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| -rw-r--r-- | doc/expr.n | 385 |
1 files changed, 171 insertions, 214 deletions
@@ -17,14 +17,14 @@ expr \- Evaluate an expression .BE .SH DESCRIPTION .PP -Concatenates \fIarg\fRs (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 +Concatenates \fIarg\fRs, separated by a space, into an expresion, and evaluates +that expression, returning its value. +The operators permitted in an expression 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). +The value of an expressions is often a numeric result, either an integer or a +floating-point value, but may also be a non-numeric value. For example, the expression .PP .CS @@ -32,78 +32,68 @@ For example, the expression .CE .PP 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 +Expressions differ from C expressions in the way that +operands are specified. Also, expressions support +non-numeric operands, string comparisons, and some additional operators not found in C. +.PP +When an expression evaluates to an integer, the value is the decimal form of +the integer, and when an expression evaluates to a floating-point number, the +value is the form produced by the \fB%g\fR format specifier of Tcl's +\fBformat\fR command. .SS OPERANDS .PP -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 \fB0b\fR), in octal -(if the first two characters of the operand are \fB0o\fR), or in hexadecimal -(if the first two characters of the operand are \fB0x\fR). For -compatibility with older Tcl releases, an octal integer value is also -indicated simply when the first character of the operand is \fB0\fR, -whether or not the second character is also \fBo\fR. -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 \fB.\fR, -the characters \fBe\fR or \fBE\fR indicating scientific notation, and -the sign characters \fB+\fR or \fB\-\fR. 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 \fBInf\fR -and \fBNaN\fR 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). -.PP -Operands may be specified in any of the following ways: +An expression consists of a combination of operands, operators, parentheses and +commas, possibly with whitespace between any of these elements, which is +ignored. +An integer operand may be specified in decimal, binary +(the first two characters are \fB0b\fR), octal +(the first two characters are \fB0o\fR), or hexadecimal +(the first two characters are \fB0x\fR) form. For +compatibility with older Tcl releases, an operand that begins with \fB0\fR is +interpreted as an octal integer even if the second character is not \fBo\fR. +A floating-point number may be specified in any of several +common decimal formats, and may use the decimal point \fB.\fR, +\fBe\fR or \fBE\fR for scientific notation, and +the sign characters \fB+\fR and \fB\-\fR. The +following are all valid floating-point numbers: 2.1, 3., 6e4, 7.91e+16. +The strings \fBInf\fR +and \fBNaN\fR, in any combination of case, are also recognized as floating point +values. An operand that doesn't have a numeric interpretation must be quoted +with either braces or with double quotes. +.PP +An operand may be specified in any of the following ways: .IP [1] As a numeric value, either integer or floating-point. .IP [2] As a boolean value, using any form understood by \fBstring is\fR \fBboolean\fR. .IP [3] -As a Tcl variable, using standard \fB$\fR notation. -The variable's value will be used as the operand. +As a variable, using standard \fB$\fR notation. +The value of the variable is then the value of the operand. .IP [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 +Backslash, variable, and command substitution are performed as described in +\fBTcl\fR. .IP [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. +The operand is treated as a braced value as described in \fBTcl\fR. .IP [6] As a Tcl command enclosed in brackets. -The command will be executed and its result will be used as -the operand. +Command substitution is performed as described in \fBTcl\fR. .IP [7] -As a mathematical function whose arguments have any of the above -forms for operands, such as \fBsin($x)\fR. See \fBMATH FUNCTIONS\fR below for +As a mathematical function such as \fBsin($x)\fR, whose arguments have any of the above +forms for operands. See \fBMATH FUNCTIONS\fR below for a discussion of how mathematical functions are handled. .PP -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. +Because \fBexpr\fR parses and performs substitutions on values that have +already been parsed and substituted by \fBTcl\fR, it is usually best to enclose +expressions in braces to avoid the first round of substitutions by +\fBTcl\fR. .PP -For some examples of simple expressions, suppose the variable -\fBa\fR has the value 3 and -the variable \fBb\fR 