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# 6 Main Loop and Break Loop

### Sections

1. Main Loop
2. View and Print
3. Break Loops
4. Variable Access in a Break Loop
5. Error
6. ErrorCount
7. Leaving GAP
8. Line Editing
9. Editing Files
10. Editor Support
11. SizeScreen

This chapter is a first of a series of chapters that describe the interactive environment in which you use GAP.

## 6.1 Main Loop

The normal interaction with GAP happens in the so-called read-eval-print loop. This means that you type an input, GAP first reads it, evaluates it, and then shows the result. Note that the term print may be confusing since there is a GAP function called `Print` (see View and Print) which is in fact not used in the read-eval-print loop, but traditions are hard to break. In the following, whenever we want to express that GAP places some characters on the standard output, we will say that GAP shows something.

The exact sequence in the read-eval-print loop is as follows.

To signal that it is ready to accept your input, GAP shows the prompt `gap>`. When you see this, you know that GAP is waiting for your input.

Note that every statement must be terminated by a semicolon. You must also enter return (i.e., strike the ``return'' key) before GAP starts to read and evaluate your input. (The ``return'' key may actually be marked with the word `Enter` and a returning arrow on your terminal.) Because GAP does not do anything until you enter return, you can edit your input to fix typos and only when everything is correct enter return and have GAP take a look at it (see Line Editing). It is also possible to enter several statements as input on a single line. Of course each statement must be terminated by a semicolon.

It is absolutely acceptable to enter a single statement on several lines. When you have entered the beginning of a statement, but the statement is not yet complete, and you enter return, GAP will show the partial prompt `>`. When you see this, you know that GAP is waiting for the rest of the statement. This happens also when you forget the semicolon `;` that terminates every GAP statement. Note that when return has been entered and the current statement is not yet complete, GAP will already evaluate those parts of the input that are complete, for example function calls that appear as arguments in another function call which needs several input lines. So it may happen that one has to wait some time for the partial prompt.

When you enter return, GAP first checks your input to see if it is syntactically correct (see Chapter The Programming Language for the definition of syntactically correct). If it is not, GAP prints an error message of the following form

    gap> 1 * ;
    Syntax error: expression expected
    1 * ;
        ^

The first line tells you what is wrong about the input, in this case the `*` operator takes two expressions as operands, so obviously the right one is missing. If the input came from a file (see Read), this line will also contain the filename and the line number. The second line is a copy of the input. And the third line contains a caret pointing to the place in the previous line where GAP realized that something is wrong. This need not be the exact place where the error is, but it is usually quite close.

Sometimes, you will also see a partial prompt after you have entered an input that is syntactically incorrect. This is because GAP is so confused by your input, that it thinks that there is still something to follow. In this case you should enter `;`return`` repeatedly, ignoring further error messages, until you see the full prompt again. When you see the full prompt, you know that GAP forgave you and is now ready to accept your next -- hopefully correct -- input.

If your input is syntactically correct, GAP evaluates or executes it, i.e., performs the required computations (see Chapter The Programming Language for the definition of the evaluation).

If you do not see a prompt, you know that GAP is still working on your last input. Of course, you can type ahead, i.e., already start entering new input, but it will not be accepted by GAP until GAP has completed the ongoing computation.

When GAP is ready it will usually show the result of the computation, i.e., the value computed. Note that not all statements produce a value, for example, if you enter a `for` loop, nothing will be printed, because the `for` loop does not produce a value that could be shown.

Also sometimes you do not want to see the result. For example if you have computed a value and now want to assign the result to a variable, you probably do not want to see the value again. You can terminate statements by two semicolons to suppress showing the result.

If you have entered several statements on a single line GAP will first read, evaluate, and show the first one, then read, evaluate, and show the second one, and so on. This means that the second statement will not even be checked for syntactical correctness until GAP has completed the first computation.

After the result has been shown GAP will display another prompt, and wait for your next input. And the whole process starts all over again. Note that if you have entered several statements on a single line, a new prompt will only be printed after GAP has read, evaluated, and shown the last statement.

