Pointers - Pointers in C Call By Reference 1 Pre-requisite...

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Pointers in C
Call By Reference

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Pre-requisite
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Basics of the C programming language
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Data type
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Variable
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Array
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Function call
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Standard Input/Output
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e.g. printf(), scanf()

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Outline
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Computer Memory Structure
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Addressing Concept
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Introduction to Pointer
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Pointer Manipulation
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Summary

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Computer Memory Revisited
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Computers store data in memory slots
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Each slot has an unique address
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Variables store their values like this:
Addr Content Addr Content Addr Content Addr Content
1000 i: 37 1001 j: 46 1002 k: 58 1003 m: 74
1004 a[0]: ‘a’ 1005 a[1]: ‘b’ 1006 a[2]: ‘c’ 1007 a[3]: ‘\0’
1008 ptr: 1001 1009 … 1010 1011

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Computer Memory Revisited
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Altering the value of a variable is indeed
changing the content of the memory
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e.g. i = 40; a[2] = ‘z’;
Addr Content Addr Content Addr Content Addr Content
1000 i: 40 1001 j: 46 1002 k: 58 1003 m: 74
1004 a[0]: ‘a’ 1005 a[1]: ‘b’ 1006 a[2]: ‘z’ 1007 a[3]: ‘\0’
1008 ptr: 1001 1009 … 1010 1011

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Addressing Concept
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Pointer stores the address of another
entity
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It refers to a memory location
int i = 5;
int * ptr; /* declare a pointer variable */
ptr = & i; /* store address-of i to ptr */
printf(“*ptr = %d\n”, *ptr ); /* refer to referee of ptr */

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What actually ptr is?

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Twin Operators
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&: Address-of operator
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Get the address of an entity
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e.g. ptr = &j;
Addr Content Addr Content Addr Content Addr Content
1000 i: 40 1001 j: 33 1002 k: 58 1003 m: 74
1004 ptr: 1001 1005 1006 1007

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Twin Operators
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*: De-reference operator
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Refer to the content of the referee
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e.g. *ptr = 99;
Addr Content Addr Content Addr Content Addr Content
1000 i: 40 1001 j: 99 1002 k: 58 1003 m: 74
1004 ptr: 1001 1005 1006 1007