Difference between revisions of "Inc and Dec"

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{{Inc}}
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{{Translate}}
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The [[Procedure|procedures]] <syntaxhighlight lang="pascal" enclose="none">inc</syntaxhighlight> and <syntaxhighlight lang="pascal" enclose="none">dec</syntaxhighlight> increment or decrement a given variable by default by one.
  
= Inc =
+
== usage ==
 +
The first parameter specifies an ordinal value variable (e.g. an [[Integer|<syntaxhighlight lang="pascal" enclose="none">integer</syntaxhighlight>]] or enumeration type) and the second optional parameter may specify a different addend/subtrahend.
  
Inc (increment) procedure in Pascal programming let you easily add 1 (or some other value) from, a variable.
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<syntaxhighlight lang="pascal" line highlight="13,18,22">
 +
program incDecDemo(input, output, stderr);
  
For example, using Inc (increment) procedure, you can increase 1 to a variable named a like this:
+
type
 +
primaryColor = (red, green, blue);
  
<syntaxhighlight>
+
var
inc( a );
+
phase: primaryColor;
 +
x: longint;
 +
 
 +
begin
 +
// enumeration type
 +
phase := red;
 +
inc(phase); // phase becomes green
 +
writeLn(phase);
 +
 +
// integer
 +
x := 1;
 +
inc(x, -1); // x becomes zero
 +
writeLn(x);
 +
 +
x := 1;
 +
dec(x); // same as above: x becomes zero
 +
writeLn(x);
 +
end.
 
</syntaxhighlight>
 
</syntaxhighlight>
  
If you want to add by two (or some other value):
+
== background ==
 +
In [[FPC]]'s [[System unit|system unit]] the {{Doc|package=RTL|unit=system|identifier=inc|text=<syntaxhighlight lang="pascal" enclose="none">inc</syntaxhighlight>}} and {{Doc|package=RTL|unit=system|identifier=dec|text=<syntaxhighlight lang="pascal" enclose="none">dec</syntaxhighlight>}} procedures are compiler procedures.
 +
They exist in order to optimize for certain architectures where faster <syntaxhighlight lang="asm" enclose="none">inc</syntaxhighlight> and <syntaxhighlight lang="asm" enclose="none">dec</syntaxhighlight> [[Assembly language|assembler]] instructions are available.
  
<syntaxhighlight>
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== special behaviors ==
inc( a, 2 );
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If <syntaxhighlight lang="pascal" enclose="none">{$rangeChecks}</syntaxhighlight> are turned on, those procedures may generate a run-time error (RTE 201).
</syntaxhighlight>
+
Also <syntaxhighlight lang="pascal" enclose="none">{$overflowChecks}</syntaxhighlight> may be generated (with a small difference in TP mode).
  
Inc procedure included [[System]] [[Unit]].
+
If [[Pointer|<syntaxhighlight lang="pascal" enclose="none">pointer</syntaxhighlight>]] arithmetics are allowed by the <syntaxhighlight lang="pascal" enclose="none">{$pointerMath}</syntaxhighlight> compiler switch, <syntaxhighlight lang="pascal" enclose="none">inc</syntaxhighlight> and <syntaxhighlight lang="pascal" enclose="none">dec</syntaxhighlight> work on pointers, too.
 +
In the case of typed pointers, e.g. a pointer to a [[Record|<syntaxhighlight lang="pascal" enclose="none">record</syntaxhighlight>]], the target's type size is considered automatically.
 +
For instance
 +
<syntaxhighlight lang="pascal" line highlight="9,10">
 +
program pointerIncDemo(input, output, stderr);
  
 +
{$pointerMath on}
  
== See also ==
+
var
 +
p: PQWord;
  
* Link to RTL documentation: [[doc:rtl/system/inc.html |inc]]
+
begin
* [[Dec]] - decrement value of integer variable.
+
inc(p);
* Link to RTL documentation: [[doc:/rtl/system/succ.html |succ]] - Return next element for an ordinal type.
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inc(p, 3);
* [[For]] control_variable := start_point [[To]] end_point [[Do]] statement.  
+
end.
 +
</syntaxhighlight>
 +
will generate (excerpt):
 +
<syntaxhighlight lang="asm" line start="13" highlight="8-11">
 +
; [pointerIncDemo.pas]
 +
; [8] begin
 +
leaq -8(%rsp),%rsp
 +
.Lc3:
 +
; Var p located in register rax
 +
call FPC_INITIALIZEUNITS
 +
movq $0,%rax
 +
; [9] inc(p);
 +
addq $8,%rax
 +
; [10] inc(p, 3);
 +
addq $24,%rax
 +
; [11] end.
 +
call FPC_DO_EXIT
 +
leaq 8(%rsp),%rsp
 +
ret
 +
</syntaxhighlight>
  
[[Category:Pascal]]
+
== see also ==
 +
* {{Doc|package=RTL|unit=system|identifier=succ|text=<syntaxhighlight lang="pascal" enclose="none">succ</syntaxhighlight>}} and {{Doc|package=RTL|unit=system|identifier=succ|text=<syntaxhighlight lang="pascal" enclose="none">pred</syntaxhighlight>}}
 +
* [[For|<syntaxhighlight lang="pascal" enclose="none">for</syntaxhighlight>-loops]]

Revision as of 15:06, 26 May 2018

Template:Translate The procedures inc and dec increment or decrement a given variable by default by one.

usage

The first parameter specifies an ordinal value variable (e.g. an integer or enumeration type) and the second optional parameter may specify a different addend/subtrahend.

 1program incDecDemo(input, output, stderr);
 2
 3type
 4	primaryColor = (red, green, blue);
 5
 6var
 7	phase: primaryColor;
 8	x: longint;
 9
10begin
11	// enumeration type
12	phase := red;
13	inc(phase); // phase becomes green
14	writeLn(phase);
15	
16	// integer
17	x := 1;
18	inc(x, -1); // x becomes zero
19	writeLn(x);
20	
21	x := 1;
22	dec(x); // same as above: x becomes zero
23	writeLn(x);
24end.

background

In FPC's system unit the inc and dec procedures are compiler procedures. They exist in order to optimize for certain architectures where faster inc and dec assembler instructions are available.

special behaviors

If {$rangeChecks} are turned on, those procedures may generate a run-time error (RTE 201). Also {$overflowChecks} may be generated (with a small difference in TP mode).

If pointer arithmetics are allowed by the {$pointerMath} compiler switch, inc and dec work on pointers, too. In the case of typed pointers, e.g. a pointer to a record, the target's type size is considered automatically. For instance

 1program pointerIncDemo(input, output, stderr);
 2
 3{$pointerMath on}
 4
 5var
 6	p: PQWord;
 7
 8begin
 9	inc(p);
10	inc(p, 3);
11end.

will generate (excerpt):

13; [pointerIncDemo.pas]
14; [8] begin
15	leaq	-8(%rsp),%rsp
16.Lc3:
17; Var p located in register rax
18	call	FPC_INITIALIZEUNITS
19	movq	$0,%rax
20; [9] inc(p);
21	addq	$8,%rax
22; [10] inc(p, 3);
23	addq	$24,%rax
24; [11] end.
25	call	FPC_DO_EXIT
26	leaq	8(%rsp),%rsp
27	ret

see also