expr , a built-in Tcl command, evaluates an expression.
expr arg ?arg arg ...?
expr concatentates its arguments, evaluates this result as a Tcl expression, and returns the value of the expression.
expr implements a little language that has a syntax separate from Tcl. An expression is composed of values and operators. Like Tcl, it interprets variable substitution, command substitution, double quotes and braces. Unlike Tcl, it interprets anything that looks like a number as a number, anything that looks like boolean as boolean, provides operators and functions, and requires that literal string values be enclosed in double quotes or braces.
A value recognized as boolean by string is boolean... can be used directly, without enclosing it in quotes or braces.
The operators permitted in Tcl expressions include most of the operators permitted in C expressions and a few additional ones. The operators have the same meaning and precedence as the corresponding C operators. Expressions can yield numeric or non-numeric results.
Functions take the form,
or
The first character of name is a letter, and remaining characters are letters, digits or underscore. I.e. name matches [A-Za-z][A-Za-z0-9_]* . Each argument is itself a complete expression.
Each argument is itself a Tcl expression. For example:
cos($x+$y)
set val1 8.2 set val2 6 expr {$val1 + $val2}
Result:
14.2
In most cases, it's best to brace or otherwise escape the argument to expr. In particular, the special Tcl characters, $, [, ", { should normally be escaped:
expr {$val1+$val2}
This allows expr to do the interpretation, rather than having Tcl interpret them first and then having expr interpret them again. See below for more details.
expr expressions differ from C expressions in the way that operands are specified. They also include some non-numeric operators for strings (comparison) and lists (membership).
Since Tcl 8.5, many operators have command-equivalents in the ::tcl::mathop namespace.
In spite of the name mathop, some of the operators are string-oriented, rather than math-oriented.
The following is a chart of operators, in order of precedence (tightest-binding to least-tight associativity):
- + ~ ! | Unary operators; specifically a negation operation, a non-negation operation, a bit-wise NOT operation (every bit in the input value gets replaced by its inverse) and a logical NOT operation (non-zero maps to zero, and zero maps to one). |
** | exponential. From Tcl 8.5 on. |
+ - | Addition and subtraction. |
<< >> | Left and right shift. Equivalent to multiplying or dividing by a suitable power of two, and then reducing the result to the range representable in an integer on the host platform. |
< > <= >= | Ordering relations: less than, greater than, less than or equal, greater than or equal. These operations work on strings as well as numbers, but where string comparison is intended, it is advisable to use the dedicated string comparison operators or string compare or string equal instead, as those are more predictable in the case of a string that looks like a number. For example, string equal considers "6" and "06" to be different strings, but the expr' operator == considers them to be equivalent numbers. |
== != | Equality and inequality. These operations work on strings as well as numbers, but see the description equality operators for notes about string comparison. |
eq ne | Since Tcl 8.4, these are string-comparison operators. "6" and "06", as well as 1 and 1.0, will compare unequal. |
in ni | checks for occurrence of an item in a list. New in Tcl 8.5. |
** | exponential. From Tcl 8.5 on. |
& | Bit-wise AND. A bit is set in the result when the corresponding bit is set in both the arguments. |
^ | Bit-wise exclusive OR. A bit is set in the result when the corresponding bit is set in precisely one of the arguments. |
| | Bit-wise OR. A bit is set in the result when the corresponding bit is set in either of the arguments. |
&& | Logical AND. The result is 1 when both of the arguments true. and 0 otherwise. This operation is a short-circuiting operation, and will only evaluate its second argument when the first argument is non-zero. This includes the expansion of Tcl commands in square brackets. Where Tcl seems not to be behaving as describe here, see double substitution. |
|| | Logical OR. The result is 0 when both of the arguments are false, and 1 otherwise. This operation is a short-circuiting operation, and will only evaluate its second argument when the first argument is zero. Where Tcl seems not to be behaving as describe here, see double substitution. |
x?y:z | If-then-else, as in C. x, y, and z are expressions. The result is y if x is true, and z otherwise. This operation is a short-circuiting operation: If x is true, z will not be evaluated, and if x is false, y will not be evaluated. Where Tcl seems not to be behaving as describe here, see double substitution. if performs just as well as this construct. The generated bytecode is identical. |
See the mathfunc man page .
