lecture 5 functions pass by value and pass by reference

Lecture 5 – Functions

Pass by Value and Pass by Reference

Recursion

Differences between Value Parameters and

Reference Parameters

Recursion

MAIN PROGRAM MEMORY

25

4000

age

If you pass only a copy of 25 to a

function, it is called “pass-by-value” and the function will not be able to

change the contents of age. It is still

25 when you return.

MAIN PROGRAM MEMORY

BUT, if you pass 4000, the address of

age to a function, it is called “pass-

by-reference” and the function will be

able to change the contents of age. It

could be 23 or 90 when you return.

25

4000

age

Value Parameter Reference Parameter

The value (25) of the

argument

is passed

to the function when

it is called.

The memory address (4000)

of the argument

is passed

to the function when

it is called.

25

4000

age

Argument

in Calling Block

reference parameters should be used

when you want your function to give a

value to, or change the value of, a

variable from the calling block.

Return more than one variables.

CALLING

BLOCK FUNCTION

CALLED

“incoming”

value of

argument

CALLING

BLOCK FUNCTION

CALLED

“incoming”

original value of

argument

“outgoing”

changed value of

argument

OR,

CALLING

BLOCK FUNCTION

CALLED

argument

has no value yet

when call occurs

“outgoing”

new value of

argument

13

int main()

{

int a = 3, b = 5;

printf("%d %d\n", a, b);

badSwap ( a, b );


return 0;

}

/* Swap the values of two variables.

*/

void badSwap ( int a, int b )

{

int temp;

temp = a;

a = b;

b = temp;


}

Output: 3 5

14

int main()

{

int a = 3, b = 5;


badSwap ( a, b );


return 0;

}


*/


{

int temp;

temp = a;

a = b;

b = temp;


}

Output: 3 5

5 3

15

int main()

{

int a = 3, b = 5;


badSwap ( a, b );


return 0;

}


*/


{

int temp;

temp = a;

a = b;

b = temp;


}

Output: 3 5

5 3

3 5

16

int main()

{

int a = 3, b = 5;


badSwap ( a, b );


return 0;

}

/* Swap the values of two variables. */


{

int temp;

temp = a;

a = b;

b = temp;


}

Output: 3 5

5 3

3 5

17

int main()

{

int a = 3, b = 5;


Swap ( &a, &b );


return 0;

}

/* Swap the values of two variables. */

void Swap ( int *a, int *b )

{

int temp;

temp = *a;

*a = *b;

*b = temp;

printf("%d %d\n", *a, *b);

}

Output: 3 5

18

int main()

{

int a = 3, b = 5;


Swap ( &a, &b );


return 0;

}


*/


{

int temp;

temp = *a;

*a = *b;

*b = temp;


}

Output: 3 5

5 3

19

int main()

{

int a = 3, b = 5;


Swap ( &a, &b );


return 0;

}


*/


{

int temp;

temp = *a;

*a = *b;

*b = temp;


}

Output: 3 5

5 3

5 3

20

int main()

{

int a = 3, b = 5;


Swap ( &a, &b );


return 0;

}


*/


{

int temp;

temp = *a;

*a = *b;

*b = temp;


}

Output: 3 5

5 3

5 3

We want to find 2 real roots for a quadratic

equation with coefficients a,b,c. Write a

prototype for a void function named

GetRoots( ) with 5 parameters. The first 3

parameters are type float. The last 2 are

reference parameters of type float.

#include <stdio.h>

#include <math.h> void GetRoots(float, float, float, float*, float*);

void main ( )

{

float a, b, c, first, second;

printf(“ Enter a b c : “); scanf(“%f %f %f”, &a , &b, &c ); GetRoots(a, b, c, &first, &second);

printf(“ Roots are %.2f %.2f \n “ , first , second );

}

// function GetRoots goes here

……..

Function Definition

In C programming, a single array element or an

entire array can be passed to a function.

Also, both one-dimensional and multi-

dimensional array can be passed to function as

argument.

#include <stdio.h>

void display(int a)

{ printf("%d",a);}

int main()

{int c[]={2,3,4};

display(c[2]); //Passing array element c[2] only.

return 0;}

Output : 4

While passing arrays to the argument, the name

of the array is passed as an argument (i.e.,

starting address of memory area is passed as

argument).

Write a C program to pass an array containing

age of 6 persons to a function. This function

should find average age and display the

average age in main function.

