1expr(n) Tcl Built-In Commands expr(n)
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6
8 expr - Evaluate an expression
9
11 expr arg ?arg arg ...?
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13
15 Concatenates args (adding separator spaces between them), evaluates the
16 result as a Tcl expression, and returns the value. The operators per‐
17 mitted in Tcl expressions include a subset of the operators permitted
18 in C expressions. For those operators common to both Tcl and C, Tcl
19 applies the same meaning and precedence as the corresponding C opera‐
20 tors. Expressions almost always yield numeric results (integer or
21 floating-point values). For example, the expression
22 expr 8.2 + 6
23 evaluates to 14.2. Tcl expressions differ from C expressions in the
24 way that operands are specified. Also, Tcl expressions support non-
25 numeric operands and string comparisons, as well as some additional
26 operators not found in C.
27 OPERANDS
29 A Tcl expression consists of a combination of operands, operators, and
30 parentheses. White space may be used between the operands and opera‐
31 tors and parentheses; it is ignored by the expression's instructions.
32 Where possible, operands are interpreted as integer values. Integer │
33 values may be specified in decimal (the normal case), in binary (if the │
34 first two characters of the operand are 0b), in octal (if the first two │
35 characters of the operand are 0o), or in hexadecimal (if the first two │
36 characters of the operand are 0x). For compatibility with older Tcl │
37 releases, an octal integer value is also indicated simply when the │
38 first character of the operand is 0, whether or not the second charac‐ │
39 ter is also o. If an operand does not have one of the integer formats │
40 given above, then it is treated as a floating-point number if that is │
41 possible. Floating-point numbers may be specified in any of several │
42 common formats making use of the decimal digits, the decimal point ., │
43 the characters e or E indicating scientific notation, and the sign │
44 characters + or -. For example, all of the following are valid float‐ │
45 ing-point numbers: 2.1, 3., 6e4, 7.91e+16. Also recognized as float‐ │
46 ing point values are the strings Inf and NaN making use of any case for │
47 each character. If no numeric interpretation is possible (note that
48 all literal operands that are not numeric or boolean must be quoted
49 with either braces or with double quotes), then an operand is left as a
50 string (and only a limited set of operators may be applied to it).
51 Operands may be specified in any of the following ways:
53 [1] As a numeric value, either integer or floating-point.
55 [2] As a boolean value, using any form understood by string is bool‐
57 ean.
58 [3] As a Tcl variable, using standard $ notation. The variable's
60 value will be used as the operand.
61 [4] As a string enclosed in double-quotes. The expression parser
63 will perform backslash, variable, and command substitutions on
64 the information between the quotes, and use the resulting value
65 as the operand
66 [5] As a string enclosed in braces. The characters between the open
68 brace and matching close brace will be used as the operand with‐
69 out any substitutions.
70 [6] As a Tcl command enclosed in brackets. The command will be exe‐
72 cuted and its result will be used as the operand.
73 [7] As a mathematical function whose arguments have any of the above
75 forms for operands, such as sin($x). See MATH FUNCTIONS below
76 for a discussion of how mathematical functions are handled.
77 Where the above substitutions occur (e.g. inside quoted strings), they
79 are performed by the expression's instructions. However, the command
80 parser may already have performed one round of substitution before the
81 expression processor was called. As discussed below, it is usually
82 best to enclose expressions in braces to prevent the command parser
83 from performing substitutions on the contents.
84 For some examples of simple expressions, suppose the variable a has the
86 value 3 and the variable b has the value 6. Then the command on the
87 left side of each of the lines below will produce the value on the
88 right side of the line:
89 expr 3.1 + $a 6.1
90 expr 2 + "$a.$b" 5.6
91 expr 4*[llength "6 2"] 8
92 expr {{word one} < "word $a"}0
93 OPERATORS
95 The valid operators (most of which are also available as commands in
96 the tcl::mathop namespace; see the mathop(n) manual page for details)
97 are listed below, grouped in decreasing order of precedence:
98 - + ~ ! Unary minus, unary plus, bit-wise NOT, logical NOT.
100 None of these operators may be applied to string
101 operands, and bit-wise NOT may be applied only to
102 integers.
103 ** Exponentiation. Valid for any numeric operands. │
105 * / % Multiply, divide, remainder. None of these opera‐
107 tors may be applied to string operands, and remain‐
108 der may be applied only to integers. The remainder
109 will always have the same sign as the divisor and
110 an absolute value smaller than the divisor.
