Provided by: tcl8.5-doc_8.5.15-2ubuntu1_all bug

NAME

       expr - Evaluate an expression

SYNOPSIS

       expr arg ?arg arg ...?
_________________________________________________________________

DESCRIPTION

       Concatenates  args  (adding  separator spaces between them), evaluates the result as a Tcl
       expression, and returns the value.  The operators permitted in Tcl expressions  include  a
       subset  of  the  operators permitted in C expressions.  For those operators common to both
       Tcl and C, Tcl applies 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
              expr 8.2 + 6
       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, as
       well as some additional operators not found in C.

   OPERANDS
       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's instructions.  Where possible, operands are interpreted  as  integer  values. │
       Integer  values may be specified in decimal (the normal case), in binary (if the first two │
       characters of the operand are 0b), in octal (if the first two characters  of  the  operand │
       are  0o),  or  in  hexadecimal  (if  the first two characters of the operand are 0x).  For │
       compatibility with older Tcl releases, an octal integer value  is  also  indicated  simply │
       when  the first character of the operand is 0, whether or not the second character is also │
       o.  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 several common formats making use of the decimal digits, the  decimal  point  ., │
       the characters e or E indicating scientific notation, and the sign characters + or -.  For │
       example, all of the following are valid floating-point numbers:  2.1, 3.,  6e4,  7.91e+16. │
       Also  recognized  as  floating  point values are the strings Inf and NaN making use of any │
       case for each character.  If no numeric interpretation is possible (note that all  literal
       operands  that are not numeric or boolean must be quoted with either braces or with double
       quotes), 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 a numeric value, either integer or floating-point.

       [2]    As a boolean value, using any form understood by string is boolean.

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

       [4]    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

       [5]    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.

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

       [7]    As a mathematical function  whose  arguments  have  any  of  the  above  forms  for
              operands,  such  as  sin($x).   See  MATH  FUNCTIONS  below for a discussion of how
              mathematical functions are handled.

       Where the above substitutions occur (e.g. inside quoted strings), they  are  performed  by
       the expression's instructions.  However, the command parser may already have performed one
       round of substitution 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 a has the value  3  and  the
       variable  b 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:
              expr 3.1 + $a           6.1
              expr 2 + "$a.$b"        5.6
              expr 4*[llength "6 2"]  8
              expr {{word one} < "word $a"}0

   OPERATORS
       The valid operators (most of which are also  available  as  commands  in  the  tcl::mathop
       namespace;  see  the  mathop(3tcl)  manual  page for details) are listed below, grouped in
       decreasing order of precedence:

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

       **                  Exponentiation.  Valid for any numeric operands.                       │

       *  /  %             Multiply, divide, remainder.  None of these operators 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 absolute value of the divisor.

                           When  applied to integers, the division and remainder operators can be
                           considered to partition the number line into a sequence of equal-sized
                           adjacent  non-overlapping  pieces  where each piece is the size of the
                           divisor; the division result identifies which piece  the  divisor  lay
                           within,  and  the  remainder result identifies where within that piece
                           the divisor lay. A consequence of this is that the result  of  “-57  /
                           10” is always -6, and the result of “-57 % 10” is always 3.

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

       <<  >>              Left and right shift.  Valid for integer operands only.  A right shift
                           always propagates the sign bit.

       <  >  <=  >=        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 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.

       eq  ne              Boolean string equal and string not equal.  Each operator  produces  a
                           zero/one result.  The operand types are interpreted only as strings.

       in  ni              List containment and negated list containment.  Each operator produces │
                           a zero/one result and treats its first argument as a  string  and  its │
                           second  argument as a Tcl list.  The in operator indicates whether the │
                           first argument is a  member  of  the  second  argument  list;  the  ni │
                           operator inverts the sense of the result.

       &                   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 boolean and numeric (integers or floating-point)
                           operands only.

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

       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 boolean or numeric value.

       See  the  C  manual  for  more  details  on  the  results  produced by each operator.  The │
       exponentiation operator promotes  types  like  the  multiply  and  divide  operators,  and │
       produces  a  result  that  is  the  same as the output of the pow function (after any type │
       conversions.)  All of the binary operators group left-to-right within the same  precedence
       level.  For example, the command
              expr {4*2 < 7}
       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
              expr {$v ? [a] : [b]}
       only  one  of  “[a]”  or  “[b]”  will actually be evaluated, depending on the value of $v.
       Note, however, that this is only true if the entire  expression  is  enclosed  in  braces;
       otherwise  the  Tcl  parser  will  evaluate  both “[a]” and “[b]” before invoking the expr
       command.

   MATH FUNCTIONS
       When the expression parser encounters a mathematical function such as sin($x), it replaces │
       it with a call to an ordinary Tcl function in the tcl::mathfunc namespace.  The processing │
       of an expression such as:                                                                  │
              expr {sin($x+$y)}                                                                   │
       is the same in every way as the processing of:                                             │
              expr {[tcl::mathfunc::sin [expr {$x+$y}]]}                                          │
       which in turn is the same as the processing of:                                            │
              tcl::mathfunc::sin [expr {$x+$y}]                                                   │

       The executor will search for  tcl::mathfunc::sin  using  the  usual  rules  for  resolving │
       functions     in     namespaces.     Either     ::tcl::mathfunc::sin     or     [namespacecurrent]::tcl::mathfunc::sin will satisfy the request, and others may as  well  (depending │
       on the current namespace path setting).                                                    │

       See the mathfunc(3tcl) manual page for the math functions that are available by default.

