expr(n) Tcl Built-In Commands expr(n)______________________________________________________________________________NAMEexpr - Evaluate an expression
SYNOPSISexpr arg ?arg arg ...?
_________________________________________________________________DESCRIPTION
Concatenates arg'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 expres‐
sion
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.
OPERANDS
A Tcl expression consists of a combination of operands, operators, and
parentheses. White space may be used between the operands and opera‐
tors 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 octal (if the
first character of the operand is 0), or in hexadecimal (if the first
two characters of the operand are 0x). 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 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
[4] 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.
[5] As a Tcl command enclosed in brackets. The command will be exe‐
cuted 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 sin($x). See below for a list of
defined functions.
Where substitutions occur above (e.g. inside quoted strings), they are
performed by the expression's instructions. 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 com‐
mand 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 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 op‐
erands, and bit-wise NOT may be applied only to
integers.
* / % Multiply, divide, remainder. None of these oper‐
ands 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 divisor.
+ - 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.
& 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 numeric value.
See the C manual for more details on the results produced by each oper‐
ator. 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 evalu‐
ate both [a] and [b] before invoking the expr command.
MATH FUNCTIONS
Tcl supports the following mathematical functions in expressions:
acos cos hypot sinh
asin cosh log sqrt
atan exp log10 tan
atan2 floor pow tanh ceil fmod sin
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 conver‐
sion between integers and floating-point numbers and the generation of
random numbers:
abs(arg)
Returns the absolute value of arg. Arg may be either integer or
floating-point, and the result is returned in the same form.
double(arg)
If arg is a floating value, returns arg, otherwise converts arg
to floating and returns the converted value.
int(arg)
If arg is an integer value, returns arg, otherwise converts arg
to integer by truncation and returns the converted value.
rand() Returns a floating point number from zero to just less than one
or, in mathematical terms, the range [0,1). The seed comes from
the internal clock of the machine or may be set manual with the
srand function.
round(arg)
If arg is an integer value, returns arg, otherwise converts arg
to integer by rounding and returns the converted value.
srand(arg)
The arg, which must be an integer, is used to reset the seed for
the random number generator. Returns the first random number
from that seed. Each interpreter has it's own seed.
In addition to these predefined functions, applications may define
additional functions using Tcl_CreateMathFunc().
TYPES, OVERFLOW, AND PRECISION
All internal computations involving integers are done with the C type
long, and 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 a Tcl error. For conver‐
sion 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. Float‐
ing-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. If one of the operands of a comparison is
a string and the other has a numeric value, the numeric operand is con‐
verted back to a string using the C sprintf format specifier %d for
integers and %g for floating-point values. 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 after the second operand
is converted to the string 18. Because of Tcl's tendency to treat val‐
ues 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 string compare 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 don't 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.
KEYWORDS
arithmetic, boolean, compare, expression, fuzzy comparison
Tcl 8.0 expr(n)