111 + - Add and subtract. Valid for any numeric operands.
113 << >> Left and right shift. Valid for integer operands
115 only. A right shift always propagates the sign
116 bit.
117 < > <= >= Boolean less, greater, less than or equal, and
119 greater than or equal. Each operator produces 1 if
120 the condition is true, 0 otherwise. These opera‐
121 tors may be applied to strings as well as numeric
122 operands, in which case string comparison is used.
123 == != Boolean equal and not equal. Each operator pro‐
125 duces a zero/one result. Valid for all operand
126 types.
127 eq ne Boolean string equal and string not equal. Each
129 operator produces a zero/one result. The operand
130 types are interpreted only as strings.
131 in ni List containment and negated list containment. │
133 Each operator produces a zero/one result and treats │
134 its first argument as a string and its second argu‐ │
135 ment as a Tcl list. The in operator indicates │
136 whether the first argument is a member of the sec‐ │
137 ond argument list; the ni operator inverts the │
138 sense of the result.
139 & Bit-wise AND. Valid for integer operands only.
141 ^ Bit-wise exclusive OR. Valid for integer operands
143 only.
144 | Bit-wise OR. Valid for integer operands only.
146 && Logical AND. Produces a 1 result if both operands
148 are non-zero, 0 otherwise. Valid for boolean and
149 numeric (integers or floating-point) operands only.
150 || Logical OR. Produces a 0 result if both operands
152 are zero, 1 otherwise. Valid for boolean and
153 numeric (integers or floating-point) operands only.
154 x?y:z If-then-else, as in C. If x evaluates to non-zero,
156 then the result is the value of y. Otherwise the
157 result is the value of z. The x operand must have
158 a boolean or numeric value.
159 See the C manual for more details on the results produced by each oper‐
161 ator. The exponentiation operator promotes types like the multiply and │
162 divide operators, and produces a result that is the same as the output │
163 of the pow function (after any type conversions.) All of the binary
164 operators group left-to-right within the same precedence level. For
165 example, the command
166 expr {4*2 < 7}
167 returns 0.
168 The &&, ||, and ?: operators have “lazy evaluation”, just as in C,
170 which means that operands are not evaluated if they are not needed to
171 determine the outcome. For example, in the command
172 expr {$v ? [a] : [b]}
173 only one of “[a]” or “[b]” will actually be evaluated, depending on the
174 value of $v. Note, however, that this is only true if the entire
175 expression is enclosed in braces; otherwise the Tcl parser will evalu‐
176 ate both “[a]” and “[b]” before invoking the expr command.
177 MATH FUNCTIONS
179 When the expression parser encounters a mathematical function such as │
180 sin($x), it replaces it with a call to an ordinary Tcl function in the │
181 tcl::mathfunc namespace. The processing of an expression such as: │
182 expr {sin($x+$y)} │
183 is the same in every way as the processing of: │
184 expr {[tcl::mathfunc::sin [expr {$x+$y}]]} │
185 which in turn is the same as the processing of: │
186 tcl::mathfunc::sin [expr {$x+$y}] │
187 The executor will search for tcl::mathfunc::sin using the usual rules │
189 for resolving functions in namespaces. Either ::tcl::mathfunc::sin or │
190 [namespace current]::tcl::mathfunc::sin will satisfy the request, and │
191 others may as well (depending on the current namespace path setting). │
192 See the mathfunc(n) manual page for the math functions that are avail‐ │
194 able by default.
195 TYPES, OVERFLOW, AND PRECISION
197 All internal computations involving integers are done calling on the │
198 LibTomMath multiple precision integer library as required so that all │
199 integer calculations are performed exactly. Note that in Tcl releases │
200 prior to 8.5, integer calculations were performed with one of the C │
201 types long int or Tcl_WideInt, causing implicit range truncation in │
202 those calculations where values overflowed the range of those types. │
203 Any code that relied on these implicit truncations will need to explic‐ │
204 itly add int() or wide() function calls to expressions at the points │
205 where such truncation is required to take place.
206 All internal computations involving floating-point are done with the C
208 type double. When converting a string to floating-point, exponent
209 overflow is detected and results in the double value of Inf or -Inf as
210 appropriate. Floating-point overflow and underflow are detected to the
211 degree supported by the hardware, which is generally pretty reliable.
