Pointers - Revision history

Jwatson: /* Assigning Pointers */

2012-09-04T03:15:31Z

Assigning Pointers

Jwatson at 03:14, 4 September 2012

2012-09-04T03:14:51Z

Jwatson at 03:14, 4 September 2012

2012-09-04T03:14:32Z

Jwatson at 03:13, 4 September 2012

2012-09-04T03:13:56Z

Jwatson at 02:24, 4 September 2012

2012-09-04T02:24:16Z

Jwatson: /* Pointers */

2012-09-04T01:50:33Z

Pointers

Jwatson: /* Pointers */

2012-09-04T01:49:51Z

Pointers

Jwatson: Created page with "General|General Programming
{{tl|1|1}}
== Pointers == {| class="wikiText" |- | Pointers are used to 'point..."

2012-09-03T20:30:51Z

Created page with "<yambe:breadcrumb self="Pointers">General|General Programming</yambe:breadcrumb> {{tl|1|1}} == Pointers == {| class="wikiText" |- | Pointers are used to 'point..."

New page

<yambe:breadcrumb self="Pointers">General|General Programming</yambe:breadcrumb>
 

{{tl|1|1}}
 

== Pointers ==
{| class="wikiText"
|-
| Pointers are used to 'point' to a variable's memory location, instead of the value stored in that location. Even if the value of the variable changes, its memory location generally stays the same. In the example below, there are three variables; a pointer integer variable called 'pointer', denoted as a pointer by the asterik * preceding its name, an integer variable called 'nopointer', and an integer called 'foo'.
|-
|<syntaxhighlight lang="ROBOTC">task main()
{
int *pointer;
int nopointer;
int foo=0;

pointer=&foo;
nopointer=foo;

foo=5;

pointer=&foo;
nopointer=foo;
}</syntaxhighlight>
|-
|Line 7 of the program sets the 'pointer' variable to the memory location of 'foo' (this is done by using the ampersand symbol (&) when setting pointer equal to foo) and line 8 sets the 'nopointer' variable to the value of 'foo'. The Local Variables debug window shows that 'pointer' now has the memory location of foo (0x2D52) while 'nopointer' holds the value of foo (0).
|-
|
|-
|Next, we change the value of 'foo' to 5 and reset the values of 'pointer' and 'nopointer' to foo. The Local Variables window again updates its values to show that nopointer now has the value in foo (5) but the pointer variable has remained unchanged. This is because '''even though the value of the variable foo has changed, its location in memory has not'''.
|-
|
|-
|}
== The House Analogy ==
{|
|A good analogy for how pointers operate is how street addresses work. When a house is going to be built, contractors must make sure that the land they are going to build the house on is big enough to contain it. Once the house is built it gets its own street address depending on where it is at on the street. Then, no matter who is living in the house, the street address will remain the same.
Likewise, when a variable is being created in a program, the program must make sure that its memory location is large enough to hold the variable. Once the variable is created it gets its own memory address depending on where in memory it is residing. Then, no matter what value is stored in the variable, the memory address will remain the same.
|}

@@ Line 67: / Line 67: @@
 ptr = foo;</syntaxhighlight>
 |-
-|Creates a pointer named ptr of the integer type, an integer named foo, and sets the value stored by foo to the pointer ptr (in this case, 0). This can cause errors if not used correctly and for the most part should be avoided.
+|Creates a pointer named ptr of the integer type, an integer named foo, and sets the value stored by foo to the pointer ptr (in this case, the memory address of 0). This can cause errors if not used correctly and for the most part should be avoided.
 |}
 <br />

@@ Line 34: / Line 34: @@
 |-
 |}
+<br />
 == The House Analogy ==
 {|
@@ Line 40: / Line 40: @@
 Likewise, when a variable is being created in a program, the program must make sure that its memory location is large enough to hold the variable. Once the variable is created it gets its own memory address depending on where in memory it is residing. Then, no matter what value is stored in the variable, the memory address will remain the same.
 |}
+<br />
 == Declaring Pointers ==
 {| style="color:black;" width="100%" cellpadding="5%" cellspacing="0" border="0"

← Older revision		Revision as of 02:24, 4 September 2012
Line 41:		Line 41:
	\|}		\|}

		+	== Declaring Pointers ==
		+	{\| style="color:black;" width="100%" cellpadding="5%" cellspacing="0" border="0"
		+	\|-
		+	\| <syntaxhighlight lang="ROBOTC">int *ptr;</syntaxhighlight>
		+	\|-
		+	\|Creates a pointer named 'ptr' of the integer type. Both the name and the type of variable must be declared, with the name being preceded by an asterisk (*).
		+	\|-
		+	\|}

		+	== Assigning Pointers ==
		+	{\|
		+	\|<syntaxhighlight lang="ROBOTC">int *ptr;
		+	int foo=0;

		+	ptr = &foo;</syntaxhighlight>
		+	\|-
		+	\|Creates a pointer named ptr of the integer type, an integer named foo, and sets the memory address of foo to the pointer ptr (using the ampersand, &).
		+	\|
		+	\|-
		+	\|<syntaxhighlight lang="ROBOTC">int *prt;
		+	int foo=0;

		+	ptr = foo;</syntaxhighlight>
		+	\|-
		+	\|Creates a pointer named ptr of the integer type, an integer named foo, and sets the value stored by foo to the pointer ptr (in this case, 0). This can cause errors if not used correctly and for the most part should be avoided.
		+	\|}
	<br />		<br />

@@ Line 28: / Line 28: @@
 |-
 |[[File:Pointers_Example_One.png]]
 |-
 |Next, we change the value of 'foo' to 5 and reset the values of 'pointer' and 'nopointer' to foo. The Local Variables window again updates its values to show that nopointer now has the value in foo (5) but the pointer variable has remained unchanged. This is because '''even though the value of the variable foo has changed, its location in memory has not'''.
 |-
 |[[File:Pointers_Example_Two.png]]
 |-
 |}

@@ Line 27: / Line 27: @@
 |Line 7 of the program sets the 'pointer' variable to the memory location of 'foo' (this is done by using the ampersand symbol (&) when setting pointer equal to foo) and line 8 sets the 'nopointer' variable to the value of 'foo'. The Local Variables debug window shows that 'pointer' now has the memory location of foo (0x2D52) while 'nopointer' holds the value of foo (0).
 |-
-|[[File:Pointer_Example_One.png]]
+|[[File:Pointers_Example_One.png]]
 |-
 |Next, we change the value of 'foo' to 5 and reset the values of 'pointer' and 'nopointer' to foo. The Local Variables window again updates its values to show that nopointer now has the value in foo (5) but the pointer variable has remained unchanged. This is because '''even though the value of the variable foo has changed, its location in memory has not'''.
 |-
-|[[File:Pointer_Example_Two.png]]
+|[[File:Pointers_Example_Two.png]]
 |-
 |}

Pointers - Revision history

Jwatson: /* Assigning Pointers */

Jwatson at 03:14, 4 September 2012

Jwatson at 03:14, 4 September 2012

Jwatson at 03:13, 4 September 2012

Jwatson at 02:24, 4 September 2012

Jwatson: /* Pointers */

Jwatson: /* Pointers */

Jwatson: Created page with "General|General Programming {{tl|1|1}} == Pointers == {| class="wikiText" |- | Pointers are used to 'point..."

Jwatson: Created page with "General|General Programming
{{tl|1|1}}
== Pointers == {| class="wikiText" |- | Pointers are used to 'point..."