In each statement that you enter, the result of the previous statement that produced a value is available in the variable `last`. The next to previous result is available in `last2` and the result produced before that is available in `last3`.

    gap> 1; 2; 3;
    1
    2
    3
    gap> last3 + last2 * last;
    7

Also in each statement the time spent by the last statement, whether it produced a value or not, is available in the variable `time`. This is an integer that holds the number of milliseconds.

## 6.2 View and Print

- `View(` obj1`,` obj2`... ) F`

  `View` shows the objects obj1, obj2... etc. in a short form on the standard output. `View` is called in the read--eval--print loop, thus the output looks exactly like the representation of the objects shown by the main loop. Note that no space or newline is printed between the objects.

- `Print(` obj1`,` obj2`... ) F`

  Also `Print` shows the objects obj1, obj2... etc. on the standard output. The difference compared to `View` is in general that the shown form is not required to be short, and that in many cases the form shown by `Print` is GAP readable.

      gap> z:= Z(2);
      Z(2)^0
      gap> v:= [ z, z, z, z, z, z, z ];
      <a GF2 vector of length 7>
      gap> Print( v );
      [ Z(2)^0, Z(2)^0, Z(2)^0, Z(2)^0, Z(2)^0, Z(2)^0, Z(2)^0 ]gap>

  Another difference is that `Print` shows strings without the enclosing quotes, so `Print` can be used to produce formatted text on the standard output (see also chapter Strings and Characters). Some characters preceded by a backslash, such as `\n`, are processed specially (see chapter Special Characters). `PrintTo` can be used to print to a file (see PrintTo).

      gap> for i in [1..5] do
      >      Print( i, " ", i^2, " ", i^3, "\n" );
      >    od;
      1 1 1
      2 4 8
      3 9 27
      4 16 64
      5 25 125

      gap> g:= SmallGroup(12,5);
      <pc group with 3 generators>
      gap> Print(g);
      Group( [ f1, f2, f3 ] )gap>
      gap> View(g);
      <pc group with 3 generators>gap>

- `ViewObj(` obj`) O`
- `PrintObj(` obj`) O`

  The functions `View` and `Print` actually call the operations `ViewObj` and `PrintObj`, respectively, for each argument. By installing special methods for these operations, it is possible to achieve special printing behavior for certain objects (see chapter Method Selection in the programmer's manual). The only exceptions are strings (see Chapter Strings and Characters), for which the default `PrintObj` and `ViewObj` methods as well as the function `View` print also the enclosing doublequotes, whereas `Print` strips the doublequotes.

  The default method for `ViewObj` is to call `PrintObj`. So it is sufficient to have a `PrintObj` method for an object in order to `View` it. If one wants to supply a ``short form'' for `View`, one can install additionally a method for `ViewObj`.

- `Display(` obj`) O`

  Displays the object obj in a nice, formatted way which is easy to read (but might be difficult for machines to understand). The actual format used for this depends on the type of obj. Each method should print a newline character as last character.

      gap> c:= CharacterTable( "A5" );
      CharacterTable( "A5" )
      gap> Display(c);
           2  2  2  .  .  .
           3  1  .  1  .  .
           5  1  .  .  1  1

             1a 2a 3a 5a 5b
          2P 1a 1a 3a 5b 5a
          3P 1a 2a 1a 5b 5a
          5P 1a 2a 3a 1a 1a

      X.1     1  1  1  1  1
      X.2     3 -1  .  A *A
      X.3     3 -1  . *A  A
      X.4     4  .  1 -1 -1
      X.5     5  1 -1  .  .

