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, parentheses
  and commas. White space may be used between the operands and operators and
  parentheses (or commas); 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. The maximum
      exponent value that Tcl can handle if the first number is an integer >
      1 is 268435455.





  
*  /  %

  
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. 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 evaluates lazily; it evaluates only one of y
      or z.





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 but
  exponentiation group left-to-right within the same precedence 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 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 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(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.


If it is necessary to include a non-constant expression string within the wider
  context of an otherwise-constant expression, the most efficient 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(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 © 1993 The Regents of the University of California.
Copyright © 1994-2000 Sun Microsystems Incorporated.
Copyright © 2005 Kevin B. Kenny <[email protected]>. All rights reserved.