C-Array-Pointer-equations
Rule |
Description |
Example |
|---|---|---|
Rule 1 |
A pointer can hold the address of a static global variable |
#include <stdio.h>
static double a = 5.12312312;
int main()
{
double *ptr;
ptr = &a;
*ptr = 7.3453; // a is now changed to 7.345300
printf("%f\n", a); // prints 7.345300
return 0;
}
|
Rule 2 |
A pointer can hold the address of a non-static global variable |
#include <stdio.h>
char a = 'A';
int main()
{
char *ptr;
ptr = &a;
*ptr = 'G'; // a is now changed to 'G'
printf("%c\n", a); // prints G
return 0;
}
|
Rule 3 |
A pointer can hold the address of a static local variable |
#include <stdio.h>
int main()
{
static int a = 500;
int *ptr;
ptr = &a;
*ptr = 1000; // a is now changed to 1000
printf("%d\n", a); // prints 1000
return 0;
}
|
Rule 4 |
A pointer can hold the address of a local variable stored on stack |
#include <stdio.h>
int main()
{
int a = 500;
int *ptr;
ptr = &a;
*ptr = 1000; // a is now changed to 1000
printf("%d\n", a); // prints 1000
return 0;
}
|
Rule 5 |
A pointer can hold the address of a memory block allocated dynamically by malloc, calloc, realloc |
#include <stdio.h>
int main()
{
int *ptr;
ptr = malloc(sizeof(int));
// ptr now points to a single block of integer of 4 bytes
*ptr = 100;
printf("%d\n", *ptr); // prints 100
free(ptr); // free the memory allocated using malloc
return 0;
}
#include <stdio.h>
#include <stdlib.h>
void display_1(int *ptr)
{
int i;
for(i = 0; i < 5; i++)
printf("%d\n", ptr[i]);
}
void display_2(int *ptr)
{
int i;
for(i = 0; i < 5; i++, ptr++)
printf("%d\n", *ptr);
}
void display_3(int *ptr)
{
int i;
for(i = 0; i < 5; i++, ptr++)
printf("%d\n", ptr[0]);
}
int main()
{
int *ptr;
ptr = malloc(sizeof(int) * 5);
// ptr now points to starting address of a 5 blocks of integers
ptr[0] = 0;
ptr[1] = 100;
ptr[2] = 200;
ptr[3] = 300;
ptr[4] = 400;
display_1(ptr);
display_2(ptr);
display_3(ptr);
free(ptr); // free the memory allocated using malloc
return 0;
}
|
Rule 6 |
More than one pointer can point to same memory location |
#include <stdio.h>
int main()
{
int a = 5;
int *p, *q, *r;
p = &a;
q = &a;
r = p;
*p = 10;
printf("%d\n", a); // a is now 10
*q = 20;
printf("%d\n", a); // a is now 20
*r = 30;
printf("%d\n", a); // a is now 30
return 0;
}
|
Rule 7 |
|
#include <stdio.h>
int x[5] = {1, 2, 3 ,4, 5};
int main()
{
x[2] = 10;
*(x + 2) = 100;
printf("%d\n", x[2]);
return 0;
}
|
Rule 8 |
|
#include <stdio.h>
int x[5] = {1, 2, 3 ,4, 5};
int *ptr = x;
int main()
{
x[2] = 10;
*(x + 2) = 100;
ptr[3] = 200;
printf("%d\n", x[2]); // --> prints 100
printf("%d\n", x[3]); // --> prints 200
printf("%d\n", ptr[3]); // --> prints 200
return 0;
}
|
Rule 9 |
|
#include <stdio.h>
int x[5] = {1, 2, 3 ,4, 5};
int *ptr = x;
int main()
{
ptr++; // ptr now points to x[1];
*ptr = 100; // x[1] is now 100
ptr[0] = 100; // x[1] is now 100
ptr = ptr + 2; // ptr now points to x[3];
*ptr = 300; // x[3] is now 300
ptr[0] = 300; // x[3] is now 300
x[4] = 400; // x[4] is now 400
*(x + 4) = 400; // x[4] is now 400
printf("%d\n", x[1]); // --> prints 100
printf("%d\n", x[3]); // --> prints 300
printf("%d\n", x[4]); // --> prints 400
return 0;
}
|
Rule 10 |
Pointers can be incremented or decremented relative to current position in a memory block |
#include <stdio.h>
int x[5] = {1, 2, 3 ,4, 5};
int *ptr = x;