      A = -E(5)-E(5)^4 = (1-ER(5))/2 = -b5

  One can assign a string to an object that `Print` will use instead of the default used by `Print`, via `SetName` (see SetName). Also, `Name` (see Name) returns the string previously assigned to an object for printing, via `SetName`. The following is an example in the current context of character tables.

      gap> c:= CharacterTable( "A5" );
      CharacterTable( "A5" )
      gap> SetName( c, "tblA5" );  c;
      tblA5
      gap> Name( c );
      "tblA5"

  ## 6.3 Break Loops

  When an error has occurred or when you interrupt GAP (usually by hitting ctrl-`C`) GAP enters a break loop, that is in most respects like the main read eval print loop (see Main Loop). That is, you can enter statements, GAP reads them, evaluates them, and shows the result if any. However those evaluations happen within the context in which the error occurred. So you can look at the arguments and local variables of the functions that were active when the error happened and even change them. The prompt is changed from `gap>` to `brk>` to indicate that you are in a break loop.

      gap> 1/0;
      Rational operations: <divisor> must not be zero
      not in any function
      Entering break read-eval-print loop ...
      you can 'quit;' to quit to outer loop, or
      you can replace <divisor> via 'return <divisor>;' to continue

  If errors occur within a break loop GAP enters another break loop at a deeper level. This is indicated by a number appended to `brk`:

      brk> 1/0;
      Rational operations: <divisor> must not be zero
      not in any function
      Entering break read-eval-print loop ...
      you can 'quit;' to quit to outer loop, or
      you can replace <divisor> via 'return <divisor>;' to continue
      brk_02>

  There are two ways to leave a break loop.

- `quit`

  The first is to quit the break loop. To do this you enter `quit;` or type the eof (end of file) character, which is usually ctrl-`D` except when using the `-e` option (see Section Command Line Options). Note that GAP code between `quit;` and the end of the input line is ignored.

      brk_02> quit;
      brk>

  In this case control returns to the break loop one level above or to the main loop, respectively. So iterated break loops must be left iteratively. Note also that if you type `quit;` from a `gap>` prompt, GAP will exit (see Leaving GAP).

  Note: If you leave a break loop with `quit` without completing a command it is possible (though not very likely) that data structures will be corrupted or incomplete data have been stored in objects. Therefore no guarantee can be given that calculations afterwards will return correct results! If you have been using options `quit`ting a break loop generally leaves the options stack with options you no longer want. The function `ResetOptionsStack` (see ResetOptionsStack) removes all options on the options stack, and this is the sole intended purpose of this function.

- `return [`obj`];`

  The other way is to return from a break loop. To do this you type `return;` or `return` expr`;`. If the break loop was entered because you interrupted GAP, then you can continue by typing `return;`. If the break loop was entered due to an error, you may have to modify the value of a variable before typing `return;` (see the example for IsDenseList) or you may have to return a value (by typing: `return` value`;`) to continue the computation; in any case, the message printed on entering the break loop will tell you which of these alternatives is possible. For example, if the break loop was entered because a variable had no assigned value, the value to be returned is often a value that this variable should have to continue the computation.

      brk> return 9;  # we had tried to enter the divisor 9 but typed 0 ...
      1/9
      gap>

- `OnBreak V`

  By default, when a break loop is entered, GAP prints a trace of the innermost 5 commands currently being executed. This behaviour can be configured by changing the value of the global variable `OnBreak`. When a break loop is entered, the value of `OnBreak` is checked. If it is a function, then it is called with no arguments. By default, the value of `OnBreak` is `Where` (see Where).

      gap> OnBreak := function() Print("Hello\n"); end;
      function(  ) ... end

      gap> Error("!\n");
      Error, !
      Hello
      Entering break read-eval-print loop ...
      you can 'quit;' to quit to outer loop, or
      you can 'return;' to continue
      brk> quit;

  In cases where a break loop is entered during a function that was called with options (see Chapter Options Stack), a `quit;` will also cause the options stack to be reset and an `Info`-ed warning stating this is emitted at `InfoWarning` level 1 (see Chapter Info functions).

  Note that for break loops entered by a call to `Error`, the lines after ```Entering break read-eval-print loop ...`'' and before the `brk>` prompt can also be customised, namely by redefining `OnBreakMessage` (see OnBreakMessage).

