IRIX Base Documentation 2001 May/SGI IRIX Base Documentation 2001 May.iso/usr/share/catman/u_man/cat3/Tcl/expr.z/expr

Text File | 1998-10-30 | 16.3 KB | 331 lines

eeeexxxxpppprrrr((((3333TTTTccccllll))))                                                          eeeexxxxpppprrrr((((3333TTTTccccllll))))



NNNNAAAAMMMMEEEE
     expr - Evaluate an expression

SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS
     eeeexxxxpppprrrr _a_r_g ?_a_r_g _a_r_g ...?


DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
     Concatenates _a_r_g's (adding separator spaces between them), evaluates the |
     result as a Tcl expression, and returns the value.  The operators
     permitted in Tcl expressions are a subset of the operators permitted in C
     expressions, and they have the same meaning and precedence as the
     corresponding C operators.  Expressions almost always yield numeric
     results (integer or floating-point values).  For example, the expression

          eeeexxxxpppprrrr 8888....2222 ++++ 6666

     evaluates to 14.2.  Tcl expressions differ from C expressions in the way
     that operands are specified.  Also, Tcl expressions support non-numeric
     operands and string comparisons.

OOOOPPPPEEEERRRRAAAANNNNDDDDSSSS
     A Tcl expression consists of a combination of operands, operators, and
     parentheses.  White space may be used between the operands and operators
     and parentheses; it is ignored by the expression processor.  Where
     possible, operands are interpreted as integer values.  Integer values may
     be specified in decimal (the normal case), in octal (if the first
     character of the operand is 0000), or in hexadecimal (if the first two
     characters of the operand are 0000xxxx).  If an operand does not have one of
     the integer formats given above, then it is treated as a floating-point
     number if that is possible.  Floating-point numbers may be specified in
     any of the ways accepted by an ANSI-compliant C compiler (except that the
     ``f'', ``F'', ``l'', and ``L'' suffixes will not be permitted in most
     installations).  For example, all of the following are valid floating-
     point numbers:  2.1, 3., 6e4, 7.91e+16.  If no numeric interpretation is
     possible, then an operand is left as a string (and only a limited set of
     operators may be applied to it).

     Operands may be specified in any of the following ways:

     [1]  As an numeric value, either integer or floating-point.

     [2]  As a Tcl variable, using standard $$$$ notation.  The variable's value
          will be used as the operand.

     [3]  As a string enclosed in double-quotes.  The expression parser will
          perform backslash, variable, and command substitutions on the
          information between the quotes, and use the resulting value as the
          operand






                                                                        PPPPaaaaggggeeee 1111






eeeexxxxpppprrrr((((3333TTTTccccllll))))                                                          eeeexxxxpppprrrr((((3333TTTTccccllll))))



     [4]  As a string enclosed in braces.  The characters between the open
          brace and matching close brace will be used as the operand without
          any substitutions.

     [5]  As a Tcl command enclosed in brackets.  The command will be executed
          and its result will be used as the operand.

     [6]  As a mathematical function whose arguments have any of the above    |
          forms for operands, such as ``ssssiiiinnnn(((($$$$xxxx))))''.  See below for a list of   |
          defined functions.

     Where substitutions occur above (e.g. inside quoted strings), they are
     performed by the expression processor.  However, an additional layer of
     substitution may already have been performed by the command parser before
     the expression processor was called.  As discussed below, it is usually
     best to enclose expressions in braces to prevent the command parser from
     performing substitutions on the contents.

     For some examples of simple expressions, suppose the variable aaaa has the
     value 3 and the variable bbbb has the value 6.  Then the command on the left
     side of each of the lines below will produce the value on the right side
     of the line:

          eeeexxxxpppprrrr 3333....1111 ++++ $$$$aaaa           6666....1111
          eeeexxxxpppprrrr 2222 ++++ """"$$$$aaaa....$$$$bbbb""""        5555....6666
          eeeexxxxpppprrrr 4444****[[[[lllllllleeeennnnggggtttthhhh """"6666 2222""""]]]]  8888
          eeeexxxxpppprrrr {{{{{{{{wwwwoooorrrrdddd oooonnnneeee}}}} <<<< """"wwwwoooorrrrdddd $$$$aaaa""""}}}}0000


OOOOPPPPEEEERRRRAAAATTTTOOOORRRRSSSS
     The valid operators are listed below, grouped in decreasing order of
     precedence:

     ----  ++++  ~~~~  !!!!          Unary minus, unary plus, bit-wise NOT, logical NOT.  |
                         None of these operands may be applied to string
                         operands, and bit-wise NOT may be applied only to
                         integers.

