Understanding Pointers and Reference Passing in C, Slides of Computer Fundamentals

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Pointers

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• Passing Pointer to func/on
• Pointers and Arrays
• Array of Pointer
• Dynamic Memory Alloca/on

Pointers

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Calling Functions by Reference

• Call by reference with pointer arguments
  • Pass address of argument using & operator
  • Allows you to change actual location in memory
  • Arrays are not passed with & because the array name is already

a pointer

• • operator
    • Used as alias/nickname for variable inside of function void double( int *number ) { *number = 2 * ( *number ); }
    • *number used as nickname for the variable passed

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Passing arguments by address

void func ( int *pValue )

{

*pValue = 6;

}

void main ( )

{

int nValue = 5; printf(“nValue = %d\n“, nValue); func (&nValue ); printf("nValue = %d\n”, nValue);

}

// 5 gets printed, then 6 gets printed

`

nValue

5

pValue

00EF

00AA1 00EF

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Outline

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fig07_10.c (Part 1 of 2)

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OutlineOutline

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fig07_10.c (Part 2 of 2)

The string before conversion is: characters and Program Output $32. The string after conversion is: CHARACTERS AND $32.

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fig07_11.c (Part 2 of 2)

Program Output The string is: print characters of a string

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fig07_12.c

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fig07_13.c

Program Output Compiling... FIG07_13.c D:\books\2003\chtp4\Examples \ch07\FIG07_13.c(15) : error C2166: l-value specifies const object Error executing cl.exe.

Changing *ptr is allowed – x is not a constant.

Changing ptr is an error – ptr is a constant pointer.

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OutlineOutline

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fig07_14.c

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Swap

temp = x ;

x = y ;

y = temp ;

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.

void swap ( int * , int * );

// function prototype

void main ( )

{

int x = 10 , y = 20 ;

printf(“Before calling swap \n”);

printf(“value of x=%d and y=%y \n”,x,y);

swap ( &x , &y ) ;

// function call

printf(“After calling swap \n”);

printf(“value of x=%d and y=%y \n”,x,y);

}

Swap using function

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• sizeof
  • Returns size of operand in bytes
  • For arrays: size of 1 element * number of elements
  • if sizeof( int ) equals 4 bytes, then int myArray[ 10 ]; printf( "%d", sizeof( myArray ) );
• will print 40
• sizeof can be used with
  • Variable names
  • Type name
  • Constant values

sizeof function

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