Difference between revisions of "Val"
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{{Val}} | {{Val}} | ||
| − | The [[Procedure|procedure]] {{Doc|package=RTL|unit=system|identifier=val|text=<syntaxhighlight lang="pascal" | + | The [[Procedure|procedure]] {{Doc|package=RTL|unit=system|identifier=val|text=<syntaxhighlight lang="pascal" inline>system.val</syntaxhighlight>}} 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. | |
== usage == | == usage == | ||
The formal signature reads:<syntaxhighlight lang="pascal">procedure val(const s: string; var v; var code: word)</syntaxhighlight> | The formal signature reads:<syntaxhighlight lang="pascal">procedure val(const s: string; var v; var code: word)</syntaxhighlight> | ||
| − | <syntaxhighlight lang="pascal" | + | <syntaxhighlight lang="pascal" inline>S</syntaxhighlight> is a [[String|string]]-type expression; |
it must be a sequence of characters that form a (possibly signed) number or enumerated type value. | it must be a sequence of characters that form a (possibly signed) number or enumerated type value. | ||
| − | <syntaxhighlight lang="pascal" | + | <syntaxhighlight lang="pascal" inline>V</syntaxhighlight> is an ordinal or [[Real|real]]-type variable. |
| − | <syntaxhighlight lang="pascal" | + | <syntaxhighlight lang="pascal" inline>Code</syntaxhighlight> is a [[Word|word]] integer variable. |
| − | If the string <syntaxhighlight lang="pascal" | + | If the string <syntaxhighlight lang="pascal" inline>s</syntaxhighlight> can not be converted to a numeric value, <syntaxhighlight lang="pascal" inline>code</syntaxhighlight> holds the index of the first character causing troubles; |
| − | otherwise <syntaxhighlight lang="pascal" | + | otherwise <syntaxhighlight lang="pascal" inline>code</syntaxhighlight> is zero, indicating success. |
| − | {{Note|In contrast to <syntaxhighlight lang="pascal" | + | {{Note|In contrast to <syntaxhighlight lang="pascal" inline>read</syntaxhighlight>, the value of <syntaxhighlight lang="pascal" inline>v</syntaxhighlight> remains unchanged if no data (i.e. empty string <syntaxhighlight lang="pascal" inline>s</syntaxhighlight>) is available. |
| − | No <syntaxhighlight lang="delphi" | + | No <syntaxhighlight lang="delphi" inline>default</syntaxhighlight> value is assigned.}} |
| − | The procedure <syntaxhighlight lang="pascal" | + | The procedure <syntaxhighlight lang="pascal" inline>val</syntaxhighlight> is so useful, since it does not trigger [[runtime error|run-time errors]] or [[Exceptions|exceptions]]. |
| − | Unlike [[Read|<syntaxhighlight lang="pascal" | + | Unlike [[Read|<syntaxhighlight lang="pascal" inline>read</syntaxhighlight>]] the user can be supplied with more useful error messages telling which character is not a numeric symbol. |
| − | The power of <syntaxhighlight lang="pascal" | + | The power of <syntaxhighlight lang="pascal" inline>val</syntaxhighlight> lies in its capability to also convert enumerated types ''by their named values''. |
<syntaxhighlight lang="pascal" highlight="3,6,11"> | <syntaxhighlight lang="pascal" highlight="3,6,11"> | ||
program valDemo(input, output, stdErr); | program valDemo(input, output, stdErr); | ||
| Line 35: | Line 35: | ||
</syntaxhighlight> | </syntaxhighlight> | ||
Note, as a general design principle this feature should not be used for user responses like this example shows, since it thwarts internationalization. | 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 <syntaxhighlight lang="pascal" | + | Localization in general is not possible with <syntaxhighlight lang="pascal" inline>val</syntaxhighlight>, most notably the decimal separator – in some regions a [[Comma|comma]], in others a [[period|period]] – can not be customized; |
it is always a period, as it is standard in your [[Standard Pascal|Pascal]] source code. | it is always a period, as it is standard in your [[Standard Pascal|Pascal]] source code. | ||
| + | However, <syntaxhighlight lang="pascal" inline>val</syntaxhighlight> still may be useful for reading and parsing ''generated'' data. | ||
== see also == | == see also == | ||
| − | * [[Str|<syntaxhighlight lang="pascal" | + | * [[Str|<syntaxhighlight lang="pascal" inline>system.str</syntaxhighlight>]] performs the reverse action |
* [[RTTI]] | * [[RTTI]] | ||
| + | * [https://BorlandPascal.FanDom.com/wiki/String_operations String operations] in ''BorlandPascal Fandom Wiki'' | ||
[[Category:Code]] | [[Category:Code]] | ||
Latest revision as of 15:52, 7 February 2023
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The procedure system.val attempts to convert a string representation of a numeric value into a numeric value variable.
This Borland Pascal extension is part of the default run-time library delivered with the FreePascal compiler, but otherwise not standardized.
usage
The formal signature reads:
procedure val(const s: string; var v; var code: word)
S is a 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-type variable.
Code is a word integer variable.
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.
Note: In contrast to read, the value of v remains unchanged if no data (i.e. empty string s) is available.
default value is assigned.The procedure val is so useful, since it does not trigger run-time errors or exceptions.
Unlike 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.
program valDemo(input, output, stdErr);
type
decision = (no, yes, maybe);
var
s: string;
r: decision;
c: word;
begin
writeLn('Are you OK?');
readLn(s);
val(s, r, c);
writeLn('So ', r, '.');
end.
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, in others a period – can not be customized;
it is always a period, as it is standard in your Pascal source code.
However, val still may be useful for reading and parsing generated data.
see also
system.strperforms the reverse action- RTTI
- String operations in BorlandPascal Fandom Wiki
