Updated 2015-10-08 15:16:39 by pooryorick

expr, a built-in Tcl command, evaluates 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
more sugar for expr
Converting numbers from arbitrary bases
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
Floating-point formatting
including how to format a floating point value without causing it to shimmer
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
How can I do math in Tcl
Importing expr functions
use expr's functions without explicitly calling that, see Importing expr functions.
a package to facilitate writing sequences of mathematical formulae in the familiar infix form
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

Documentation  edit

officieal reference
mathop man page
Tcl_GetBoolean man page

Synopsis  edit

expr arg ?arg arg ...?

Description  edit

expr concatentates its arguments, evaluates this result as an 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,

name(argument,argument ...)`

For example:

Usage  edit

set val1 8.2
set val2 6
expr {$val1 + $val2}


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).

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-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).
* / % 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 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.
** exponential. From Tcl 8.5 on.
in ni checks for occurrence 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:
Absolute value (negate if negative).
Inverse cosine (result in radians).
Inverse sine (result in radians).
Inverse tangent (result in radians).
Inverse tangent. Can handle cases which plain atan() can't (due to division by zero) and has a larger output range (result in radians).
Accepts any valid boolean value and returns the corresponding boolean value 0 or 1.
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.
Cosine (input in radians).
Hyperbolic cosine.
Convert number to floating point.
Like int(x), but there is no limit to the input or output value.
Exponential function. Returns e to the power or x, where e is the base of natural logarithms.
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.
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).
Convert number to integer by truncation. Limited by the size of long in C.
Compute the integer part of the square root of x.
Natural logarithm.
Logarithm with respect to base 10.
Return the one argument with the greatest value.
Return the one argument with the least value.
Power function. x to the power of y.
Random number. Uses uniform distribution over the range [0,1). This RNG is not suitable for cryptography.
Round to nearest whole number. Not suitable for financial rounding.
Sine (input in radians).
Hyperbolic sine.
Square root (well, the positive square root only. And Tcl doesn't do complex math, so the input had better be positive).
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.
Tangent (input in radians.)
Hyperbolic tangent.kjh
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] ;# ==> 8

But 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}

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

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} {}]}] ;# 16

In 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 ;# -> 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 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

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

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)
() 61 % expr int(36.370*100)

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.

pow() vs **  edit

LES 2005-07-23:
% expr pow(5,6)

% expr 5**6

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.

Precision  edit

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.

Nan and Inf  edit

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
% info patchlevel
% expr {Inf+1}
[phil@becca ~]$ tclsh8.5 
% expr sqrt(-1)
domain error: argument not in valid range
% ::tcl::mathfunc::sqrt -1
% info patchlevel
% expr {Inf+1}

Interactions with locale  edit

parsing of decimals in expr 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} ;# 1

But if the arguments are not bracketed, it is a syntax error:
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

Another 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}

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.

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.

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 expressions 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:
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 the parser for expr', not recommended practice. Another example is substitution of operators:
set op +
expr 4 $op 5
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) 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.

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) 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

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

% expr (1<<31)-1

% expr 2147483647 + 2147483647

% expr sqrt((1<<31)-1)

expr 46341*46341

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.

Suggestion to reparse expr variables  edit

RS suggests that arguments to expr could be reparsed so that full mathematical 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
% let j = $i + 1
% let k = {$i + $j}
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.

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: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?