   TYPES, OVERFLOW, AND PRECISION
       All  internal  computations involving integers are done calling on the LibTomMath multiple │
       precision integer library as required so  that  all  integer  calculations  are  performed │
       exactly.  Note that in Tcl releases prior to 8.5, integer calculations were performed with │
       one of the C types long int or Tcl_WideInt, causing implicit  range  truncation  in  those │
       calculations  where  values  overflowed the range of those types.  Any code that relied on │
       these implicit truncations will need to explicitly add int() or wide() function  calls  to │
       expressions at the points where such truncation is required to take place.

       All  internal computations involving floating-point are done with the C type double.  When
       converting a string to floating-point, exponent overflow is detected and  results  in  the
       double  value  of  Inf  or -Inf as appropriate.  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,
              expr {5 / 4}
       returns 1, while
              expr {5 / 4.0}
              expr {5 / ( [string length "abcd"] + 0.0 )}
       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,
              expr {20.0/5.0}
       returns 4.0, not 4.

   STRING OPERATIONS
       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, i.e., when all
       arguments to the operator allow numeric interpretations, except in the case of the eq  and
       ne  operators.   If  one  of  the operands of a comparison is a string and the other has a
       numeric value, a canonical string representation of the numeric operand value is generated
       to compare with the string operand.  Canonical string representation for integer values is
       a decimal string format.  Canonical string representation  for  floating-point  values  is
       that  produced  by  the  %g  format  specifier  of Tcl's format command.  For example, the
       commands
              expr {"0x03" > "2"}
              expr {"0y" > "0x12"}
       both return 1.  The first comparison is done using integer comparison, and the  second  is
       done  using  string  comparison.   Because  of  Tcl's  tendency to treat values as numbers
       whenever possible, it is not generally a good idea to  use  operators  like  ==  when  you
       really  want  string  comparison and the values of the operands could be arbitrary;  it is
       better in these cases to use the eq or ne operators, or the string command instead.

PERFORMANCE CONSIDERATIONS

       Enclose expressions in braces for the best speed and the  smallest  storage  requirements.
       This allows the Tcl bytecode compiler to generate the best code.

       As  mentioned above, expressions are substituted twice: once by the Tcl parser and once by
       the expr command.  For example, the commands
              set a 3
              set b {$a + 2}
              expr $b*4
       return 11, not a multiple of 4.  This is because the Tcl parser will first substitute $a +
       2 for the variable b, then the expr command will evaluate the expression $a + 2*4.

       Most expressions do not require a second round of substitutions.  Either they are enclosed
       in braces or, if not, their variable and command substitutions yield  numbers  or  strings
       that do not themselves require substitutions.  However, because a few unbraced expressions
       need two rounds of substitutions, the bytecode compiler must emit additional  instructions
       to  handle  this  situation.  The most expensive code is required for unbraced expressions
       that contain command substitutions.  These expressions must be implemented  by  generating
       new  code  each time the expression is executed.  When the expression is unbraced to allow │
       the substitution of a function or operator, consider using the commands documented in  the │
       mathfunc(3tcl) or mathop(3tcl) manual pages directly instead.

EXAMPLES

       Define a procedure that computes an “interesting” mathematical function:
              proc tcl::mathfunc::calc {x y} {
                  expr { ($x**2 - $y**2) / exp($x**2 + $y**2) }
              }

       Convert polar coordinates into cartesian coordinates:
              # convert from ($radius,$angle)
              set x [expr { $radius * cos($angle) }]
              set y [expr { $radius * sin($angle) }]

       Convert cartesian coordinates into polar coordinates:
              # convert from ($x,$y)
              set radius [expr { hypot($y, $x) }]
              set angle  [expr { atan2($y, $x) }]

       Print a message describing the relationship of two string values to each other:
              puts "a and b are [expr {$a eq $b ? {equal} : {different}}]"

       Set a variable to whether an environment variable is both defined at all and also set to a
       true boolean value:
              set isTrue [expr {
                  [info exists ::env(SOME_ENV_VAR)] &&
                  [string is true -strict $::env(SOME_ENV_VAR)]
              }]

       Generate a random integer in the range 0..99 inclusive:
              set randNum [expr { int(100 * rand()) }]

SEE ALSO

       array(3tcl),   for(3tcl),   if(3tcl),   mathfunc(3tcl),   mathop(3tcl),   namespace(3tcl),
       proc(3tcl), string(3tcl), Tcl(3tcl), while(3tcl)

KEYWORDS

       arithmetic, boolean, compare, expression, fuzzy comparison

COPYRIGHT

       Copyright (c) 1993 The Regents of the University of California.
       Copyright (c) 1994-2000 Sun Microsystems Incorporated.
       Copyright (c) 2005 by Kevin B. Kenny <kennykb@acm.org>. All rights reserved.