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: +Below are some examples of simple expressions where the value of \fBa\fR is 3 +and the value of \fBb\fR is 6. The command on the left side of each line +produces the value on the right side. .PP .CS .ta 6c @@ -114,34 +104,41 @@ will produce the value on the right side of the line: .CE .SS OPERATORS .PP -The valid operators (most of which are also available as commands in -the \fBtcl::mathop\fR namespace; see the \fBmathop\fR(n) manual page -for details) are listed below, grouped in decreasing order of precedence: +For operators having both a numeric mode and a string mode, the numeric mode is +chosen when all operands have a numeric interpretation. The integer +interpretation of an operand is preferred over the floating-point +interpretation. To ensure string operations on arbitrary values it is generally a +good idea to use \fBeq\fR, \fBne\fR, or the \fBstring\fR command instead of +more versatile operators such as \fB==\fR. +.PP +Unless otherwise specified, operators accept non-numeric operands. The value +of a boolean operation is 1 if true, 0 otherwise. See also \fBstring is\fR +\fBboolean\fR. The valid operators, most of which are also available as +commands in the \fBtcl::mathop\fR namespace (see \fBmathop\fR(n)), are listed +below, grouped in decreasing order of precedence: .TP 20 \fB\-\0\0+\0\0~\0\0!\fR . -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. +Unary minus, unary plus, bit-wise NOT, logical NOT. These operators +may only be applied to numeric operands, and bit-wise NOT may only be +applied to integers. .TP 20 \fB**\fR . -Exponentiation. Valid for any numeric operands. +Exponentiation. Valid for numeric operands. .TP 20 \fB*\0\0/\0\0%\fR . -Multiply, divide, remainder. None of these operators may be -applied to string operands, and remainder may be applied only -to integers. -The remainder always has the same sign as the divisor and -an absolute value smaller than the absolute value of the divisor. +Multiply and divide, which are valid for numeric operands, and remainder, which +is valid for integers. The remainder, an absolute value smaller than the +absolute value of the divisor, has the same sign as the divisor. .RS .PP -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 dividend lies within, and the -remainder result identifies where within that piece the dividend lies. A +When applied to integers, division and remainder can be +considered to partition the number line into a sequence of +adjacent non-overlapping pieces, where each piece is the size of the divisor; +the quotient identifies which piece the dividend lies within, and the +remainder identifies where within that piece the dividend lies. A consequence of this is that the result of .QW "-57 \fB/\fR 10" is always -6, and the result of @@ -151,177 +148,157 @@ is always 3. .TP 20 \fB+\0\0\-\fR . -Add and subtract. Valid for any numeric operands. +Add and subtract. Valid for numeric operands. .TP 20 \fB<<\0\0>>\fR . -Left and right shift. Valid for integer operands only. +Left and right shift. Valid for integers. A right shift always propagates the sign bit. .TP 20 \fB<\0\0>\0\0<=\0\0>=\fR . -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 less than, greater than, less than or equal, and greater than or equal. .TP 20 \fB==\0\0!=\fR . -Boolean equal and not equal. Each operator produces a zero/one result. -Valid for all operand types. +Boolean equal and not equal. .TP 20 \fBeq\0\0ne\fR . -Boolean string equal and string not equal. Each operator produces a -zero/one result. The operand types are interpreted only as strings. +Boolean string equal and string not equal. .TP 20 \fBin\0\0ni\fR . -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 \fBin\fR operator indicates -whether the first argument is a member of the second argument list; -the \fBni\fR operator inverts the sense of the result. +List containment and negated list containment. The first argument is +interpreted as a string, the second as a list. \fBin\fR tests for membership +in the list, and \fBni\fR is the inverse. .TP 20 \fB&\fR . -Bit-wise AND. Valid for integer operands only. +Bit-wise AND. Valid for integer operands. .TP 20 \fB^\fR . -Bit-wise exclusive OR. Valid for integer operands only. +Bit-wise exclusive OR. Valid for integer operands. .TP 20 \fB|\fR . -Bit-wise OR. Valid for integer operands only. +Bit-wise OR. Valid for integer operands. .TP 20 \fB&&\fR . -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 AND. If both operands are true, the result is 1, or 0 otherwise. + .TP 20 \fB||\fR . -Logical OR. Produces a 0 result if both operands are zero, 1 otherwise. -Valid for boolean and numeric (integers or floating-point) operands only. +Logical OR. If both operands are false, the result is 0, or 1 otherwise. .TP 20 \fIx\fB?