300 lines
11 KiB
Plaintext
300 lines
11 KiB
Plaintext
'\"
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'\" Copyright (c) 1993 The Regents of the University of California.
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'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
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'\"
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'\" See the file "license.terms" for information on usage and redistribution
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'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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'\"
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'\" SCCS: @(#) expr.n 1.17 96/03/14 10:54:40
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'\"
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.so man.macros
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.TH expr n 7.4 Tcl "Tcl Built-In Commands"
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.BS
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'\" Note: do not modify the .SH NAME line immediately below!
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.SH NAME
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expr \- Evaluate an expression
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.SH SYNOPSIS
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\fBexpr \fIarg \fR?\fIarg arg ...\fR?
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.BE
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.SH DESCRIPTION
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.PP
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.VS
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Concatenates \fIarg\fR's (adding separator spaces between them),
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evaluates the result as a Tcl expression, and returns the value.
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.VE
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The operators permitted in Tcl expressions are a subset of
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the operators permitted in C expressions, and they have the
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same meaning and precedence as the corresponding C operators.
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Expressions almost always yield numeric results
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(integer or floating-point values).
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For example, the expression
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.CS
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\fBexpr 8.2 + 6\fR
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.CE
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evaluates to 14.2.
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Tcl expressions differ from C expressions in the way that
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operands are specified. Also, Tcl expressions support
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non-numeric operands and string comparisons.
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.SH OPERANDS
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.PP
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A Tcl expression consists of a combination of operands, operators,
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and parentheses.
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White space may be used between the operands and operators and
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parentheses; it is ignored by the expression processor.
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Where possible, operands are interpreted as integer values.
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Integer values may be specified in decimal (the normal case), in octal (if the
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first character of the operand is \fB0\fR), or in hexadecimal (if the first
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two characters of the operand are \fB0x\fR).
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If an operand does not have one of the integer formats given
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above, then it is treated as a floating-point number if that is
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possible. Floating-point numbers may be specified in any of the
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ways accepted by an ANSI-compliant C compiler (except that the
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``f'', ``F'', ``l'', and ``L'' suffixes will not be permitted in
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most installations). For example, all of the
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following are valid floating-point numbers: 2.1, 3., 6e4, 7.91e+16.
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If no numeric interpretation is possible, then an operand is left
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as a string (and only a limited set of operators may be applied to
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it).
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.PP
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Operands may be specified in any of the following ways:
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.IP [1]
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As an numeric value, either integer or floating-point.
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.IP [2]
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As a Tcl variable, using standard \fB$\fR notation.
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The variable's value will be used as the operand.
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.IP [3]
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As a string enclosed in double-quotes.
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The expression parser will perform backslash, variable, and
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command substitutions on the information between the quotes,
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and use the resulting value as the operand
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.IP [4]
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As a string enclosed in braces.
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The characters between the open brace and matching close brace
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will be used as the operand without any substitutions.
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.IP [5]
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As a Tcl command enclosed in brackets.
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The command will be executed and its result will be used as
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the operand.
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.IP [6]
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.VS
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As a mathematical function whose arguments have any of the above
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forms for operands, such as ``\fBsin($x)\fR''. See below for a list of defined
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functions.
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.VE
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.LP
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Where substitutions occur above (e.g. inside quoted strings), they
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are performed by the expression processor.
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However, an additional layer of substitution may already have
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been performed by the command parser before the expression
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processor was called.
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As discussed below, it is usually best to enclose expressions
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in braces to prevent the command parser from performing substitutions
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on the contents.
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.PP
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For some examples of simple expressions, suppose the variable
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\fBa\fR has the value 3 and
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the variable \fBb\fR has the value 6.