The following is a list of builtin functions:
Simple addition:
set a [expr {1 + 2}]
mathematical functions
set a [expr {sqrt(4)}]
martin Lemburg: The following returns 1 because " 2 " will be interpreted as 2: as to 2:
set a [expr {" 2 " == [string trim " 2 "]}]
To ensure that expression evaluates to a floating point number, use double() or floor():
set a 1 set b 2 expr {double($a)/$b}
or, to get an integer:
expr {entier($a/$b)}
int() would also have worked, but entier() is more general
The following returns returns 4, rather than -4 as some might expect:
set a [expr {-2**2}]
The following returns 1 because 2==2 is evaluated first: returns 1 because 2==2 is evaluated first
set a [expr {5&2==2}]
AMG: The order of bitwise operations (|, &, and ^) may seem totally bogus, but it's inherited from C, which in turn inherited it from an early prototype version of C which lacked separate logical operators (&& and ||) [L1 ]. I wouldn't cry if a new language (not Tcl) decided to break compatibility with C in this respect.
expr tries to interpret operands as numeric values, but doesn't parse expr tries to interpret operands as numeric values, but it does not try to interpret variable values as complete numeric expressons, so a value "2*3" will be interpreted as a string:
set y 2*3; expr {$y} ;# ==> 2*3 set y 2*3; expr {$y+2} ;# ==> can't use non-numeric string as operand of "+"
To pass a complete expression stored in a variable, omit the braces so that Tcl To pass a complete expression stored in a variable, take advantage of double substitution by omitting braces:
set y 2*3; expr $y ;# ==> 6 set y 2*3; puts [expr $y+2] ;# ==> 8
But be careful not to introduce an injection attack vulnerability. See double substitution.
expr implements a little language distinct from the language described in the rules of Tcl. One expr implements a little language distinct from standard Tcl. One
% if {joe eq mike} {puts wow} syntax error in expression "joe eq mike": variable references require preceding $ % if {"joe" eq "mike"} {puts wow} % if {{joe} eq {mike}} {puts wow}
To insert a literal value when templating an expression, use an identity function like lindex:
set expr {[lindex @val1@] eq [lindex @val2@]} set expr [string map [list @val@ [list $var1] @val2@ [list $var2]] $expr] expr $expr
If the return value of expr is numeric, it is transformed into a canonical If the value of an expr is numeric, it will transformed into a canonical
set val 0x10 puts $val ;# 0x10 set val [expr {$val}] puts $val ;# 16
puts [expr {[join {0 x 1 0} {}]}] ;# 16
In other words, expr may mutate strings that can be interpreted as numbers, which is a potential gotcha when using string functionality numbers, which is a potential gotcha for the programmer using string functions interpretation.
See Tcl and Octal Numbers for details.
rjm: Why does expr return integers with leading zeroes as hex?, e.g.
expr 0100 ;# -> 64
AMG: The leading zero makes 0100 octal. The 1 is in the 64's place, hence the result is 64 in decimal. That's not hexadecimal. expr always returns a decimal value; you have to use format if you want it in some other base.
RJM: I came around this as I was going to do calculus on a four-digit formatted number in an entry field. I had to apply scan $var %d to get rid formatted number in an entry field. I had to apply scan $var %d to get rid language...
expr uses floating point arithmetic, so strings representing decimal fractions that don't have a precise floating-point representation will be given to a close-enough representation. In the following example, 36.37 gets a to a close-enough representation. In the following example, "36.37" gets a
expr {int(36.37*100)}
If that value is subsequently used as a string, it becomes necessary to somehow convert it. Over the years, the default string conversion has varied. For Tcl version 8.5.13, it looks like
3636.9999999999995
RS points out that version 8.4.9 provided the following results, and that that braced or not, expr returns the same (string rep of) double as well as integer, so the issue of bracing one's expressions is not relevant to the issue of floating-point to string conversion.
