**expr**, a built-in Tcl command, is used to evaluate an expression

## See Also edit

- A little math language
- adds features & sugar to [expr]
- A real problem
- Additional math functions
- Arbitrary precision math procedures
- Brace your expr-essions
- compute
- more sugar for [expr]
- Converting numbers from arbitrary bases
- DebuggingExprCalls
- rwm sometimes it is difficult to debug [expr] calls where the operands are variables. DebuggingExprCalls explains how to wrap expr to help with these cases.
- double substitution
- gotcha
- where [expr] figures prominently
- Tcl and octal numbers
- The Octal Bug
- expr problems with int
- historical (pre-8.5) information on the limits of number representation (both integer and float) inherited from C
- for
- How can I do math in Tcl
- if
- Importing expr functions
- use [expr]'s functions without explicitly calling that, see Importing expr functions.
- Math function help
- Modeling COND with expr
- Braced expressions can span several lines
- Numerical Analysis in Tcl
- Sample Math Programs
- extremely simple math examples
- Tcl help
- TIP #123
- Adding an Exponentiation Operator to [expr]
- TIP #174
- Math Operators as Commands
- TIP #182
- Add 'expr bool' Math Function
- TIP #201
- Add 'in' Operator to expr
- TIP #201
- Creating New Math Functions for [expr] (tcl::mathfunc)
- TIP #237
- Arbitrary-Precision Integers for Tcl
- while

## Documentation edit

## Synopsis edit

**expr**

*arg*?

*arg arg ...*?

## Description edit

[expr] implements a mini language that has a syntax separate from the Tcl . It supports some of the same constructs as Tcl, such as variable substitution, command substitution, and braces. It adds additional syntax for mathematical operators and functions, and unlike Tcl, does not accept character strings that are not enclosed in double quotes or braces.The [expr] conditions of commands such as [if] and [while] expect the expression to evaluate to a boolean, i.e., an integer or one of the following string values:- true, on, yes
- false, off, no

## Usage edit

[expr] [concat]enates its arguments, evaluates this result as a Tcl expression, and returns the value. 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 almost always yield numeric results (integer or floating-point values). For example, the expression,set val1 8.2 set val2 6 expr $val1 + $val2returns 14.2.Usually, though, it's best to brace the argument:

expr {$val1+$val2}Any argument that can be interpreted in some way, e.g., variable or a command expansion, should normally be enclosed in brackets. This allows the [expr] command to do the interpretation, rather than having the caller interpret the arguments before handing them off to [expr]. See below for more details.[expr]essions differ from C expressions in the way that operands are specified. They also include some non-numeric operators for strings (comparison) and lists (membership).

## Operators edit

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, rather than math-oriented.The following is a chart of operators, in order of precedence (tightest-binding to least-tight binding):

- + ~ ! | 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). |

* / % | Multiplication, division and integer remainder (see fmod() below.) |

+ - | 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 [expr]'s == 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. |

** | exponential. From Tcl 8.5 on. |

in ni | checks for occurance of an item in a list. New in Tcl 8.5. |

& | 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. |

## Functions edit

See the mathfunc man page.The following is a list of builtin functions:**abs**(x)- Absolute value (negate if negative).
**acos**(x)- Inverse cosine (result in radians).
**asin**(x)- Inverse sine (result in radians).
**atan**(x)- Inverse tangent (result in radians).
**atan2**(y,x)- Inverse tangent. Can handle cases which plain atan() can't (due to division by zero) and has a larger output range (result in radians).
**bool**(x)- Accepts any valid boolean value and returns the corresponding boolean value 0 or 1.
**ceil**(x)- Ceiling (defined over floating point numbers.) If the input value is not a whole number, return the next
*larger*whole number.**Surprise**: The return value is a float, not an integer. **cos**(x)- Cosine (input in radians).
**cosh**(x)- Hyperbolic cosine.
**double**(x)- Convert number to floating point.
**entier**(x)- Like int(x), but there is no limit to the input or output value.
**exp**(x)- Exponential function. Returns e to the power or x, where e is the base of natural logarithms.
**floor**(x)- Floor (defined over floating-point numbers.) If the input value is not a whole number, return the next
*smaller*whole number as a floating-point number. **fmod**(x, y)- Floating-point remainder of x divided by y.
**hypot**(x,y)- Hypotenuse calculator. Assumes boring old Euclidean geometry. If the projection of a straight line segment onto the X axis is x units long, and the projection of that line segment onto the Y axis is y units long, then the line segment is hypot(x,y) units long. Equivalent to sqrt(x*x+y*y).
**int**(x)- Convert number to integer by truncation. Limited by the size of long in C.
**isqrt**(x)- Compute the integer part of the square root of x.
**log**(x)- Natural logarithm.
**log10**(x)- Logarithm with respect to base 10.
**max**(x,...)- Return the one argument with the greatest value.
**min**(x,...)- Return the one argument with the least value.
**pow**(x,y)- Power function. x to the power of y.
**rand**()- Random number. Uses uniform distribution over the range [0,1).
*This RNG is not suitable for cryptography.* **round**(x)- Round to nearest whole number.
*Not suitable for financial rounding.* **sin**(x)- Sine (input in radians).
**sinh**(x)- Hyperbolic sine.
**sqrt**(x)- Square root (well, the positive square root only. And Tcl doesn't do complex math, so the input had better be positive).
**srand**(x)- Seeds the random number generator with the given value. Each interpreter has its own random number generator, which starts out seeded with the current time.
**tan**(x)- Tangent (input in radians.)
**tanh**(x)- Hyperbolic tangent.kjh
**wide**(x)- Take any numeric value, and return the low order 64 bits of the integer value of the argument

## Mathematical Expressions edit

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 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

## Order of Precedence edit

The following returns returns 4, rather than -4 as some might expect:set a [expr {-2**2}]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 ||) [1]. I wouldn't cry if a new language (not Tcl) decided to break compatibility with C in this respect.