\fIy\fB:\fIz\fR . -If-then-else, as in C. If \fIx\fR -evaluates to non-zero, then the result is the value of \fIy\fR. -Otherwise the result is the value of \fIz\fR. -The \fIx\fR operand must have a boolean or numeric value. -.PP -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 \fBpow\fR 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 +If-then-else, as in C. If \fIx\fR is false , the result is the value of +\fIy\fR. Otherwise the result is the value of \fIz\fR. +.PP +The exponentiation operator promotes types in the same way as the multiply +and divide operators, and the result is is the same as the result of +\fBpow\fR. +exponentiation groups right-to-left within a precedence level. Other binary +operators group left-to-right. For example, the value of .PP .CS \fBexpr\fR {4*2 < 7} .CE .PP -returns 0, while +is 0, while the value of .PP .CS \fBexpr\fR {2**3**2} .CE .PP -returns 512. +is 512. .PP -The \fB&&\fR, \fB||\fR, and \fB?:\fR operators have +\fB&&\fR, \fB||\fR, and \fB?:\fR feature .QW "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 +not needed to determine the outcome. For example, in .PP .CS \fBexpr\fR {$v ? [a] : [b]} .CE .PP -only one of -.QW \fB[a]\fR -or -.QW \fB[b]\fR -will actually be evaluated, -depending on the value of \fB$v\fR. Note, however, that this is -only true if the entire expression is enclosed in braces; otherwise -the Tcl parser will evaluate both -.QW \fB[a]\fR -and -.QW \fB[b]\fR -before invoking the \fBexpr\fR command. +only one of \fB[a]\fR or \fB[b]\fR is evaluated, +depending on the value of \fB$v\fR. This is not true of the normal Tcl parser, +so it is normally recommended to enclose the arguments to \fBexpr\fR in braces. +Without braces, as in +\fBexpr\fR $v ? [a] : [b] +both \fB[a]\fR and \fB[b]\fR are evaluated before \fBexpr\fR is even called. +.PP +For more details on the results +produced by each operator, see the documentation for C. .SS "MATH FUNCTIONS" .PP -When the expression parser encounters a mathematical function -such as \fBsin($x)\fR, it replaces it with a call to an ordinary -Tcl function in the \fBtcl::mathfunc\fR namespace. The processing -of an expression such as: +A mathematical function such as \fBsin($x)\fR is replaced with a call to an ordinary +Tcl command in the \fBtcl::mathfunc\fR namespace. The evaluation +of an expression such as .PP .CS \fBexpr\fR {sin($x+$y)} .CE .PP -is the same in every way as the processing of: +is the same in every way as the evaluation of .PP .CS \fBexpr\fR {[tcl::mathfunc::sin [\fBexpr\fR {$x+$y}]]} .CE .PP -which in turn is the same as the processing of: +which in turn is the same as the evaluation of .PP .CS tcl::mathfunc::sin [\fBexpr\fR {$x+$y}] .CE .PP -The executor will search for \fBtcl::mathfunc::sin\fR using the usual -rules for resolving functions in namespaces. Either -\fB::tcl::mathfunc::sin\fR or \fB[namespace -current]::tcl::mathfunc::sin\fR will satisfy the request, and others -may as well (depending on the current \fBnamespace path\fR setting). +\fBtcl::mathfunc::sin\fR is resolved as described in +\fBNAMESPACE RESOLUTION\fR in the \fBnamespace\fR documentation. Given the +default value of \fBnamespace path\fR, \fB[namespace +current]::tcl::mathfunc::sin\fR or \fB::tcl::mathfunc::sin\fR are the typical +resolutions. .PP -Some mathematical functions have several arguments, separated by commas like in C. Thus: +As in C, a mathematical function may accept multiple arguments separated by commas. Thus, .PP .CS \fBexpr\fR {hypot($x,$y)} .CE .PP -ends up as +becomes .PP .CS tcl::mathfunc::hypot $x $y .CE .PP -See the \fBmathfunc\fR(n) manual page for the math functions that are +See the \fBmathfunc\fR(n) documentation for the math functions that are available by default. .SS "TYPES, OVERFLOW, AND PRECISION" .PP -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 +When needed to guarantee exact performance, internal computations involving +integers use the LibTomMath multiple precision integer library. In Tcl releases +prior to 8.5, integer calculations were performed using one of the C types \fIlong int\fR or \fITcl_WideInt\fR, 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 \fBint()\fR or \fBwide()\fR function calls to expressions at the points -where such truncation is required to take place. +Any code that relied on these implicit truncations should instead call +\fBint()\fR or \fBwide()\fR, which do truncate. .PP -All internal computations involving floating-point are -done with the C type \fIdouble\fR. +Internal floating-point computations are +performed using the C type \fIdouble\fR. When converting a string to floating-point, exponent overflow is detected and results in the \fIdouble\fR value of \fBInf\fR or \fB\-Inf\fR as appropriate. Floating-point overflow and underflow are detected to the degree supported by the hardware, which is generally -pretty reliable. +fairly reliable. .PP -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, +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 values are used. For example, .PP .CS \fBexpr\fR {5 / 4} @@ -335,82 +312,62 @@ returns 1, while .CE .PP both return 1.25. -Floating-point values are always returned with a +A floating-point result can be distinguished from an integer result by the +presence of either .QW \fB.