% expr 36.37*100 3637.0 ;#-- good enough... % expr {36.37*100} 3637.0 ;#-- the same % expr {int(36.37*100)} 3636 ;#-- Hmm % expr int(36.37*100) 3636 ;#-- the same % info pa 8.4.9
One way to get 3637 would be to use round(): One way to get "3637" would be to use round():
expr {round(36.37*100)}
format can also be useful, but the main point is to remain aware of the context and decide if and how to use floating-point operations.
LV: My response on comp.lang.tcl was that I thought it was a shame that expr (or perhaps it is Tcl) didn't use the same mechanism for both calculations of 36.37 * 100 ; that way, the results would at least be consistent. Even if they were consistently wrong, one would be able to at least to live within the law of least surprise. As it is, until one experiments, one won't know which way that Tcl is going to round results.
EPSJ: This may be a side effect of the IEEE floating point standard. This is done in hardware to guarantee the convergence in the case of a series of math algorithms. The rule is that the mantissa of a floating point number must be rounded to the nearest even number. As 36.37 cannot be represented exactly in float point it ends up being a small fraction below the intended number. On the other side 36.38 moves on the other direction. Look the following result:
() 60 % expr int(36.380*100) 3638 () 61 % expr int(36.370*100) 3636
x86 floating point hardware allows this to be configurable to nearest even, nearest odd, and a few more options. But usually nearest even is the default. The result may seem inconsistent, but it is intentional.
LES 2005-07-23:
% expr pow(5,6) 15625.0 % expr 5**6 15625
Two syntaxes, two slightly different results. Is that intentional?
RS: Yes. While pow() always goes for double logarithms, ** tries to do integer exponentiation where possible.
davou: What is the precision of functions in expr, and how can it be expanded upon?
Lars H: That's generally determined by the C library functions that implement them. It depends on where (and against what) Tcl is compiled. For real numbers, that means doubles, which are floating-point numbers of typically about 17 decimal digits precision (but how many of these are correct varies between functions and platforms). For integers Tcl has traditionally used longs, which in most cases means 32-bit two's complement integers ($tcl_platform(wordSize) tells you the actual number of bytes), but as of Tcl 8.5, it supports (almost) arbitrarily large integers (googol magnitude is no problem anymore, whereas googolplex magnitude wouldn't fit in the computer memory anyway). As for extending what the core provides, tcllib provides math::bignum and math::bigfloat.
At least as of Tcl 8.5, NaN and Inf are potential values returning from expr.
Philip Smolen I've never seen expr return NaN. I wish it would!
[phil@harvey ~]$ tclsh8.6 % expr sqrt(-1) domain error: argument not in valid range % ::tcl::mathfunc::sqrt -1 -NaN % info patchlevel 8.6.4 % expr {Inf+1} Inf %
[phil@becca ~]$ tclsh8.5 % expr sqrt(-1) domain error: argument not in valid range % ::tcl::mathfunc::sqrt -1 -NaN % info patchlevel 8.5.14 % expr {Inf+1} Inf %
parsing of decimals in expr may be hampered by locale - you might get
expr concatenates its arguments into an expression. Consider the following expr resolves variables in the context of its caller, and each variable value becomes exactly one operand in the expression. Consider the following:
expr 5 > {} ;# -> missing operand at _@_ set color2 green expr {$color1} eq {$color2} ;# 1 Concatenated, the arguments form the script, `green eq green`, in which the two But if the arguments are not bracketed, it is a syntax error: Another example illustrating the same point:
#wrong expr $color1 eq $color2
invalid bareword "green" in expression "green eq green"; should be "$green" or "{green}" or "green(...)" or ...
This is because in `expr` syntax, strings should be quoted or bracketed
set a abc set a "abc" expr $a in $b # invalid bareword "abc" # in expression "abc in 123 abcd xyz lmnop"; # should be "$abc" or "{abc}" or "abc(...)" or ...
expr {$a in $b} ;#-> 0 expr {$a in $b} # 0 expr {$a ni $b} ;#-> 1 expr {$a ni $b} # 1
% expr $a eq "foo" ? true : false % expr $a == "foo" ? true : false % expr {$a eq "foo" ? true : false} % expr {$a == "foo" ? true : false}
When exactly one unconcatenated value is passed to expr, the argument can be expr is much more performant when it has exactly one argument. The argument must either be brace-quoted or be a single variable substitution, since this allows byte-compilation.