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Then the command on the left side of each of the lines below
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will produce the value on the right side of the line:
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.CS
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.ta 6c
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\fBexpr 3.1 + $a 6.1
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expr 2 + "$a.$b" 5.6
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expr 4*[llength "6 2"] 8
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expr {{word one} < "word $a"} 0\fR
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.CE
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.SH OPERATORS
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.PP
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The valid operators are listed below, grouped in decreasing order
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of precedence:
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.TP 20
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\fB\-\0\0+\0\0~\0\0!\fR
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.VS
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Unary minus, unary plus, bit-wise NOT, logical NOT. None of these operands
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.VE
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may be applied to string operands, and bit-wise NOT may be
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applied only to integers.
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.TP 20
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\fB*\0\0/\0\0%\fR
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Multiply, divide, remainder. None of these operands may be
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applied to string operands, and remainder may be applied only
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to integers.
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.VS
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The remainder will always have the same sign as the divisor and
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an absolute value smaller than the divisor.
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.VE
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.TP 20
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\fB+\0\0\-\fR
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Add and subtract. Valid for any numeric operands.
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.TP 20
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\fB<<\0\0>>\fR
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Left and right shift. Valid for integer operands only.
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A right shift always propagates the sign bit.
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.TP 20
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\fB<\0\0>\0\0<=\0\0>=\fR
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Boolean less, greater, less than or equal, and greater than or equal.
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Each operator produces 1 if the condition is true, 0 otherwise.
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These operators may be applied to strings as well as numeric operands,
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in which case string comparison is used.
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.TP 20
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\fB==\0\0!=\fR
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Boolean equal and not equal. Each operator produces a zero/one result.
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Valid for all operand types.
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.TP 20
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\fB&\fR
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Bit-wise AND. Valid for integer operands only.
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.TP 20
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\fB^\fR
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Bit-wise exclusive OR. Valid for integer operands only.
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.TP 20
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\fB|\fR
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Bit-wise OR. Valid for integer operands only.
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.TP 20
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\fB&&\fR
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Logical AND. Produces a 1 result if both operands are non-zero, 0 otherwise.
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Valid for numeric operands only (integers or floating-point).
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.TP 20
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\fB||\fR
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Logical OR. Produces a 0 result if both operands are zero, 1 otherwise.
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Valid for numeric operands only (integers or floating-point).
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.TP 20
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\fIx\fB?\fIy\fB:\fIz\fR
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If-then-else, as in C. If \fIx\fR
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evaluates to non-zero, then the result is the value of \fIy\fR.
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Otherwise the result is the value of \fIz\fR.
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The \fIx\fR operand must have a numeric value.
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.LP
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See the C manual for more details on the results
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produced by each operator.
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All of the binary operators group left-to-right within the same
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precedence level. For example, the command
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.CS
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\fBexpr 4*2 < 7\fR
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.CE
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returns 0.
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.PP
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The \fB&&\fR, \fB||\fR, and \fB?:\fR operators have ``lazy
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evaluation'', just as in C,
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which means that operands are not evaluated if they are
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not needed to determine the outcome. For example, in the command
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.CS
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\fBexpr {$v ? [a] : [b]}\fR
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.CE
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only one of \fB[a]\fR or \fB[b]\fR will actually be evaluated,
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depending on the value of \fB$v\fR. Note, however, that this is
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only true if the entire expression is enclosed in braces; otherwise
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the Tcl parser will evaluate both \fB[a]\fR and \fB[b]\fR before
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invoking the \fBexpr\fR command.
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.SH "MATH FUNCTIONS"
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.PP
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.VS
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Tcl supports the following mathematical functions in expressions:
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.DS
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.ta 3c 6c 9c
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\fBacos\fR \fBcos\fR \fBhypot\fR \fBsinh\fR
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\fBasin\fR \fBcosh\fR \fBlog\fR \fBsqrt\fR
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\fBatan\fR \fBexp\fR \fBlog10\fR \fBtan\fR
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\fBatan2\fR \fBfloor\fR \fBpow\fR \fBtanh\fR
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\fBceil\fR \fBfmod\fR \fBsin\fR
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.DE
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Each of these functions invokes the math library function of the same
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name; see the manual entries for the library functions for details
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on what they do. Tcl also implements the following functions for
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conversion between integers and floating-point numbers:
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.TP
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\fBabs(\fIarg\fB)\fR
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Returns the absolute value of \fIarg\fR. \fIArg\fR may be either
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integer or floating-point, and the result is returned in the same form.