  Also, note that one can achieve the effect of changing `OnBreak` locally. As mentioned above, the default value of `OnBreak` is `Where`. Thus, a call to `Error` (see Error) generally gives a trace back up to five levels of calling functions. Conceivably, we might like to have a function like `Error` that does not trace back without globally changing `OnBreak`. Such a function we might call `ErrorNoTraceBack` and here is how we might define it. (Note `ErrorNoTraceBack` is not a GAP function.)

      gap> ErrorNoTraceBack := function(arg) # arg is a special variable that GAP
      >                                      # knows to treat as a list of arg'ts
      >      local SavedOnBreak, ENTBOnBreak;
      >      SavedOnBreak := OnBreak;        # save the current value of OnBreak
      >
      >      ENTBOnBreak := function()       # our `local' OnBreak
      >      local s;
      >        for s in arg do
      >          Print(s);
      >        od;
      >        OnBreak := SavedOnBreak;      # restore OnBreak afterwards
      >      end;
      >
      >      OnBreak := ENTBOnBreak;
      >      Error();
      >    end;
      function( arg ) ... end

  Here is a somewhat trivial demonstration of the use of `ErrorNoTraceBack`.

      gap> ErrorNoTraceBack("Gidday!", " How's", " it", " going?\n");
      Error, Gidday! How's it going?
      Entering break read-eval-print loop ...
      you can 'quit;' to quit to outer loop, or
      you can 'return;' to continue
      brk> quit;

  Now we call `Error` with the same arguments to show the difference.

      gap> Error("Gidday!", " How's", " it", " going?\n");
      Error, Gidday! How's it going?
      Hello
      Entering break read-eval-print loop ...
      you can 'quit;' to quit to outer loop, or
      you can 'return;' to continue
      brk> quit;

  Observe that the value of `OnBreak` before the `ErrorNoTraceBack` call was restored. However, we had changed `OnBreak` from its default value; to restore `OnBreak` to its default value, we should do the following.

      gap> OnBreak := Where;;

- `OnBreakMessage V`

  When a break loop is entered by a call to `Error` (see Error) the message after the ```Entering break read-eval-print loop ...`'' line is produced by the function `OnBreakMessage`, which just like `OnBreak` (see OnBreak) is a user-configurable global variable that is a function with no arguments.

      gap> OnBreakMessage(); # By default, OnBreakMessage prints the following
      you can 'quit;' to quit to outer loop, or
      you can 'return;' to continue

  Perhaps you are familiar with what's possible in a break loop, and so don't need to be reminded. In this case, you might wish to do the following (the first line just makes it easy to restore the default value later).

      gap> NormalOnBreakMessage := OnBreakMessage;; # save the default value
      gap> OnBreakMessage := function() end;        # do-nothing function
      function(  ) ... end

  With `OnBreak` still set away from its default value, calling `Error` as we did above, now produces:

      gap> Error("!\n");
      Error, !
      Hello
      Entering break read-eval-print loop ...
      brk> quit; # to get back to outer loop

  However, suppose you are writing a function which detects an error condition and `OnBreakMessage` needs to be changed only locally, i.e., the instructions on how to recover from the break loop need to be specific to that function. The same idea used to define `ErrorNoTraceBack` (see OnBreak) can be adapted to achieve this. The function `CosetTableFromGensAndRels` (see CosetTableFromGensAndRels) is an example in the GAP code where the idea is actually used.

- `Where( [`nr`] ) F`

  shows the last nr commands on the execution stack during whose execution the error occurred. If not given, nr defaults to 5. (Assume, for the following example, that after the last example `OnBreak` (see OnBreak) has been set back to its default value.)

      gap> StabChain(SymmetricGroup(100)); # After this we typed ^C
      user interrupt at
      bpt := S.orbit[1];
       called from
      SiftedPermutation( S, (g * rep) ^ -1 ) called from
      StabChainStrong( S.stabilizer, [ sch ], options ); called from
      StabChainStrong( S.stabilizer, [ sch ], options ); called from
      StabChainStrong( S, GeneratorsOfGroup( G ), options ); called from
      StabChainOp( G, rec(
           ) ) called from
      ...
      Entering break read-eval-print loop ...
      you can 'quit;' to quit to outer loop, or
      you can 'return;' to continue
      brk> Where(2);
       called from
      SiftedPermutation( S, (g * rep) ^ -1 ) called from
      StabChainStrong( S.stabilizer, [ sch ], options ); called from
      ...