AMG: More precisely, the argument must not contain multiple substitutions or be the concatenation of substitutions and literal text. The goal is for there If expr has to concatenate its arguments (i.e. it is passed more than one argument), or if Tcl has to concatenate the results of multiple substitutions and literal substrings, then the math expression will be in a temporary Tcl_Obj which must be regenerated every time expr is called.
Fast:
expr {2 + 2} ; # Preferred expr 2+2 ; # Valid but lazy (1) expr "2 + 2" ; # Valid but not preferred (2) expr 2\ +\ 2 ; # Valid but ugly (3) expr $expression ; # Valid expr [expression] ; # Valid
(1) This style is easy to type and is fine for interactive use, but you will lose performance (and correctness and security) if you use this notation in combination with variable and script substitutions.
(2) Same problems as (1). Use braces instead.
(3) Same problems as (1), plus you might forget a backslash before a space, thereby forcing expr to concatenate its arguments.
Slow:
expr 2 + 2 ; # Slow since [expr] must concatenate its arguments expr 2 + $x ; # Slow since [expr] must concatenate its arguments, also unsafe expr 2+$x ; # Slow since Tcl must concatenate to determine argument, also unsafe expr "2 + $x" ; # Slow since Tcl must concatenate to determine argument, also unsafe
AMG: The security problems of unbraced expressions are very similar to SQL injection attacks. Notice how sqlite's Tcl binding does its own SQL injection attacks. Notice how sqlite's Tcl binding does its own this problem as well because the default is to apply multiple passes of interpretation.
See also double substitution. See also double substitution
AMG: The above speed and security concerns also apply to if, for, and while since they share the expression engine with expr.
Additionally, for and while really do need their expressions to be braced. In order for the loop to execute a nonzero but finite number of times, the expression's value must not be constant; but if they're not braced, their value is determined before the loop can begin.
The exception is when the expression (as opposed to value) is contained in a variable, in which case it must not be brace-quoted, or else the command would try to treat the expression as a (string) value and almost certainly fail to convert it to a boolean value.
DKF: But even then, for if, for and while you must still brace the expression to avoid being stuck in the compilation slow lane. Putting an expr inside can help:
set ex {$f > 42} while {[expr $ex]} { puts "This is the body: f is now [incr f -1]" }
Consider what would happen if this script were actually working with user input:
#DON'T EXECUTE THIS SCRIPT!!! set x {[exec format C:\\]} set j {[puts Sucker!]} #C:\ get formatted in the next command set k [expr $x / $j.]
On the other hand,
set k [expr { $x / double($j) }]
gives a much more reasonable result:
argument to math function didn't have numeric value while executing "expr { $x / double($y) }" invoked from within "set k [expr { $x / double($y) }] " (file "foo.tcl" line 3)
Unless you know exactly what you are doing, unbraced expressions are not recommended. Nevertheles...
With unbraced expressions, . (\x2e) can be appended to a variable to get expr With unbraced expressions, a "." can be appended to a variable to get expr
set x 1; set j 2 # works (but don't do this) expr $x/$j. #an accepted way to do it expr {double($x)/$j} # error: syntax error in expression "$x/$j." (expr parser) expr {$x/$j.}
It's faster, too:
set script1 { set x 1 set j 2 set k [expr $x / $j.] } set script2 { set x 1 set j 2 set k [expr { $x / double($j) }] } foreach v {script1 script2} { foreach v { script1 script2 } { } #script1: 38 microseconds per iteration #script2: 9 microseconds per iteration #[pyk] 2012-11-28: what a difference a few years makes (an "old" 3.06Ghz Intel Core 2 Duo): #script1: 4.4767364 microseconds per iteration #script2: 0.7374299 microseconds per iteration
RS: This was just to demonstrate the differences between the regular Tcl parser and the parser for expr', not recommended practice. Another example is substitution of operators:
set op + expr 4 $op 5 9 expr {4 $op 5} syntax error in expression "4 $op 5"
See the for page on a case where that helped.