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.TP
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\fBdouble(\fIarg\fB)\fR
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If \fIarg\fR is a floating value, returns \fIarg\fR, otherwise converts
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\fIarg\fR to floating and returns the converted value.
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.TP
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\fBint(\fIarg\fB)\fR
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If \fIarg\fR is an integer value, returns \fIarg\fR, otherwise converts
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\fIarg\fR to integer by truncation and returns the converted value.
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.TP
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\fBround(\fIarg\fB)\fR
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If \fIarg\fR is an integer value, returns \fIarg\fR, otherwise converts
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\fIarg\fR to integer by rounding and returns the converted value.
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.PP
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In addition to these predefined functions, applications may
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define additional functions using \fBTcl_CreateMathFunc\fR().
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.VE
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.SH "TYPES, OVERFLOW, AND PRECISION"
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.PP
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All internal computations involving integers are done with the C type
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\fIlong\fR, and all internal computations involving floating-point are
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done with the C type \fIdouble\fR.
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When converting a string to floating-point, exponent overflow is
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detected and results in a Tcl error.
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For conversion to integer from string, detection of overflow depends
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on the behavior of some routines in the local C library, so it should
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be regarded as unreliable.
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In any case, integer overflow and underflow are generally not detected
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reliably for intermediate results. Floating-point overflow and underflow
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are detected to the degree supported by the hardware, which is generally
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pretty reliable.
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.PP
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Conversion among internal representations for integer, floating-point,
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and string operands is done automatically as needed.
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For arithmetic computations, integers are used until some
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floating-point number is introduced, after which floating-point is used.
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For example,
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.CS
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\fBexpr 5 / 4\fR
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.CE
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returns 1, while
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.CS
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\fBexpr 5 / 4.0\fR
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\fBexpr 5 / ( [string length "abcd"] + 0.0 )\fR
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.CE
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both return 1.25.
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.VS
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Floating-point values are always returned with a ``.''
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or an ``e'' so that they will not look like integer values. For
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example,
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.CS
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\fBexpr 20.0/5.0\fR
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.CE
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returns ``4.0'', not ``4''. The global variable \fBtcl_precision\fR
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determines the the number of significant digits that are retained
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when floating values are converted to strings (except that trailing
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zeroes are omitted). If \fBtcl_precision\fR
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is unset then 6 digits of precision are used.
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To retain all of the significant bits of an IEEE floating-point
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number set \fBtcl_precision\fR to 17; if a value is converted to
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string with 17 digits of precision and then converted back to binary
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for some later calculation, the resulting binary value is guaranteed
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to be identical to the original one.
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.VE
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.SH "STRING OPERATIONS"
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.PP
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String values may be used as operands of the comparison operators,
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although the expression evaluator tries to do comparisons as integer
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or floating-point when it can.
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If one of the operands of a comparison is a string and the other
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has a numeric value, the numeric operand is converted back to
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a string using the C \fIsprintf\fR format specifier
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\fB%d\fR for integers and \fB%g\fR for floating-point values.
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For example, the commands
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.CS
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\fBexpr {"0x03" > "2"}\fR
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\fBexpr {"0y" < "0x12"}\fR
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.CE
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both return 1. The first comparison is done using integer
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comparison, and the second is done using string comparison after
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the second operand is converted to the string ``18''.
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.VS
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Because of Tcl's tendency to treat values as numbers whenever
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possible, it isn't generally a good idea to use operators like \fB==\fR
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when you really want string comparison and the values of the
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operands could be arbitrary; it's better in these cases to use the
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\fBstring compare\fR command instead.
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.VE
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.SH KEYWORDS
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arithmetic, boolean, compare, expression
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