Ubuntu Manpage: expr - Evaluate an expression

Provided by: tcl8.6-doc_8.6.13+dfsg-2_all

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()) }]

KEYWORDS

       arithmetic, boolean, compare, expression, fuzzy comparison

COPYRIGHT

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