     ****  ////  %%%%             Multiply, divide, remainder.  None of these operands
                         may be applied to string operands, and remainder may
                         be applied only to integers.  The remainder will     |
                         always have the same sign as the divisor and an      |
                         absolute value smaller than the divisor.

     ++++  ----                Add and subtract.  Valid for any numeric operands.

     <<<<<<<<  >>>>>>>>              Left and right shift.  Valid for integer operands
                         only.

     <<<<  >>>>  <<<<====  >>>>====        Boolean less, greater, less than or equal, and
                         greater than or equal.  Each operator produces 1 if
                         the condition is true, 0 otherwise.  These operators



                                                                        PPPPaaaaggggeeee 2222






eeeexxxxpppprrrr((((3333TTTTccccllll))))                                                          eeeexxxxpppprrrr((((3333TTTTccccllll))))



                         may be applied to strings as well as numeric
                         operands, in which case string comparison is used.

     ========  !!!!====              Boolean equal and not equal.  Each operator produces
                         a zero/one result.  Valid for all operand types.

     &&&&                   Bit-wise AND.  Valid for integer operands only.

     ^^^^                   Bit-wise exclusive OR.  Valid for integer operands
                         only.

     ||||                   Bit-wise OR.  Valid for integer operands only.

     &&&&&&&&                  Logical AND.  Produces a 1 result if both operands
                         are non-zero, 0 otherwise.  Valid for numeric
                         operands only (integers or floating-point).

     ||||||||                  Logical OR.  Produces a 0 result if both operands are
                         zero, 1 otherwise.  Valid for numeric operands only
                         (integers or floating-point).

     _x????_y::::_z               If-then-else, as in C.  If _x evaluates to non-zero,
                         then the result is the value of _y.  Otherwise the
                         result is the value of _z.  The _x operand must have a
                         numeric value.

     See the C manual for more details on the results produced by each
     operator.  All of the binary operators group left-to-right within the
     same precedence level.  For example, the command

          eeeexxxxpppprrrr 4444****2222 <<<< 7777

     returns 0.

     The &&&&&&&&, ||||||||, and ????:::: operators have ``lazy evaluation'', just as in C,
     which means that operands are not evaluated if they are not needed to
     determine the outcome.  For example, in the command

          eeeexxxxpppprrrr {{{{$$$$vvvv ???? [[[[aaaa]]]] :::: [[[[bbbb]]]]}}}}

     only one of [[[[aaaa]]]] or [[[[bbbb]]]] will actually be evaluated, depending on the value
     of $$$$vvvv.  Note, however, that this is only true if the entire expression is
     enclosed in braces;  otherwise the Tcl parser will evaluate both [[[[aaaa]]]] and
     [[[[bbbb]]]] before invoking the eeeexxxxpppprrrr command.

MMMMAAAATTTTHHHH FFFFUUUUNNNNCCCCTTTTIIIIOOOONNNNSSSS
     Tcl supports the following mathematical functions in expressions:        |

          aaaaccccoooossss        ccccoooossss         hhhhyyyyppppooootttt      ssssiiiinnnnhhhh                             |
          aaaassssiiiinnnn        ccccoooosssshhhh        lllloooogggg        ssssqqqqrrrrtttt                             |
          aaaattttaaaannnn        eeeexxxxpppp         lllloooogggg11110000      ttttaaaannnn                              |
          aaaattttaaaannnn2222       fffflllloooooooorrrr       ppppoooowwww        ttttaaaannnnhhhh                             |



                                                                        PPPPaaaaggggeeee 3333






eeeexxxxpppprrrr((((3333TTTTccccllll))))                                                          eeeexxxxpppprrrr((((3333TTTTccccllll))))



          cccceeeeiiiillll        ffffmmmmoooodddd        ssssiiiinnnn                                         |

     Each of these functions invokes the math library function of the same    |
     name;  see the manual entries for the library functions for details on   |
     what they do.  Tcl also implements the following functions for conversion|
     between integers and floating-point numbers:

     aaaabbbbssss((((_a_r_g))))
          Returns the absolute value of _a_r_g.  _A_r_g may be either integer or    |
          floating-point, and the result is returned in the same form.

     ddddoooouuuubbbblllleeee((((_a_r_g))))
          If _a_r_g is a floating value, returns _a_r_g, otherwise converts _a_r_g to  |
          floating and returns the converted value.

     iiiinnnntttt((((_a_r_g))))
          If _a_r_g is an integer value, returns _a_r_g, otherwise converts _a_r_g to  |
          integer by truncation and returns the converted value.

     rrrroooouuuunnnndddd((((_a_r_g))))
          If _a_r_g is an integer value, returns _a_r_g, otherwise converts _a_r_g to  |
          integer by rounding and returns the converted value.                |

     In addition to these predefined functions, applications may define       |
     additional functions using TTTTccccllll____CCCCrrrreeeeaaaatttteeeeMMMMaaaatttthhhhFFFFuuuunnnncccc().