AMG: When expr's argument is properly braced, the expression can be bytecoded for significant performance gains. However, the performance is not always quite as good as one would hope. Use tcl::unsupported::disassemble to see this in action:
% tcl::unsupported::disassemble script {expr {4 / 3. * acos(-1) * $r ** 3}} ByteCode 0x00000000025FADB0, refCt 1, epoch 16, interp 0x00000000026365B0 (epoch 16) Source "expr {4 / 3. * acos(-1) * $r ** 3}" Cmds 1, src 34, inst 17, litObjs 5, aux 0, stkDepth 3, code/src 0.00 Commands 1: 1: pc 0-15, src 0-33 Command 1: "expr {4 / 3. * acos(-1) * $r ** 3}" (0) push1 0 # "1.3333333333333333" (2) push1 1 # "tcl::mathfunc::acos" (4) push1 2 # "-1" (6) invokeStk1 2 (8) mult (9) push1 3 # "r" (11) loadStk (12) push1 4 # "3" (14) expon (15) mult (16) done
This shows that 4 / 3. is precomputed to 1.3333333333333333, but acos(-1) is not precomputed to 3.141592653589793. While it would seem ideal to fold the constants together into 4.1887902047863905, doing so would skip invoking tcl::mathfunc::acos, which might have a trace on it. Tcl optimizations always favor correctness over speed, so this shortcut is not available. This shows that "4 / 3." is precomputed to "1.3333333333333333", but "acos(-1)" is not precomputed to "3.141592653589793". While it would seem ideal to fold the constants together into "4.1887902047863905", doing so would skip invoking tcl::mathfunc::acos, which might have a trace on it. Tcl optimizations always favor correctness over speed, so this shortcut is not available. Here's the above again, but with local variables which provide a large speed boost by avoiding looking up the variable by name:
% tcl::unsupported::disassemble lambda {{} {expr {4 / 3. * acos(-1) * $r ** 3}}} ByteCode 0x00000000027A1080, refCt 1, epoch 16, interp 0x00000000026E65B0 (epoch 16) Source "expr {4 / 3. * acos(-1) * $r ** 3}" Cmds 1, src 34, inst 16, litObjs 4, aux 0, stkDepth 3, code/src 0.00 Proc 0x00000000026A8620, refCt 1, args 0, compiled locals 1 slot 0, scalar, "r" Commands 1: 1: pc 0-14, src 0-33 Command 1: "expr {4 / 3. * acos(-1) * $r ** 3}" (0) push1 0 # "1.3333333333333333" (2) push1 1 # "tcl::mathfunc::acos" (4) push1 2 # "-1" (6) invokeStk1 2 (8) mult (9) loadScalar1 %v0 # var "r" (11) push1 3 # "3" (13) expon (14) mult (15) done
(For disassembly readouts, it's not necessary to list the variables as arguments to the lambda. They'll be assigned slots in the compiled locals table either way. You're not actually running the code, so it doesn't matter if the variable exists.)