  Note that the variables displayed even in the first line of the `Where` list (after the `called from` line) may be already one environment level higher and `DownEnv` (see DownEnv) may be necessary to access them.

  At the moment this backtrace does not work from within compiled code (this includes the method selection which by default is compiled into the kernel). If this creates problems for debugging, call GAP with the `-M` option (see Advanced Features of GAP) to avoid loading compiled code.

  (Function calls to `Info` and methods installed for binary operations are handled in a special way. In rare circumstances it is possible therefore that they do not show up in a `Where` log but the log refers to the last proper function call that happened before.)

  The command line option `-T` to GAP disables the break loop. This is mainly intended for testing purposes and for special applications. If this option is given then errors simply cause GAP to return to the main loop.

  ## 6.4 Variable Access in a Break Loop

  In a break loop access to variables of the current break level and higher levels is possible, but if the same variable name is used for different objects or if a function calls itself recursively, of course only the variable at the lowest level can be accessed.

- `DownEnv( [`nr`] ) F`
- `UpEnv( [`nr`] ) F`

  `DownEnv` moves up nr steps in the environment and allows one to inspect variables on this level; if nr is negative it steps down in the environment again; nr defaults to 1 if not given. `UpEnv` acts similarly to `DownEnv` but in the reverse direction. (The names of `DownEnv` and `UpEnv` are the wrong way 'round; I guess it all depends on which direction defines is ``up'' -- just use `DownEnv` and get used to that.)

      gap> OnBreak := function() Where(0); end;; # eliminate back-tracing on
      gap>                                       # entry to break loop
      gap> test:= function( n )
      >    if n > 3 then Error( "!\n" ); fi; test( n+1 ); end;;
      gap> test( 1 );
      Error, !
      Entering break read-eval-print loop ...
      you can 'quit;' to quit to outer loop, or
      you can 'return;' to continue
      brk> Where();
       called from
      test( n + 1 ); called from
      test( n + 1 ); called from
      test( n + 1 ); called from
      <function>( <arguments> ) called from read-eval-loop
      brk> n;
      4
      brk> DownEnv();
      brk> n;
      3
      brk> Where();
       called from
      test( n + 1 ); called from
      test( n + 1 ); called from
      <function>( <arguments> ) called from read-eval-loop
      brk> DownEnv( 2 );
      brk> n;
      1
      brk> Where();
       called from
      <function>( <arguments> ) called from read-eval-loop
      brk> DownEnv( -2 );
      brk> n;
      3
      brk> quit;
      gap> OnBreak := Where;; # restore OnBreak to its default value

  Note that the change of the environment caused by `DownEnv` only affects variable access in the break loop. If you use `return` to continue a calculation GAP automatically jumps to the right environment level again.

  Note also that search for variables looks first in the chain of outer functions which enclosed the definition of a currently executing function, before it looks at the chain of calling functions which led to the current invocation of the function.

      gap> foo := function()
      > local x; x := 1;
      > return function() local y; y := x*x; Error("!!\n"); end;
      > end;
      function(  ) ... end
      gap> bar := foo();
      function(  ) ... end
      gap> fun := function() local x; x := 3; bar(); end;
      function(  ) ... end
      gap> fun();
      Error, !!
       called from
      bar(  ); called from
      <function>( <arguments> ) called from read-eval-loop
      Entering break read-eval-print loop ...
      you can 'quit;' to quit to outer loop, or
      you can 'return;' to continue
      brk> x;
      1
      brk> DownEnv(1);
      brk> x;
      3

  Here the `x` of `foo` which contained the definition of `bar` is found before that of `fun` which caused its execution. Using `DownEnv` we can access the `x` from `fun`.

  ## 6.5 Error

- `Error(` messages`... ) F`

  `Error` signals an error from within a function. First the messages messages are printed, this is done exactly as if `Print` (see View and Print) were called with these arguments. Then a break loop (see Break Loops) is entered, unless the standard error output is not connected to a terminal. You can leave this break loop with `return;` to continue execution with the statement following the call to `Error`.

