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expr 8.2 + 6
evaluates to 14.2. Tcl expressions differ from C expressions in the way that operands are specified. Also, Tcl expressions support non-numeric operands and string comparisons.
Operands may be specified in any of the following ways:
[1] As an numeric value, either integer or floating-point.
[2] As a Tcl variable, using standard $ notation. The
variable's value will be used as the operand.
[3] As a string enclosed in double-quotes. The expression parser will perform backslash, variable, and command substitutions on the information between the quotes, and use the resulting value as the operand
[4] As a string enclosed in braces. The characters between the open brace and matching close brace will be used as the operand without any substitutions.
[5] As a Tcl command enclosed in brackets. The command will be executed and its result will be used as the operand.
[6] As a mathematical function whose arguments have any of the above forms for operands, such as ``sin($x)''. See below for a list of defined functions.
Where substitutions occur above (e.g. inside quoted strings), they are performed by the expression processor. However, an additional layer of substitution may already have been performed by the command parser before the expression processor was called. As discussed below, it is usually best to enclose expressions in braces to prevent the command parser from performing substitutions on the contents.
For some examples of simple expressions, suppose the variable a has the value 3 and the variable b has the value 6. Then the command on the left side of each of the lines below will produce the value on the right side of the line:
< > <= >= Boolean less, greater, less than or equal, and greater than or equal. Each operator produces 1 if the condition is true, 0 otherwise. These operators may be applied to strings as well as numeric operands, in which case string comparison is used.
== != Boolean equal and not equal. Each operator produces a zero/one result. Valid for all operand types.
expr 4*2 < 7
returns 0.
The &&, ||, and ?: operators have ``lazy evaluation'', just as in C, which means that operands are not evaluated if they are not needed to determine the outcome. For example, in the command
expr {$v ? [a] : [b]}
only one of [a] or [b] will actually be evaluated, depending on the value of $v. Note, however, that this is only true if the entire expression is enclosed in braces; otherwise the Tcl parser will evaluate both [a] and [b] before invoking the expr command.
acos cos hypot sinh asin cosh log sqrt atan exp log10 tan atan2 floor pow tanh ceil fmod sin
Each of these functions invokes the math library function of the same name; see the manual entries for the library func- tions for details on what they do. Tcl also implements the following functions for conversion between integers and floating-point numbers:
abs(arg)
|
Returns the absolute value of arg. Arg may be either
integer or floating-point, and the result is returned
in the same form.
double(arg)
|
If arg is a floating value, returns arg, otherwise con-
verts arg to floating and returns the converted value.
int(arg)
|
If arg is an integer value, returns arg, otherwise con-
verts arg to integer by truncation and returns the con-
verted value.
round(arg)
|
If arg is an integer value, returns arg, otherwise con- verts arg to integer by rounding and returns the con- verted value.
In addition to these predefined functions, applications may define additional functions using Tcl_CreateMathFunc().
Conversion among internal representations for integer, floating-point, and string operands is done automatically as needed. For arithmetic computations, integers are used until some floating-point number is introduced, after which floating-point is used. For example,
expr 5 / 4
returns 1, while
expr 5 / 4.0
expr 5 / ( [string length "abcd"] + 0.0 )
both return 1.25. Floating-point values are always returned with a ``.'' or an ``e'' so that they will not look like integer values. For example,
expr 20.0/5.0
returns ``4.0'', not ``4''. The global variable tcl_precision determines the the number of significant digits that are retained when floating values are converted to strings (except that trailing zeroes are omitted). If tcl_precision is unset then 6 digits of precision are used. To retain all of the significant bits of an IEEE floating- point number set tcl_precision to 17; if a value is con- verted to string with 17 digits of precision and then con- verted back to binary for some later calculation, the resulting binary value is guaranteed to be identical to the original one.
expr {"0x03" > "2"}
expr {"0y" < "0x12"}
both return 1. The first comparison is done using integer comparison, and the second is done using string comparison after the second operand is converted to the string ``18''. Because of Tcl's tendency to treat values as numbers when- ever possible, it isn't generally a good idea to use opera- tors like == when you really want string comparison and the values of the operands could be arbitrary; it's better in these cases to use the string compare command instead.