Common subexpressions cannot be optimized because this would bypass some potential traces on variable access and procedure invocation. If expr could know in advance that particular procedures and variables don't have traces, it would have greater freedom to perform common subexpression elimination. Knowing that a procedure is a pure function (its result depends only on its arguments), plus knowing that its definition will not change throughout the execution of the program, would let expr treat acos(-1) as a constant. Common subexpressions cannot be optimized because this would bypass some potential traces on variable access and procedure invocation. If expr could know in advance that particular procedures and variables don't have traces, it would have greater freedom to perform common subexpression elimination. Knowing that a procedure is a pure function (its result depends only on its arguments), plus knowing that its definition will not change throughout the execution of the program, would let expr treat acos(-1) as a constant. Now rearrange the expression to put the division at the end. Algebraically, this should produce an identical result. But because of potential floating point precision issues (non-commutativity of operations), Tcl must play it safe and do the operations in the order specified:
% tcl::unsupported::disassemble script {expr {4 * acos(-1) * $r ** 3 / 3.}} ByteCode 0x00000000025F91B0, refCt 1, epoch 16, interp 0x00000000026365B0 (epoch 16) Source "expr {4 * acos(-1) * $r ** 3 / 3.}" Cmds 1, src 34, inst 20, litObjs 6, aux 0, stkDepth 3, code/src 0.00 Commands 1: 1: pc 0-18, src 0-33 Command 1: "expr {4 * acos(-1) * $r ** 3 / 3.}" (0) push1 0 # "4" (2) push1 1 # "tcl::mathfunc::acos" (4) push1 2 # "-1" (6) invokeStk1 2 (8) mult (9) push1 3 # "r" (11) loadStk (12) push1 4 # "3" (14) expon (15) mult (16) push1 5 # "3." (18) div (19) done
Back to common subexpression elimination: It may seem that the solution is for the programmer to manually precompute common subexpressions and reference their values via variables. This generally helps, so long as the subexpressions aren't too simple, but you must use local variables or else performance will suffer:
% proc a {x} {expr {cos($x * acos(-1)) + sin($x * acos(-1))}} % proc b {x} {set y [expr {$x * acos(-1)}]; expr {cos($y) + sin($y)}} % proc c {x} {set ::y [expr {$x * acos(-1)}]; expr {cos($::y) + sin($::y)}} % set x 12.3 % time {a $x} 1000000 1.536581 microseconds per iteration % time {b $x} 1000000 1.333106 microseconds per iteration % time {c $x} 1000000 1.994305 microseconds per iteration
In Embedded vs. separate commands , 1992-12-38, JO published the voting results 37:8 in favor of embedded functions() vs. separate [commands]
Addition
% expr (1<<31)-1 2147483647 % expr 2147483647 + 2147483647 -2
Multiplication
% expr sqrt((1<<31)-1) 46340.9500011 expr 46341*46341 -2147479015
These are results of Tcl 8.4 and older versions using a 32-bit representation for integers.
Tcl 8.5 features abritrary-precision integers. See TIP #237 .
expressions in variable values would interpreted as such
RS 2003-04-24: Here's a tiny wrapper for friends of infix assignment:
proc let {var = args} { uplevel 1 set $var \[expr $args\] } ;#RS
% let i = 1 1 % let j = $i + 1 2 % let k = {$i + $j} 3
set y 2*3; puts [expr $y+0] ;# ==> 6
AM: The problem with variables whose values are actually expressions is that they change the whole expression in which they are used. The performance gain for caching the parsed expression will then be lost.
AMG: This reopens the door to all the security, performance, and correctness problems solved by bracing one's expressions.
Wookie: I had some trouble recently using expr to calculate time offsets. I had 2 time stamps in the form hh:mm
So I had 4 variables h1, m1, h2, m2 and one of my expr functions was
set result [expr {$m1 + $m2}]
As many of you may be thinking, you fool! what about 08 and 09, which will get treated as invalid octal. So after some grumbling I thought okay so I have to trimleft them. Bit verbose but who cares:
set m1 [string trimleft $m1 0] set m2 [string trimleft $m2 0] set result [expr ($m1 + $m2)]
Now what could possibly go wrong with that... well obviously 00 becomes the empty string, which causes unexpected closed parameter in the expression. So now I have to check for the empty string. So...
set m1 [string trimleft $m1 0] if {$m1=={}} {set m1 0} set m2 [string trimleft $m2 0] if {$m2=={}} {set m2 0} set result [expr {$m1 + $m2}]
... and then repeat it for the hours. It all seemed very clumsy. So I came up with this, which may solve many of the conversion issues in this section.
scan "$h1:$m1 $h2:$m2" "%d:%d %d:%d" h1 m1 h2 m2 set result [expr {$m1 + $m2}]
All the conversions to int have been done and leading 0's have been stripped and returns 0 if the value is all 0s. This works for float and probably double (though I've not tried). Can anyone see any problems with this approach?
glennj: No, scan is definitely the way to parse numbers out of dates and times. However, for date arithmetic, nothing beats clock.
# adding a delta to a time set h1 12; set m1 45 set h2 3; set m2 30 clock format [clock add [clock scan "$h1:$m1" -format "%H:%M"] $h2 hours $m2 minutes] -format %T ;# ==> 16:15:00
What are you trying to do with your two times?