212 Conversion among internal representations for integer, floating-point,
214 and string operands is done automatically as needed. For arithmetic
215 computations, integers are used until some floating-point number is
216 introduced, after which floating-point is used. For example,
217 expr {5 / 4}
218 returns 1, while
219 expr {5 / 4.0}
220 expr {5 / ( [string length "abcd"] + 0.0 )}
221 both return 1.25. Floating-point values are always returned with a “.”
222 or an “e” so that they will not look like integer values. For example,
223 expr {20.0/5.0}
224 returns 4.0, not 4.
225 STRING OPERATIONS
227 String values may be used as operands of the comparison operators,
228 although the expression evaluator tries to do comparisons as integer or
229 floating-point when it can, except in the case of the eq and ne opera‐
230 tors. If one of the operands of a comparison is a string and the other
231 has a numeric value, a canonical string representation of the numeric
232 operand value is generated to compare with the string operand. Canoni‐
233 cal string representation for integer values is a decimal string for‐
234 mat. Canonical string representation for floating-point values is that
235 produced by the %g format specifier of Tcl's format command. For exam‐
236 ple, the commands
237 expr {"0x03" > "2"}
238 expr {"0y" < "0x12"}
239 both return 1. The first comparison is done using integer comparison,
240 and the second is done using string comparison after the second operand
241 is converted to the string 18. Because of Tcl's tendency to treat val‐
242 ues as numbers whenever possible, it is not generally a good idea to
243 use operators like == when you really want string comparison and the
244 values of the operands could be arbitrary; it is better in these cases
245 to use the eq or ne operators, or the string command instead.
246
248 Enclose expressions in braces for the best speed and the smallest stor‐
249 age requirements. This allows the Tcl bytecode compiler to generate
250 the best code.
251 As mentioned above, expressions are substituted twice: once by the Tcl
253 parser and once by the expr command. For example, the commands
254 set a 3
255 set b {$a + 2}
256 expr $b*4
257 return 11, not a multiple of 4. This is because the Tcl parser will
258 first substitute $a + 2 for the variable b, then the expr command will
259 evaluate the expression $a + 2*4.
260 Most expressions do not require a second round of substitutions.
262 Either they are enclosed in braces or, if not, their variable and com‐
263 mand substitutions yield numbers or strings that do not themselves
264 require substitutions. However, because a few unbraced expressions
265 need two rounds of substitutions, the bytecode compiler must emit addi‐
266 tional instructions to handle this situation. The most expensive code
267 is required for unbraced expressions that contain command substitu‐
268 tions. These expressions must be implemented by generating new code
269 each time the expression is executed. When the expression is unbraced │
270 to allow the substitution of a function or operator, consider using the │
271 commands documented in the mathfunc(n) or mathop(n) manual pages │
272 directly instead.
273
275 Define a procedure that computes an “interesting” mathematical func‐
276 tion:
277 proc tcl::mathfunc::calc {x y} {
278 expr { ($x**2 - $y**2) / exp($x**2 + $y**2) }
279 }
280 Convert polar coordinates into cartesian coordinates:
282 # convert from ($radius,$angle)
283 set x [expr { $radius * cos($angle) }]
284 set y [expr { $radius * sin($angle) }]
285 Convert cartesian coordinates into polar coordinates:
287 # convert from ($x,$y)
288 set radius [expr { hypot($y, $x) }]
289 set angle [expr { atan2($y, $x) }]
290 Print a message describing the relationship of two string values to
292 each other:
293 puts "a and b are [expr {$a eq $b ? {equal} : {different}}]"
294 Set a variable to whether an environment variable is both defined at
296 all and also set to a true boolean value:
297 set isTrue [expr {
298 [info exists ::env(SOME_ENV_VAR)] &&
299 [string is true -strict $::env(SOME_ENV_VAR)]
300 }]
301 Generate a random integer in the range 0..99 inclusive:
303 set randNum [expr { int(100 * rand()) }]
304
306 array(n), for(n), if(n), mathfunc(n), mathop(n), namespace(n), proc(n),
307 string(n), Tcl(n), while(n)
308
310 arithmetic, boolean, compare, expression, fuzzy comparison
311
313 Copyright (c) 1993 The Regents of the University of California.
314 Copyright (c) 1994-2000 Sun Microsystems Incorporated.
315 Copyright (c) 2005 by Kevin B. Kenny <kennykb@acm.org>. All rights reserved.
316Tcl 8.5 expr(n)