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ESc101: Fundamentals of Com utin

2011-12-Monsoon Semester

Lecture #20, September 19, 2011

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Announcements

Mid-sem exam co ies will be returned in Wednesda Tutorial.

Lec-20 1Dheeraj Sanghi, CSE Dept., IIT Kanpur

ESc101, 2011-12-Monsoon

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Recap

String fucntions

Array initialization



Recap: Memory Model

Memory is a list ofbytes

Each byte has an address

All variables are stored in memory

As many bytes are allocated to the variable as the type ofthat variable requires

Memory for a single variable is contiguous

o guarantees w et er two consecut ve y ec are

variables will get contiguous memory areas (bytes)

Address of a variable is the address of the first byte where

it is stored

Lec-20 Dheeraj Sanghi, CSE Dept., IIT Kanpur


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Recap: Memory Model

ci

0 1 2 3 4 5 6 7 8 9 10 12

int i;

char c;



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Address of i is 1. It occupies bytes 1 to 4.

Address of c is 7.

It only occupies byte 7.

Pointer

variable.

A pointer is denoted by the symbol *

int i; // i is a variable of type int

int *p; // p is a pointer to some integer

The address of a variable is denoted by the symbol &

p = &i; // p is now address of i (1 in our example)

p stores the address of i, i.e., &i

*p denotes the content pointed by p, i.e., i.



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Why we need to know the addresses

,

the same memory areas in both callee and called functions.

Like what happened with arrays

We did not have to copy large arrays during function calls

When we want the changes made to memory areas in

called functions to be reflected in callee function, knowing

.

We will also learn about better memory management by

asking computer to allocate only as much memory as

needed at the run time through the use of pointers.



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Have we seen the pointers before

When a variable is read using scanf ( )

Operator & gives location where scanf ( ) should store value

Notice the difference between scanfand printf:

We wanted the parameter variable to change after scanf The changes made within scanfshould be visible in the callee

function

Hence we passed on the address of the variable and not the content

or va ue o e var a e

In printf, we did not want the function to make any changes

The value should simply be printed and no changes to be done

Hence only the content or value of the variable needed to be passed



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Have we seen pointers before

Arrays are passed as address

Array names are essentially pointers

a is a pointer to the first element of the array, i.e., a[0]

*a is equivalent to a[0]

Since array elements are stored contiguously in memory

(a + i) is a pointer to a[i]

*(a + i) is equivalent to a[i]

Notice that pointer arithmetic is different from normal

arithmetic

When we add 1 to a, we are actually telling the computer to get

the address of the next integer (or whatever type array is of)



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More on pointers and arrays

,

of (a + i) and accesses the variable in that address

If the computer has 4-byte integers, then (a+i) would

actually translate to a + 4i, where a is the address of a[0] No error checking on array boundaries is done

Therefore, a[-2], a [12], etc., become legal in C language

thou h ma be wron as er the lo ic of our ro ram

Pointers can be assigned array names:

int *p;

p = a;



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Swapping two integers

void swap (int *pa, int *pb)

{

int t = *pa;

* = *

*pb = t;

}

int main ( )

{

int x = 3, y = 4;

int *a, *b;

= = , , ,

a = &x; b = &y;

swap (a, b);

printf(x = %d, y = %d\n, x, y);

}



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Any Questions?



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