  ## 6.6 ErrorCount

- `ErrorCount() F`

  `ErrorCount` returns a count of the number of errors (including user interruptions) which have occurred in the GAP session so far. This count is reduced modulo 228 on 32 bit systems, 260 on 64 bit systems. The count is incremented by each error, even if GAP was started with the `-T` option to disable the break loop.

  ## 6.7 Leaving GAP

  The normal way to terminate a GAP session is to enter either `quit;` (note the semicolon) or an end-of-file character (usually ctrl-D) at the `gap>` prompt in the main read eval print loop.

  An emergency way to leave GAP is to enter

- `QUIT`

  at any `gap>` or `brk>` or `brk_`nn`>` prompt.

- `InstallAtExit(` func`) F`
- `QUITTING V`

  Before actually terminating, GAP will call (with no arguments) all of the functions that have been installed using `InstallAtExit`. These typically perform tasks such as cleaning up temporary files created during the session, and closing open files. If an error occurs during the execution of one of these functions, that function is simply abandoned, no break loop is entered.

      gap> InstallAtExit(function() Print("bye\n"); end);
      gap> quit;
      bye

  During execution of these functions, the global variable `QUITTING` will be set to `true` if GAP is exiting because the user typed `QUIT` and `false` otherwise. Since `QUIT` is considered as an emergency measure, different action may be appropriate.

- `SaveOnExitFile V`

  If, when GAP is exiting due to a `quit` or end-of-file (ie not due to a `QUIT`) the variable `SaveOnExitFile` is bound to a string value, then the system will try to save the workspace to that file.

  ## 6.8 Line Editing

  GAP allows one you to edit the current input line with a number of editing commands. Those commands are accessible either as control keys or as escape keys. You enter a control key by pressing the ctrl key, and, while still holding the ctrl key down, hitting another key key. You enter an escape key by hitting esc and then hitting another key key. Below we denote control keys by ctrl-key and escape keys by esc-key. The case of key does not matter, i.e., ctrl-`A` and ctrl-`a` are equivalent.

  Typing ctrl-key or esc-key for characters not mentioned below always inserts ctrl-key resp. esc-key at the current cursor position.

  The first few commands allow you to move the cursor on the current line.

  ctrl-`A` move the cursor to the beginning of the line.

  esc-`B` move the cursor to the beginning of the previous word.

  ctrl-`B` move the cursor backward one character.

  ctrl-`F` move the cursor forward one character.

  esc-`F` move the cursor to the end of the next word.

  ctrl-`E` move the cursor to the end of the line.

  The next commands delete or kill text. The last killed text can be reinserted, possibly at a different position, with the ``yank'' command ctrl-`Y`.

  ctrl-`H` or del delete the character left of the cursor.

  ctrl-`D` delete the character under the cursor.

  ctrl-`K` kill up to the end of the line.

  esc-`D` kill forward to the end of the next word.

  esc-del kill backward to the beginning of the last word.

  ctrl-`X` kill entire input line, and discard all pending input.

  ctrl-`Y` insert (yank) a just killed text.

  The next commands allow you to change the input.

  ctrl-`T` exchange (twiddle) current and previous character.

  esc-`U` uppercase next word.

  esc-`L` lowercase next word.

  esc-`C` capitalize next word.

  The tab character, which is in fact the control key ctrl-`I`, looks at the characters before the cursor, interprets them as the beginning of an identifier and tries to complete this identifier. If there is more than one possible completion, it completes to the longest common prefix of all those completions. If the characters to the left of the cursor are already the longest common prefix of all completions hitting tab a second time will display all possible completions.

  tab complete the identifier before the cursor.

  The next commands allow you to fetch previous lines, e.g., to correct typos, etc. This history is limited to about 8000 characters.

  ctrl-`L` insert last input line before current character.

  ctrl-`P` redisplay the last input line, another ctrl-`P` will redisplay the line before that, etc. If the cursor is not in the first column only the lines starting with the string to the left of the cursor are taken.

  ctrl-`N` Like ctrl-`P` but goes the other way round through the history.

  esc-`<` goes to the beginning of the history.

  esc-`>` goes to the end of the history.

  ctrl-`O` accepts this line and perform a ctrl-`N`.

