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Practical Programming in Tcl & Tk, Third Edition
By Brent B. Welch

Table of Contents
Chapter 1.  Tcl Fundamentals


Math Expressions

The Tcl interpreter itself does not evaluate math expressions. Tcl just does grouping, substitutions and command invocations. The expr command is used to parse and evaluate math expressions.

Example 1-4 Simple arithmetic.
expr 7.2 / 4
=> 1.8

The math syntax supported by expr is the same as the C expression syntax. The expr command deals with integer, floating point, and boolean values. Logical operations return either 0 (false) or 1 (true). Integer values are promoted to floating point values as needed. Octal values are indicated by a leading zero (e.g., 033 is 27 decimal). Hexadecimal values are indicated by a leading 0x. Scientific notation for floating point numbers is supported. A summary of the operator precedence is given on page 20.

You can include variable references and nested commands in math expressions. The following example uses expr to add the value of x to the length of the string foobar. As a result of the innermost command substitution, the expr command sees 6 + 7, and len gets the value 13:

Example 1-5 Nested commands.
set x 7
set len [expr [string length foobar] + $x]
=> 13

The expression evaluator supports a number of built-in math functions. (For a complete listing, see page 21.) Example 1-6 computes the value of pi:

Example 1-6 Built-in math functions.
set pi [expr 2*asin(1.0)]
=> 3.1415926535897931

The implementation of expr is careful to preserve accurate numeric values and avoid conversions between numbers and strings. However, you can make expr operate more efficiently by grouping the entire expression in curly braces. The explanation has to do with the byte code compiler that Tcl uses internally, and its effects are explained in more detail on page 15. For now, you should be aware that these expressions are all valid and run a bit faster than the examples shown above:

Example 1-7 Grouping expressions with braces.
expr {7.2 / 4}
set len [expr {[string length foobar] + $x}]
set pi [expr {2*asin(1.0)}]

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