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EvaluateExpression
Introduction and Overview
The EvaluateExpression module provides the SWI OS_EvaluateExpression. This call attempts to calculate the value of the string passed to it. Although the SWI itself is not normally used by the programmer, its effects are seen most commonly through the *SetEval and *If commands.
Technical details
Operators
OS_EvaluateExpression is defined to operate using a number of specified operations. These are unchanged since RISC OS 2 (and possibly earlier). Dependent on their context, the operations are applied to either numbers or strings, with the preference being for string operations.
| ( ) | Grouping operators, applied using correct mathematical precedence |
| - * / | Arithmetic operators |
| + | Arithmetic operator, or if used on strings a concatenation operator |
| = | Equality (either numeric or string dependent on its context, the same condition applies to all the comparison operators) |
| <> | Inequality |
| < | Less than |
| > | Greater than |
| <= | Less than or equal to |
| >= | Greater than or equal to |
| >>> | Logical shift right (ignores sign) |
| >> | Arithmetic shift right (sign extending) |
| << | Shift left (arithmetic shift and logical shift are the same) |
| AND | Logical AND (both conditions must be true) |
| OR | Logical OR (either condition may be true) |
| EOR | Logical exclusive OR (one or the other may be true, but not both) |
| NOT | Negate (inverts the bitpattern of a number) |
| MOD | Modulo (remainder) |
| RIGHT | Obtain the rightmost characters from a string |
| LEFT | Obtain the leftmost characters from a string |
| STR | Convert a number to a string |
| VAL | Convert a string to a number |
| LEN | Find the length of a string |
In addition, the EvaluateExpression module also provides a number of extensions to these operators :
LEAFNAME
Return the leaf name from the path specified.
Example : LEAFNAME "ADFS::4.$.Directory.File"
Evaluates to: "File"
DIRNAME
Return the directory name from the path specified.
Example : DIRNAME "ADFS::4.$.Directory.File"
Evaluates to: "ADFS::4.$.Directory"
CANONICALISE
Convert the path specified to its canonical form.
Example : CANONICALISE "ADFS::4.$.Directory.File"
Evaluates to: "ADFS::HardDisc4.$.Directory.File"
TIMEFORMAT
Format the current time in the format specified.
Example : TIMEFORMAT "%24-%MI-%SE"
Evaluates to: "18-27-22"
SET
Returns boolean value of whether a system variable is set, -1 if set, 0 if not set.
Example : SET "Not$Set"
Evaluates to : 0
MODULEVERSION
Returns the version number of a module multiplied by 100 or -1 if not present.
Example : MODULEVERSION "WindowManager"
Evaluates to: 607
Change in parsing rules
The operator parsing rules have changed slightly in version 0.13 of the EvaluateExpression module. Previous versions would have given the following sequence of results :
*set LENGTH 77
*eval LENGTH
Unknown operand
*set GTH 12
*eval LENGTH
Result is an integer, value :2
*eval length
Result is a string, value :7
That is, the operator prefix on a variable name would always be processed first, even if the operator was completely alphabetically named. The new parser will only treat alphabetic operators as being complete if they are followed by non-alphabetic characters. Thus, the above sequence of commands now produces :
*set LENGTH 7
*eval LENGTH
Result is a string, value :7
*set GTH 12
*eval LENGTH
Result is a string, value :7
*eval length
Result is a string, value :7
This is both more obvious and more deterministic. Where developers have used operators in this way in the past it was necessary to insert whitespace between the operator name and its parameter. It is not expected, due to the non-obvious behaviour of such evaluation, that this will cause any problems for the majority of developers.
Practical examples
*SetEval Obey$AppName (LEAFNAME Obey$Dir) RIGHT (LEN (LEAFNAME Obey$Dir)-1)
| Obey file 'which'
IfThere Run:%0 Then SetEval Which CANONICALISE "Run:%0" Else Set Which Not on run path
Echo <Which>
Unset Which
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![[Select]](../imgs/seldoc.png)