## Composing Expressions edit

[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, take advantage of double substitution by omitting braces:

set y 2*3; expr $y ;# ==> 6 set y 2*3; puts [expr $y+2] ;# ==> 8But be careful not to introduce an injection attack vulnerability. See double substitution.

## Literal String Operands edit

[expr] implements a little language distinct from standard Tcl. One difference is that [expr] requires literal strings to be quoted:% 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}If you prefer consistency, then throughout your Tcl program, always quote values you consider to be "strings". (AMG: This is a totally arbitrary and stylistic distinction because everything is a string.) Note that at some point [expr] may grow the feature of accepting bareword strings. (AMG: Or it might choose to interpret barewords as variable substitutions.)

## Canonical Number Form edit

If the value of an [expr] is numeric, it will transformed into a canonical numeric form:set val 0x10 puts $val ;# 0x10 set val [expr {$val}] puts $val ;# 16

puts [expr {[join {0 x 1 0} {}]}] ;# 16In other words, [expr] may mutate strings that can be interpreted as numbers, which is a potential gotcha for the programmer using string functions of [expr] while not considering that they may have a numeric interpretation.

## The Octal Bug edit

See Tcl and Octal Numbers for details.rjm: Why does [expr] return integers with leading zeroes as hex?, e.g.expr 0100 ;# -> 64AMG: 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 of the leading zeroes - as would be

**explicitly**necessary in every typed language...

## Floating-Point Arithmetic edit

[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 floating-point representation that approaches 36.37: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

3636.9999999999995RS 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 8.4.9One way to get "3637" would be to use round():

expr {round(36.37*100)}The [format] command 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) 3636x86 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.

## pow() vs ** edit

LES 2005-07-23:% expr pow(5,6) 15625.0 % expr 5**6 15625Two syntaxes, two slightly different results. Is that intentional?RS: Yes. While pow() always goes for double logarithms, ** tries to do integer exponentiation where possible.

## Precision edit

davou: What is the precision of [expr]'s functions, 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**double**s, 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

**long**s, 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.

## Nan and Inf edit

At least as of Tcl 8.5, NaN and Inf are potential values returning from [expr].## Interactions with locale edit

[expr]'s parsing of decimals may be hampered by locale - you might get*syntax error in expression 1.0*

## Brace Your Expressions For Syntax edit

[expr] resolves variables in the context of its caller, and each variable value becomes exactly one operand in the expression. Consider the following:set color1 green set color2 green expr {$color1} eq {$color2} ;# 1But if the arguments are not bracketed, it is a syntax error:

#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 bracketedAnother example illustrating the same point:

set a "abc" set b [list 123 abcd xyz lmnop] 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 ni $b} # 1

% expr $a == "foo" ? true : false invalid bareword "abc" % expr {$a == "foo" ? true : false} false

## Brace Your Expressions for Performance edit

[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 to be a persistent Tcl_Obj in which to store the compiled math expression. 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

## Brace Your Expressions for Security edit

AMG: The security problems of unbraced [expr]essions are very similar to SQL injection attacks. Notice how sqlite's Tcl binding does its own variable expansion to avoid this very problem. Many, many sh scripts have this problem as well because the default is to apply multiple passes of interpretation.See also double substitution## Remember [if], [for], and [while] edit

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)

## The 'Dot' Trick for Unbraced Expressions edit

Unless you know exactly what you are doing, unbraced expressions are not recommended. Nevertheles...With unbraced expressions, a "." can be appended to a variable to get [expr] to interpret the value as a float, but double() is a better alternative: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 } { puts "$v: [time [set $v] 10000]" } #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 [expr]'s parser, 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.

## Bytecode compilation and performance edit

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) doneThis 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.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) doneBack 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

## History edit

In Embedded vs. separate commands, 1992-12-38, JO published the voting results 37 : 8 in favor of embedded functions() vs. separate [commands]## (Tcl 8.4 and older) 32-bit integer limitations edit

Addition% expr (1<<31)-1 2147483647 % expr 2147483647 + 2147483647 -2Multiplication

% expr sqrt((1<<31)-1) 46340.9500011 expr 46341*46341 -2147479015These 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.

## Suggestion to reparse expr variables edit

RS suggests [expr]'s arguments could be reparsed so that full mathematical expressions in variable values would interpreted as suchRS 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] ;# ==> 6AM: 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.

## Unsuitability of expr for time offset calculations edit

[Wookie]: I had some trouble recently using [expr] to calculate time offsets. I had 2 time stamps in the form hh:mmSo I had 4 variables h1, m1, h2, m2 and one of my [expr] functions wasset 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 [expr]ession. 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 ;# ==> 16:15:00What are you trying to do with your two times?