TTTTYYYYPPPPEEEESSSS,,,, OOOOVVVVEEEERRRRFFFFLLLLOOOOWWWW,,,, AAAANNNNDDDD PPPPRRRREEEECCCCIIIISSSSIIIIOOOONNNN
     All internal computations involving integers are done with the C type
     _l_o_n_g, and all internal computations involving floating-point are done
     with the C type _d_o_u_b_l_e.  When converting a string to floating-point,
     exponent overflow is detected and results in a Tcl error.  For conversion
     to integer from string, detection of overflow depends on the behavior of
     some routines in the local C library, so it should be regarded as
     unreliable.  In any case, integer overflow and underflow are generally
     not detected reliably for intermediate results.  Floating-point overflow
     and underflow are detected to the degree supported by the hardware, which
     is generally pretty reliable.

     Conversion among internal representations for integer, floating-point,
     and string operands is done automatically as needed.  For arithmetic
     computations, integers are used until some floating-point number is
     introduced, after which floating-point is used.  For example,

          eeeexxxxpppprrrr 5555 //// 4444

     returns 1, while

          eeeexxxxpppprrrr 5555 //// 4444....0000
          eeeexxxxpppprrrr 5555 //// (((( [[[[ssssttttrrrriiiinnnngggg lllleeeennnnggggtttthhhh """"aaaabbbbccccdddd""""]]]] ++++ 0000....0000 ))))

     both return 1.25.  Floating-point values are always returned with a ``.''|
     or an ``e'' so that they will not look like integer values.  For example,|



                                                                        PPPPaaaaggggeeee 4444






eeeexxxxpppprrrr((((3333TTTTccccllll))))                                                          eeeexxxxpppprrrr((((3333TTTTccccllll))))



          eeeexxxxpppprrrr 22220000....0000////5555....0000                                                       |

     returns ``4.0'', not ``4''.  The global variable ttttccccllll____pppprrrreeeecccciiiissssiiiioooonnnn determines|
     the the number of significant digits that are retained when floating     |
     values are converted to strings (except that trailing zeroes are         |
     omitted).  If ttttccccllll____pppprrrreeeecccciiiissssiiiioooonnnn is unset then 6 digits of precision are used.|
     To retain all of the significant bits of an IEEE floating-point number   |
     set ttttccccllll____pppprrrreeeecccciiiissssiiiioooonnnn to 17;  if a value is converted to string with 17      |
     digits of precision and then converted back to binary for some later     |
     calculation, the resulting binary value is guaranteed to be identical to |
     the original one.


SSSSTTTTRRRRIIIINNNNGGGG OOOOPPPPEEEERRRRAAAATTTTIIIIOOOONNNNSSSS
     String values may be used as operands of the comparison operators,
     although the expression evaluator tries to do comparisons as integer or
     floating-point when it can.  If one of the operands of a comparison is a
     string and the other has a numeric value, the numeric operand is
     converted back to a string using the C _s_p_r_i_n_t_f format specifier %%%%dddd for
     integers and %%%%gggg for floating-point values.  For example, the commands

          eeeexxxxpppprrrr {{{{""""0000xxxx00003333"""" >>>> """"2222""""}}}}
          eeeexxxxpppprrrr {{{{""""0000yyyy"""" <<<< """"0000xxxx11112222""""}}}}

     both return 1.  The first comparison is done using integer comparison,
     and the second is done using string comparison after the second operand
     is converted to the string ``18''.  Because of Tcl's tendency to treat   |
     values as numbers whenever possible, it isn't generally a good idea to   |
     use operators like ======== when you really want string comparison and the     |
     values of the operands could be arbitrary;  it's better in these cases to|
     use the ssssttttrrrriiiinnnngggg ccccoooommmmppppaaaarrrreeee command instead.


KKKKEEEEYYYYWWWWOOOORRRRDDDDSSSS
     arithmetic, boolean, compare, expression




















                                                                        PPPPaaaaggggeeee 5555