
sections to delimit text that you should enter into GAP or that GAP
prints in response. When the whole section has been displayed the normal
GAP prompt gap>
is shown and normal GAP interaction resumes.
When there are several manual sections that match section a list of topics as described under Help Index is displayed.
Section Reading Sections tells you what actions you can perform
while you are reading a section. You tell GAP to display this
section by entering ?Reading Sections
, without quotes.
Section Format of Sections describes the format of sections and the
conventions used,
Browsing through the Sections lists the commands you use to flip
through sections, Redisplaying a Section describes how to read a
section again, Abbreviating Section Names tells you how to avoid typing
the long section names, and Help Index describes the index command.
If the section is longer than the size of your screen, GAP stops after displaying a full screen and displays
-- <space> for more, <q> to quit --
If you press space GAP displays the next full screen of lines of the
section and then stops again.
This goes on until the whole section has been displayed,
at which point GAP will return immediately to the main GAP loop.
Pressing f
has the same effect as space.
You can also press b
which will scroll back to the previous full screen
of lines of the section.
If you press b
when GAP is displaying the top of a section,
GAP will ring the bell.
You can also press q
to quit and return immediately back to the main
GAP loop without reading the rest of the section.
The size of the screen is set using information obtained from the
operating system, if possible, or to 24 lines if not.
If this does not produce the correct results for your system,
you may wish to set the number of lines with the -y
rows option
(see Command Line Options) when you start GAP.
This section describes the format of sections when they are displayed on the screen and the special conventions used. For general conventions about manual sections and the format of sections in the printed manual, see Manual Conventions.
As you can see, GAP prints a header line containing the name of the section on the left and the name of the chapter on the right.
(If this header line ends in (not loaded)
the documentation belongs to a
share package, which has not yet been loaded. See section Loading a Share Package for further information.)
<text>Text enclosed in angle brackets is used for arguments in the descriptions of functions and for other place holders. It means that you should not actually enter this text into GAP but replace it by appropriate text depending on what you want to do. For example when we write that you should enter
?
section
to see the section with the name section,
section serves as a place holder, indicating that you can enter the
name of the section that you want to see at this place.
`text'Text enclosed in single quotes is used for names of variables and functions and other text that you may actually enter into your computer and see on your screen. The text enclosed in single quotes may contain place holders enclosed in angle brackets as described above. For example when the help text for
IsPrime
says that the form of the call is
IsPrime(
n )
this means that you should actually
enter the strings ``IsPrime('' and ``)'', without the quotes,
but replace the
n
with the number (or expression)
that you want to test.
"text"Text enclosed in double quotes is used for cross references to other parts of the manual. So the text inside the double quotes is the name of another section of the manual. This is used to direct you to other sections that describe a topic or a function used in this section. So, for example, Browsing through the Sections is a cross reference to the next section.
> Oper( <arg1>, <arg2>[, <opt>] ) Fstarts a subsection on the command
Oper
that takes two arguments arg1
and arg2 and an optional third argument opt.
The letter F
at the end
indicates that the command is a simple function.
The letters A
, P
, O
, C
, R
, and V
indicate
``Attribute'', ``Property'', ``Operation'', ``Category'', ``Representation''
(see Chapter Types of Objects), or ``Variable'', respectively.
_
and ^
In mathematical formulas the underscore and the caret are used to denote
subscription and superscription. Ordinarily they apply only to the very
next character following, unless a whole expression enclosed in
parentheses follows. So, for example, x_1^(i+1)
denotes the variable x
with subscript 1 raised to the power i+1
.
Longer examples are usually paragraphs of their own.
Everything on the lines with the prompts gap>
and >
, except
the prompts themselves of course, is the input you have to type;
everything else is GAP's response.
gap> ?Format of Sections Format of Sections ______________________________________ Environment This section describes the format of sections when they are displayed on the screen and the special conventions used. ...
2.4 Browsing through the Sections
The help sections are organized like a book into chapters. This should not surprise you, since the same source is used both for the printed manual and the online help. Just as you can flip through the pages of a book there are special commands to browse through the help sections.
?>
?<
The two help commands ?<
and ?>
correspond to the flipping of pages.
?<
takes you to the section preceding the current section and displays
it, and ?>
takes you to the section following the current section.
?>>
?<<
?<<
is like ?<
, only more so. It takes you back to the first
section of the current chapter, which gives an overview of the sections
described in this chapter. If you are already in this section ?<<
takes you to the first section of the previous chapter. ?>>
takes you
to the first section of the next chapter.
?-
?+
GAP remembers the sections that you have read. ?-
takes you to the
one that you have read before the current one, and displays it again.
Further applications of ?-
take you further back in this history. ?+
reverses this
process, i.e., it takes you back to the section that you have read
after the current one. It is important to note that ?-
and ?+
do
not alter the history like the other help commands.
2.5 Changing the Way the Help Pages are Displayed
If you have installed an html version of the manual you can alternatively use an html browser to display the manual sections. The command
SetHelpViewer(
device)
will select a method described by the string device to display the help
pages online. Currently device can be "screen"
for the default built-in
text browser, "less"
(only under UNIX) to use less
for paging,
"netscape"
for the netscape HTML browser and "lynx"
for the
lynx HTML browser. Both HTML browsers will only work under UNIX.
On an Apple Macintosh you can use an HTML browser by calling SetHelpViewer
with the parameter "Internet Config"
.
See section Features of GAP for MacOS for details about this.
If you want the HTML help to be the default, you should call this function
in your .gaprc
file (see the sections on operating
system dependent features in chapter Installing GAP).
Note that you need to set up the symbol
font properly in your web browser
to get a correct display of mathematical formulae (see section HTML Font Setup).
?
The help command ?
followed by no section name redisplays the last help
section again. So if you reach the bottom of a long help section and have
already forgotten what was mentioned at the beginning, or, for example, the
examples do not seem to agree with your interpretation of the
explanations, use ?
to read the whole section again from the beginning.
When ?
is used before any section has been read, GAP displays a
Welcome to GAP
.
2.7 Abbreviating Section Names
Upper and lower case in section are not distinguished, so typing either
?Abbreviating Section Names
or ?abbreviating section names
will show
the section you are currently reading.
Each word in section may be abbreviated. So instead of typing
?abbreviating section names
you may also type ?abb sec nam
, or even ?a
s n
. You must not omit the spaces separating the words. For each word in
the section name you must give at least the first character. As another
example you may type ?el oper for int
instead of ?elementary operations
for integers
, which is especially handy when you can not remember whether
it was operations
or operators
.
If an abbreviation matches multiple section names a list of all these section names is displayed.
??
topic
The operator ??
looks up topic in GAP's index and prints all the
index entries that contain the substring topic.
Then you can decide which section is the one you are actually interested
in and request this one.
gap> ??read Help: several entries match this topic [1] reference:read [2] reference:reading sections [3] reference:readlib [4] reference:isreadablefile [5] reference:read ...
The first part of each line is a reference number. Then follows the part of the manual which contains the section and finally the actual name of the (sub)section. All names are converted to lower case.
The order of the sections corresponds to their order in the GAP manual, so that related sections should be adjacent.
You can then either refer to the desired subsection by its name or simply
use ?
nr to look at the topic with the reference number nr. So in the
above example
?3
would display the section on ReadLib
.
When referring to sections by their name you can usually omit the part of the manual unless several parts contain the same section names.
If there are several subsections which have exactly the same name, a number in parentheses is added to the name to distinguish these.
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