Val: what changed
Revision 0 to 1. Removed lines have a minus sign; added lines have a plus sign.
Title: Val
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| 1 | + | The [[Procedure|procedure]] [`system.val`](https://www.freepascal.org/docs-html/rtl/system/val.html) attempts to convert a string representation of a numeric value into a numeric value variable. This [[Borland Pascal]] extension is part of the default [[RTL|run-time library]] delivered with the [[FPC|FreePascal compiler]], but otherwise not standardized. | |
| 2 | + | ||
| 3 | + | ## usage | |
| 4 | + | ||
| 5 | + | The formal signature reads: | |
| 6 | + | ||
| 7 | + | ``` pascal | |
| 8 | + | procedure val(const s: string; var v; var code: word) | |
| 9 | + | ``` | |
| 10 | + | ||
| 11 | + | `S` is a [[String|string]]-type expression; it must be a sequence of characters that form a (possibly signed) number or enumerated type value. `V` is an ordinal or [[Real|real]]-type variable. `Code` is a [[Word|word]] integer variable. | |
| 12 | + | ||
| 13 | + | If the string `s` can not be converted to a numeric value, `code` holds the index of the first character causing troubles; otherwise `code` is zero, indicating success. | |
| 14 | + | ||
| 15 | + | > **Note:** In contrast to `read`, the value of `v` remains unchanged if no data (i.e. empty string `s`) is available. No `default` value is assigned. | |
| 16 | + | ||
| 17 | + | The procedure `val` is so useful, since it does not trigger [[runtime error|run-time errors]] or [[Exceptions|exceptions]]. Unlike [[Read|read]] the user can be supplied with more useful error messages telling which character is not a numeric symbol. The power of `val` lies in its capability to also convert enumerated types *by their named values*. | |
| 18 | + | ||
| 19 | + | ``` pascal | |
| 20 | + | program valDemo(input, output, stdErr); | |
| 21 | + | type | |
| 22 | + | decision = (no, yes, maybe); | |
| 23 | + | var | |
| 24 | + | s: string; | |
| 25 | + | r: decision; | |
| 26 | + | c: word; | |
| 27 | + | begin | |
| 28 | + | writeLn('Are you OK?'); | |
| 29 | + | readLn(s); | |
| 30 | + | val(s, r, c); | |
| 31 | + | writeLn('So ', r, '.'); | |
| 32 | + | end. | |
| 33 | + | ``` | |
| 34 | + | ||
| 35 | + | Note, as a general design principle this feature should not be used for user responses like this example shows, since it thwarts internationalization. Localization in general is not possible with `val`, most notably the decimal separator – in some regions a [[Comma|comma]], in others a [[period]] – can not be customized; it is always a period, as it is standard in your [[Standard Pascal|Pascal]] source code. However, `val` still may be useful for reading and parsing *generated* data. | |
| 36 | + | ||
| 37 | + | ## see also | |
| 38 | + | ||
| 39 | + | - [[Str|system.str]] performs the reverse action | |
| 40 | + | - [[RTTI]] | |
| 41 | + | - [String operations](https://BorlandPascal.FanDom.com/wiki/String_operations) in *BorlandPascal Fandom Wiki* | |
| 42 | + | ||
| 43 | + | --- | |
| 44 | + | *From the Free Pascal wiki, page "Val", as it stood on 2023-02-07 after a change by Kai Burghardt, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).* | |
| 45 | + | Category: Code |