int main()
{
ptr++; // ptr now points to x[1];
*ptr = 100; // x[1] is now 100
ptr[0] = 100; // x[1] is now 100
ptr = ptr + 2; // ptr now points to x[3];
*ptr = 300; // x[3] is now 300
ptr[0] = 300; // x[3] is now 300
ptr--; // ptr now points to x[2]
*ptr = 200;
ptr[0] = 200;
x[4] = 400; // x[4] is now 400
*(x + 4) = 400; // x[4] is now 400
printf("%d\n", x[1]); // --> prints 100
printf("%d\n", x[1]); // --> prints 200
printf("%d\n", x[3]); // --> prints 300
printf("%d\n", x[4]); // --> prints 400
return 0;
}
|
Rule 11 |
Pointers can be indexed with positive or negative index relative to current position in a memory block |
#include <stdio.h>
int x[5] = {1, 2, 3 ,4, 5};
int *ptr = x;
int main()
{
ptr++; // ptr now points to x[1];
*ptr = 100; // x[1] is now 100
ptr[0] = 100; // x[1] is now 100
ptr = ptr + 2; // ptr now points to x[3];
*ptr = 300; // x[3] is now 300
ptr[0] = 300; // x[3] is now 300
ptr[-1] = 200; // x[2] is now 200
ptr[1] = 400; // x[4] is now 400
printf("%d\n", x[1]); // --> prints 100
printf("%d\n", x[1]); // --> prints 200
printf("%d\n", x[3]); // --> prints 300
printf("%d\n", x[4]); // --> prints 400
return 0;
}
|
Rule 12 |
|
|
Rule 13 |
Subtraction of two pointers is allowed, when two pointers point to two blocks in contiguous memory |
|
Rule 14 |
Addition of two pointers is NOT allowed |
|
Rule 15 |
Multiplication of two pointers is NOT allowed |
|
Rule 16 |
Division of two pointers is NOT allowed |
|
Rule 17 |
& and * on same side of the equation, cancel each other |
|
Rule 18 |
* on LHS and * on RHS can cancel each other to get address |
|
Rule 19 |
|
#include <stdio.h>
void display_1(int arr[], int size)
{
int i;
printf("Iterate using array index\n");
for(i = 0; i < size; i++)
printf("%d\n", arr[i]);
}
void display_2(int *arr, int size)
{
int i;
printf("Iterate array as pointer\n");
for(i = 0; i < size; i++, arr++)
printf("%d\n", *arr);
}
void display_3(int *arr, int size)
{
int i;
printf("Iterate using array index\n");
for(i = 0; i < size; i++, arr++)
printf("%d\n", arr[0]);
}
int main()
{
int a[5] = {100, 200, 300, 400, 500};
display_1(a, 5);
display_2(&a[0], 5);
display_3(a + 2, 3);
return 0;
}
|
Rule 20 |
|
#include<stdio.h>
int main()
{
int arr[3][4] = {
{1,2,3,4},
{5,6,7,8},
{1,6,7,4}
};
int i, j;
int (*p)[4];
printf("Iterate array using arr and [] notation\n");
for(i = 0; i < 3; i++)
{
for(j = 0; j < 4; j++)
{
printf("arr[%d][%d]=%d\n", i, j, arr[i][j] );
}
printf("\n\n");
}
printf("Iterate array using arr and * notation\n");
for(i = 0; i < 3; i++)
{
for(j = 0; j < 4; j++)
{
printf("arr[%d][%d]=%d\n", i, j, *( *(arr + i) + j) );
}
printf("\n\n");
}
p = arr;
printf("Iterate array using pointer to an array\n");
for(i = 0; i < 3; i++)
{
for(j = 0; j < 4; j++)
{
printf("arr[%d][%d]=%d\n", i, j, *( *(p + i) + j) );
}
printf("\n\n");
}
return 0;
}
|
Rule 21 |
|
#include<stdio.h>
int main()
{
int A[2][3][4] =
{
{
{1,2,3,4},
{5,6,7,8},
{9,10,11,12},
},
{
{13,14,15,16},
{17,18,19,20},
{21,22,23,24},
}
};
int (*p)[3][4] = A;
int i, j, k;
printf("Iterate using Array index\n");
for (i = 0; i < 2; i++) {
for (j =0; j < 3; j++) {
for (k =0; k < 4; k++)
printf("%d\t", A[i][j][k]);
printf("\n");
}
}
printf("Iterate using pointer to array\n");
for (i = 0; i < 2; i++) {
for (j = 0; j < 3; j++) {
for (k = 0; k < 4; k++)
