Difference between revisions of "Val"

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(Created page with "Procedure '''Val''' converts the String value '''S''' to its numeric representation. S is a string-type expression; it must be a sequence of characters that form a signed...")
 
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Procedure '''Val''' converts the [[String]] value '''S''' to its numeric representation.
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{{Val}}
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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.
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This [[Borland Pascal]] extension is part of the default [[RTL|run-time library]] delivered with the [[FPC|FreePascal compiler]], but otherwise not standardized.
  
S is a string-type expression; it must be a sequence of characters that form a signed whole number. V is an integer-type or real-type variable. Code is a variable of type Integer. If the string is invalid, the index of the offending character is stored in Code; otherwise, Code is set to zero.  
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== usage ==
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The formal signature reads:<syntaxhighlight lang="pascal">procedure val(const s: string; var v; var code: word)</syntaxhighlight>
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<syntaxhighlight lang="pascal" inline>S</syntaxhighlight> is a [[String|string]]-type expression;
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it must be a sequence of characters that form a (possibly signed) number or enumerated type value.
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<syntaxhighlight lang="pascal" inline>V</syntaxhighlight> is an ordinal or [[Real|real]]-type variable.
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<syntaxhighlight lang="pascal" inline>Code</syntaxhighlight> is a [[Word|word]] integer variable.
  
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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;
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otherwise <syntaxhighlight lang="pascal" inline>code</syntaxhighlight> is zero, indicating success.
  
Declaration
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{{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.
procedure Val(S; var V; var Code: Integer);
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No <syntaxhighlight lang="delphi" inline>default</syntaxhighlight> value is assigned.}}
  
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The procedure <syntaxhighlight lang="pascal" inline>val</syntaxhighlight> is so useful, since it does not trigger [[runtime error|run-time errors]] or [[Exceptions|exceptions]].
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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.
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The power of <syntaxhighlight lang="pascal" inline>val</syntaxhighlight> lies in its capability to also convert enumerated types ''by their named values''.
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<syntaxhighlight lang="pascal" highlight="3,6,11">
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program valDemo(input, output, stdErr);
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type
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decision = (no, yes, maybe);
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var
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s: string;
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r: decision;
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c: word;
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begin
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writeLn('Are you OK?');
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readLn(s);
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val(s, r, c);
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writeLn('So ', r, '.');
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end.
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</syntaxhighlight>
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Note, as a general design principle this feature should not be used for user responses like this example shows, since it thwarts internationalization.
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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;
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it is always a period, as it is standard in your [[Standard Pascal|Pascal]] source code.
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However, <syntaxhighlight lang="pascal" inline>val</syntaxhighlight> still may be useful for reading and parsing ''generated'' data.
  
See also:
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== see also ==
* [[Str]]
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* [[Str|<syntaxhighlight lang="pascal" inline>system.str</syntaxhighlight>]] performs the reverse action
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* [[RTTI]]
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* [https://BorlandPascal.FanDom.com/wiki/String_operations String operations] in ''BorlandPascal Fandom Wiki''
  
[[Category:Pascal]]
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[[Category:Code]]

Latest revision as of 14:52, 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.

Light bulb  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.

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