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expr(n)                      Tcl Built-In Commands                     expr(n)



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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  per-
       mitted  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 opera-
       tors.  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,
       parentheses  and  commas.  White space may be used between the operands
       and operators and parentheses (or commas); it is ignored by the expres-
       sion's instructions.  Where possible, operands are interpreted as inte-
       ger 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 speci-
       fied in any of several common formats making use of the decimal digits,
       the decimal point ., the characters e or E indicating scientific  nota-
       tion,  and the sign characters + or -.  For example, all of the follow-
       ing 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 with-
              out any substitutions.

       [6]    As a Tcl command enclosed in brackets.  The command will be exe-
              cuted 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(n) 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.
                           The maximum exponent value that Tcl can  handle  if
                           the first number is an integer > 1 is 268435455.

       *  /  %             Multiply,  divide, remainder.  None of these opera-
                           tors may be applied to string operands, and remain-
                           der 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  remain-
                           der  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 remain-
                           der 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  opera-
                           tors  may  be applied to strings as well as numeric
                           operands, in which case string comparison is  used.

       ==  !=              Boolean  equal  and  not equal.  Each operator pro-
                           duces 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 argu-
                           ment as a Tcl  list.   The  in  operator  indicates
                           whether  the first argument is a member of the sec-
                           ond 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.
                           This  operator  evaluates lazily; it only evaluates
                           its second operand if it must in order to determine
                           its result.

       ||                  Logical  OR.   Produces a 0 result if both operands
                           are zero,  1  otherwise.   Valid  for  boolean  and
                           numeric (integers or floating-point) operands only.
                           This operator evaluates lazily; it  only  evaluates
                           its second operand if it must in order to determine
                           its result.

       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.  This operator evalu-
                           ates lazily; it evaluates only one of y or z.

       See the C manual for more details on the results produced by each oper-
       ator.  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 but exponentiation group left-to-right within the same prece-
       dence  level;  exponentiation  groups  right-to-left.  For example, the
       command


              expr {4*2 < 7}

       returns 0, while

              expr {2**3**2}

       returns 512.

       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 evalu-
       ate 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 command  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
       [namespace  current]::tcl::mathfunc::sin  will satisfy the request, and
       others may as well (depending on the current namespace path setting).

       Some mathematical functions have several arguments, separated by commas
       like in C. Thus:

              expr {hypot($x,$y)}

       ends up as

              tcl::mathfunc::hypot $x $y

       See  the mathfunc(n) manual page for the math functions that are avail-
       able 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 explic-
       itly  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  ten-
       dency to treat values as numbers whenever possible, it is not generally
       a good idea to use operators like == when you really want  string  com-
       parison  and the values of the operands could be arbitrary;  it is bet-
       ter 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 stor-
       age 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 com-
       mand 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 addi-
       tional  instructions to handle this situation.  The most expensive code
       is required for unbraced expressions  that  contain  command  substitu-
       tions.   These  expressions  must be implemented by generating new code
       each time the expression is executed.

       If it is necessary to include a non-constant expression  string  within
       the  wider  context of an otherwise-constant expression, the most effi-
       cient technique is to put the varying part inside a recursive expr,  as
       this  at  least allows for the compilation of the outer part, though it
       does mean that the varying part must itself be evaluated as a  separate
       expression.  Thus,  in  this  example  the  result  is 20 and the outer
       expression benefits from fully cached bytecode compilation.

              set a 3
              set b {$a + 2}
              expr {[expr $b] * 4}

       When the expression is unbraced to allow the substitution of a function
       or  operator, consider using the commands documented in the mathfunc(n)
       or mathop(n) manual pages directly instead.


EXAMPLES

       Define a procedure that computes an  "interesting"  mathematical  func-
       tion:

              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(n), for(n), if(n), mathfunc(n), mathop(n), namespace(n), proc(n),
       string(n), Tcl(n), while(n)


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 Kevin B. Kenny <kennykb@acm.org>. All rights reserved.



Tcl                                   8.5                              expr(n)

tcl 8.6.11 - Generated Wed Jan 20 14:39:03 CST 2021
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