printf("%d\t", *(*(*(p +i) + j) +k));
printf("\n");
}
}
printf("\n");
return 0;
}
|
Rule 22 |
|
|
Rule 23 |
|
#include <stdio.h>
int main()
{
int X = 5;
int *P;
int **Q;
P = &X;
Q = &P;
Q[0][0] = 100;
printf("%d\n", X); // prints 100
*Q[0] = 200;
printf("%d\n", X); // prints 200
**Q = 300;
printf("%d\n", X); // prints 300
return 0;
}
|
Rule 24 |
|
#include <stdio.h>
int main()
{
int X = 5;
int *P;
int **Q;
int ***R;
P = &X;
Q = &P;
R = &Q;
Q[0][0] = 100;
printf("%d\n", X); // prints 100
*Q[0] = 200;
printf("%d\n", X); // prints 200
**Q = 300;
printf("%d\n", X); // prints 300
R[0][0][0] = 1000;
printf("%d\n", X); // prints 1000
*R[0][0] = 2000;
printf("%d\n", X); // prints 2000
**R[0] = 3000;
printf("%d\n", X); // prints 3000
***R = 4000;
printf("%d\n", X); // prints 4000
return 0;
}
|
Rule 25 |
|
#include <stdio.h>
struct abc {
int x;
} a[5];
int main()
{
a[2].x = 5;
(a + 2)->x = 10;
(*(a + 2)).x = 20;
return 0;
}
|
Rule 26 |
|
#include <stdio.h>
struct abc {
int x;
} b[3][4];
int main()
{
b[1][2].x = 5;
(*(b[1] + 2)).x = 5;
(b[1] + 2)->x = 5;
(*(b + 1))[2].x = 5;
((*(b + 1)) + 2)->x = 5;
(*((*(b + 1)) + 2)).x = 5;
return 0;
}
|
Rule 27 |
|
#include <stdio.h>
struct abc {
int x;
} c[2][3][4];
int main()
{
c[1][2][3].x = 100;
(c[1][2] + 3)->x = 100;
(*(c[1][2] + 3)).x = 100;
( (*(c[1] + 2)) + 3)->x = 100;
( (*( (*(c + 1)) + 2)) + 3)->x = 100;
(*( (*(c + 1)) + 2))[3].x = 100;
(* ( (*( (*(c + 1)) + 2)) + 3)).x = 100;
return 0;
}
|
Rule 28 |
|
#include <stdio.h>
struct abc {
int x;
};
struct abc X; struct abc *P = &X;
struct abc A[5]; struct abc *Q = A;
int main()
{
X.x = 5;
P[0].x = 100;
P->x = 100;
(*P).x = 100;
A[2].x = 5;
(A + 2)->x = 10;
(*(A + 2)).x = 20;
Q[2].x = 200;// changes A[2].x to 200
(Q + 2)->x = 200;
(*(Q + 2)).x = 200;
printf("%d\n", X.x); //prints 100
printf("%d\n", A[2].x); //prints 200
return 0;
}
|
Rule 29 |
|
#include <stdio.h>
struct abc {
int x;
};
struct abc X;
struct abc *P = &X;
struct abc **Q = &P;
int main()
{
X.x = 5;
P[0].x = 100;
P->x = 100;
(*P).x = 100;
printf("%d\n", X.x); //prints 100
(*(*Q)).x = 200;
printf("%d\n", X.x); //prints 200
(*(Q[0])).x = 300;
printf("%d\n", X.x); //prints 300
Q[0][0].x = 400;
printf("%d\n", X.x); //prints 400
Q[0]->x = 500;
printf("%d\n", X.x); //prints 500
(*Q)->x = 600;
printf("%d\n", X.x); //prints 600
return 0;
}
|
Rule 30 |
|
#include <stdio.h>
struct abc {
int x;
};
struct abc X;
struct abc *P = &X;
struct abc **Q = &P;
struct abc ***R = &Q;
int main()
{
X.x = 5;
P[0].x = 100;
P->x = 100;
(*P).x = 100;
printf("%d\n", X.x); //prints 100
(*(*Q)).x = 200;
printf("%d\n", X.x); //prints 200
(*(Q[0])).x = 300;
printf("%d\n", X.x); //prints 300
Q[0][0].x = 400;
printf("%d\n", X.x); //prints 400
Q[0]->x = 500;
printf("%d\n", X.x); //prints 500
(*Q)->x = 600;
printf("%d\n", X.x); //prints 600
(*(*(*R))).x = 1000;
printf("%d\n", X.x); //prints 1000
(*(*R))->x = 2000;
printf("%d\n", X.x); //prints 2000
(*(*R))[0].x = 3000;
printf("%d\n", X.x); //prints 3000
(*R)[0][0].x = 4000;
printf("%d\n", X.x); //prints 4000
(*R)[0]->x = 5000;
printf("%d\n", X.x); //prints 5000
R[0][0][0].x = 6000;
printf("%d\n", X.x); //prints 6000
R[0][0]->x = 7000;
printf("%d\n", X.x); //prints 7000
(*(R[0][0])).x = 8000;
printf("%d\n", X.x); //prints 8000
return 0;
}
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