#include <stdio.h> float average(float a[]); int main() { float avg, c[]={23.4, 55, 22.6, 3, 40.5, 18}; avg=average(c);/* Only name of array is passed as argument. */ printf("Average age=%.2f",avg); return 0; } float average(float a[]) {int i;float avg, sum=0.0; for(i=0;i<6;i++) {sum+=a[i];} avg =(sum/6); return avg; }

To pass two-dimensional array to a function as

an argument, starting address of memory area

reserved is passed as in one dimensional array.

#include <stdio.h>

void Function(int c[2][2]);

int main()

{int c[2][2],i,j;

printf("Enter 4 numbers:\n");

for(i=0;i<2;++i)for(j=0;j<2;++j)

{scanf("%d",&c[i][j]);}

Function(c); /*passing multi-dimensional array to function */

return 0;

}

void Function(int c[2][2])

{/* Instead to above line, void Function(int c[][2]){ is also valid */

int i,j;

printf("Displaying:\n");

for(i=0;i<2;++i)

for(j=0;j<2;++j)

printf("%d\n",c[i][j]);

}

What is recursion?

When one function calls ITSELF directly or

indirectly.

A function is a recursive if a statement in the

body of the function calls the function that

contains it.

Why learn recursion?

New mode of thinking.

Powerful programming tool.

Divide-and-conquer paradigm.

Many computations are naturally self- referential

A recursive definition is made up of two parts. ◦ Base case that tells us directly what the answer is.

◦ Recursive case that defines the answer in terms of the answer to a subproblem.

For example, in factorial: ◦ Base case is factorial(0)=1.

◦ Recursive case is factorial(n) = n * factorial(n-1).

Recursive Function Multiply

int multiply ( int m, int n)

{

int ans;

If (n==1)

ans=m;

else

ans= m + multiply (m, n-1);

return (ans);

}

m is 6

n is 3

3 == 1 is false

ans is 6 + multiply (6,2)

return (ans)

m is 6

n is 2

2 == 1 is false

ans is 6 + multiply (6,1)

return (ans)

m is 6

n is 1

1 == 1 is true

ans is 6

return (ans)

6

12

18

multiply (6,3) int multiply ( int m, int n) { int ans; If (n==1) ans=m; else ans= m + multiply (m, n-

1); return (ans); }

When a function calls itself, new local variables

are allocated storage on the stack, and the

function code is executed with these new

variables from the beginning .

A recursive call does not make a new copy of

the function. Only the arguments are new.

#include <stdio.h>

int sum(int n)

{ if(n==0)

return n;

else return n+sum(n-1);

}

int main()

{ int num,add;

printf("Enter a positive integer:\n");

scanf("%d",&num);

add=sum(num);

printf("sum=%d",add);

}

#include <stdio.h>

int sum(int n);

int main()

{ int num,add;

printf("Enter a positive integer:\n");

scanf("%d",&num);

add=sum(num);

printf("sum=%d",add);

}

int sum(int n)

{ if(n==0)

return n;

else return n+sum(n-1);

}

sum(5)

=5+sum(4)

=5+4+sum(3)

=5+4+3+sum(2)

=5+4+3+2+sum(1)

=5+4+3+2+1+sum(0)

=5+4+3+2+1+0

=5+4+3+2+1

=5+4+3+3

=5+4+6

=5+10

=15

Functions are very often defined recursively. The classic example is the factorial function. ◦ factorial(0) = 1 ◦ factorial(n) = n * factorial(n-1) [for n>0]

Let's compute factorial(3). ◦ factorial(3) = 3 * factorial(2) = 3 * 2 * factorial(1) = 3 * 2 * 1 * factorial(0) = 3 * 2 * 1 * 1 = 6

int factorial(int n)

{

if (n == 0) return 1 ;

else return n * factorial(n-1) ;

}

Base Cases: fib(0) = 0, fib(1) = 1

Recursive Case (two recursive calls): fib(n) = fib(n-1) + fib(n-2) [for n>1]

fib(4)= fib(3) + fib(2)

= fib(2) + fib(1) + fib(1) + fib(0)

= fib(1) + fib (0) + 1 + 1 + 0

= 1 +0 + 1 + 1 + 0

= 3

int fib(int n)

{

if (n == 0) return 0;

if (n == 1) return 1;

else return (fib(n-1) + fib(n-2));

}

43

F(X) = 2 *X + F ( X – 2) ;

F(0) = F(1) = 0;

lecture 5 functions pass by value and pass by reference

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