\fR -or an +or .QW \fBe\fR -so that they will not look like integer values. For example, +.PP +. For example, .PP .CS \fBexpr\fR {20.0/5.0} .CE .PP returns \fB4.0\fR, not \fB4\fR. -.SS "STRING OPERATIONS" -.PP -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 \fBeq\fR and \fBne\fR 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 \fB%g\fR format specifier of Tcl's -\fBformat\fR command. For example, the commands -.PP -.CS -\fBexpr\fR {"0x03" > "2"} -\fBexpr\fR {"0y" > "0x12"} -.CE -.PP -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 \fB==\fR -when you really want string comparison and the values of the -operands could be arbitrary; it is better in these cases to use -the \fBeq\fR or \fBne\fR operators, or the \fBstring\fR command instead. .SH "PERFORMANCE CONSIDERATIONS" .PP -Enclose expressions in braces for the best speed and the smallest -storage requirements. -This allows the Tcl bytecode compiler to generate the best code. -.PP -As mentioned above, expressions are substituted twice: -once by the Tcl parser and once by the \fBexpr\fR command. -For example, the commands +Where an expression contains syntax that Tcl would otherwise perform +substitutions on, enclosing an expression in braces or otherwise quoting it +so that it's a static value allows the Tcl compiler to generate bytecode for +the expression, resulting in better speed and smaller storage requirements. +This also avoids issues that can arise if Tcl is allowed to perform +substitution on the value before \fBexpr\fR is called. .PP +In the following example, the value of the expression is 11 because the Tcl parser first +substitutes \fB$b\fR and \fBexpr\fR then substitutes \fB$a\fR. Enclosing the +expression in braces would result in a syntax error. .CS set a 3 set b {$a + 2} \fBexpr\fR $b*4 .CE .PP -return 11, not a multiple of 4. -This is because the Tcl parser will first substitute \fB$a + 2\fR for -the variable \fBb\fR, -then the \fBexpr\fR command will evaluate the expression \fB$a + 2*4\fR. -.PP -Most expressions do not require a second round of substitutions. -Either they are enclosed in braces or, if not, -their variable and command substitutions yield numbers or strings -that do not themselves require substitutions. -However, because a few unbraced expressions -need two rounds of substitutions, -the bytecode compiler must emit -additional instructions to handle this situation. -The most expensive code is required for -unbraced expressions that contain command substitutions. -These expressions must be implemented by generating new code -each time the expression is executed. -When the expression is unbraced to allow the substitution of a function or -operator, consider using the commands documented in the \fBmathfunc\fR(n) or -\fBmathop\fR(n) manual pages directly instead. + +When an expression is generated at runtime, like the one above is, the bytcode +compiler must ensure that new code is generated each time the expression +is evaluated. This is the most costly kind of expression from a performance +perspective. In such cases, consider directly using the commands described in +the \fBmathfunc\fR(n) or \fBmathop\fR(n) documentation instead of \fBexpr\fR. + +Most expressions are not formed at runtime, but are literal strings or contain +substitutions that don't introduce other substitutions. To allow the bytecode +compiler to work with an expression as a string literal at compilation time, +ensure that it contains no substitutions or that it is enclosed in braces or +otherwise quoted to prevent Tcl from performing substitutions, allowing +\fBexpr\fR to perform them instead. .SH EXAMPLES .PP +A numeric comparison whose result is 1: +.CS +\fBexpr\fR {"0x03" > "2"} +.CE +.PP +A string comparison whose result is 1: +.CS +\fBexpr\fR {"0y" > "0x12"} +.CE +.PP Define a procedure that computes an .QW interesting mathematical function: @@ -444,8 +401,8 @@ each other: puts "a and b are [\fBexpr\fR {$a eq $b ? {equal} : {different}}]" .CE .PP -Set a variable to whether an environment variable is both defined at -all and also set to a true boolean value: +Set a variable indicating whether an environment variable is defined and has +value of true: .PP .CS set isTrue [\fBexpr\fR { |