5 Main Loop and Break Loop

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

Sections

  1. Main Loop
  2. View and Print
  3. Break Loops
  4. Variable Access in a Break Loop
  5. Error
  6. Leaving GAP
  7. Line Editing
  8. Edit
  9. SizeScreen

5.1 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 before GAP starts to read and evaluate your input. 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.

5.2 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 prg: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.

    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
    

  • Name( obj ) A

    The name of an object is used only for viewing the object via this name.

    There are no methods installed for computing names of objects, but the name may be set for suitable objects, using SetName.

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

    5.3 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
    Entering break read-eval-print loop, you can 'quit;' to quit to outer loop,
    or you can return a new 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
    Entering break read-eval-print loop, you can 'quit;' to quit to outer loop,
    or you can return a new 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. 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).

    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 has been stored in objects. Therefore no guarantee can be given that calculations afterwards will return correct results!

  • return [obj];

    The other way is to return from a break loop. To do this you enter return; or return expr;. If the break loop was entered because you interrupted GAP, then you can continue by entering return;. If the break loop was entered due to an error, you usually have to return a value to continue the computation. For example, if the break loop was entered because a variable had no assigned value, the value to be returned is the 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("Hallo\n"); end;
    function(  ) ... end
    gap> Error("!");
    Error !
    Hallo
    Entering break read-eval-print loop, you can 'quit;' to quit to outer loop,
    or you can return to continue
    brk> 
    

  • 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.

    gap> StabChain(SymmetricGroup(100));
    # Here ^C was pressed
    user interrupt at
    S := S.stabilizer;
    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
    StabChainImmutable( arg[1] ) 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> Where(2);
    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 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.)

    5.4 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

    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.

    gap> test:= function( n )
    >    if n > 3 then Error( "!" ); fi; test( n+1 ); end;;
    gap> test( 1 );
    Error ! at
    Error( "!" );
    Entering break read-eval-print loop,
    you can 'quit;' to quit to outer loop,
    or you can return to continue
    brk> Where();
    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();
    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();
    <function>( <arguments> ) called from read-eval-loop
    brk> DownEnv( -2 );
    brk> n;
    3
    

    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("!!"); end; 
    > end;  
    function(  ) ... end
    gap> bar := foo();
    function(  ) ... end
    gap> fun := function() local x; x := 3; bar(); end;
    function(  ) ... end
    gap> fun();
    Error !! at
    Error( "!!" );
    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.

    5.5 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.

    5.6 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.

    5.7 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.

    5.8 Edit

  • 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).

    5.9 SizeScreen

  • SizeScreen() F

    In this form 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.

  • SizeScreen( [ x, y ] ) F

    In this form SizeScreen sets the size of the screen. x is the length of each line, y is the number of lines. Either value may be missing, 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 Chapter Installing GAP).

    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
    July 1999