  Finally there are a few miscellaneous commands.

  ctrl-`V` enter next character literally, i.e., enter it even if it is one of the control keys.

  ctrl-`U` execute the next line editing command 4 times.

  esc-num execute the next line editing command num times.

  esc-ctrl-`L` redisplay input line.

  The four arrow keys (cursor keys) can be used instead of ctrl-`B`, ctrl-`F`, ctrl-`P`, and ctrl-`N`, respectively.

  ## 6.9 Editing Files

  In most cases, it is preferable to create longer input (in particular GAP programs) separately in an editor, and to read in the result via `Read`. Note that `Read` by default reads from the directory in which GAP was started (respectively under Windows the directory containing the GAP binary), so you might hav eto give an absolute path to the file.

  If you cannot create several windows, the `Edit` command may be used to leave GAP, start an editor, and read in the edited file automatically.

- `Edit(` filename`) F`

  `Edit` starts an editor with the file whose filename is given by the string filename, and reads the file back into GAP when you exit the editor again. You should set the GAP variable `EDITOR` to the name of the editor that you usually use, e.g., `/usr/ucb/vi`. This can for example be done in your `.gaprc` file (see the sections on operating system dependent features in Chapter Installing GAP).

  ## 6.10 Editor Support

  In the `etc` subdirectory of the GAP installation we provide some setup files for the editors `vim` and `emacs`/`xemacs`.

  `vim` is a powerful editor that understands the basic `vi` commands but provides much more functionality. You can find more information about it (and download it) from http://www.vim.org.

  To get support for GAP syntax in vim, create in your home directory a directory `.vim` and a subdirectory `.vim/indent` (If you are not using Unix, refer to the `vim` documentation on where to place syntax files). Then copy the file `etc/gap.vim` in this`.vim` directory and copy the file `etc/gap_indent.vim` to `.vim/indent/gap.vim`.

  Then edit the `.vimrc` file in your home directory. Add lines as in the following example:

      if has("syntax")
        syntax on             " Default to no syntax highlightning
      endif

      " For GAP files
      augroup gap
        " Remove all gap autocommands
        au!
        autocmd BufRead,BufNewFile *.g,*.gi,*.gd source ~/.vim/gap.vim
      autocmd BufRead,BufNewFile *.g,*.gi,*.gd set filetype=gap comments=s:##\ \ ,m:##\ \ ,e:##\ \ b:#

      " I'm using the external program `par' for formating comment lines starting
      " with `##  '. Include these lines only when you have par installed.
        autocmd BufRead,BufNewFile *.g,*.gi,*.gd set formatprg="par w76p4s0j"
        autocmd BufWritePost,FileWritePost *.g,*.gi,*.gd set formatprg="par w76p0s0j"
      augroup END

  See the headers of the two mentioned files for additional comments. Adjust details according to your personal taste.

  Setup files for `(x)emacs` are contained in the `etc/emacs` subdirectory.

  ## 6.11 SizeScreen

- `SizeScreen() F`
- `SizeScreen( [` x`,` y`] ) F`

  With no arguments, `SizeScreen` returns the size of the screen as a list with two entries. The first is the length of each line, the second is the number of lines.

  With one argument that is a list, `SizeScreen` sets the size of the screen; x is the length of each line, y is the number of lines. Either value x or y may be missing (i.e. left unbound), to leave this value unaffected. It returns the new values. Note that those parameters can also be set with the command line options `-x` x`` and `-y` y`` (see Section Command line options).

  To check/change whether line breaking occurs for files and streams see PrintFormattingStatus and SetPrintFormattingStatus.

  The screen width must be between 20 and 256 characters (inclusive) and the depth at least 10 lines. Values outside this range will be adjusted to the nearest endpoint of the range.

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  GAP 4 manual
  May 2002