CSO101: Computer Programming

Pointers - Arrays - Functions

​

​

O.S.L. Bhavana

[ ] Array subscript operator

• [] should be used with pointers/arrays
• This operator the effect of pointer arithmetic and dereferencing (in places other than array declaration)
• a[b] has the effect of *(a+b)
• Ex: int a[5]={0}; a[2] // the expression a[2] has the effect of *(a+2); infact a[2] or 2[a] have the same effect
• [ ] operator can also be used with pointers
• Ex: int a[5]={0}, *ptr=a; ptr[2] also has the effect of *(ptr+2)
• int a[3][4]; a[i][j] = *(a[i]+j) = *(*(a+i)+j)
• Precedence of [] is higher than * and &, therefore parentheses is must
• Associativity of [] is left-to-right, associativity of * and & is right-to-left
• x[y] = *(x+y) by the definition of the index/subscript operator

​

Pointer to an array

• int (*ptr)[5]; // In this declaration
• (*ptr) emphasises ptr is a pointer
• (*ptr)[5] → ptr points to an 1D-array of 5 elements
• int (*ptr)[5] → ptr points to an 1D-array of 5 integer elements
• i^th element of this 1D array is given by (*ptr)[i]
• Value of ptr+1 is value of ptr + 20bytes
• int (*ptr)[5][2];
• (*ptr) emphasises ptr is a pointer
• (*ptr)[5][2] → ptr points to a 2D-array of 5*2 elements
• int (*ptr)[5][2] → ptr points to an 2D-array of 5*2 integer elements
• (*ptr)[i] - accesses the i^th row
• (*ptr)[i][j] - accesses the j^th element in i^th row
• Value of ptr+1 is value of ptr + 40bytes

​

Pointer to an array

Pointer to an array

Pointer to an array

Recall

• int arrA[m]; // arrA[i] = *(arrA+i)
• int arrB[m][n];
• // arrB[i][j] = *(*(arrB+i)+j);
• = *((int *)arrB + i*n +j)
• int arrC[m][n][p];
• arrC[i][j][k] = *((int *)arrC + i*n*p + j*p + k)
• = *(*(*(arrC+i) + j) + k)
• arrC - pointer to arrC[0]
• arrC+i - pointer to arrC[i]
• *(arrC+i)

Pointer Arrays:

• What is the output of the following code?
• int *pa[4]; //declares pa is an array of 4 pointers

• ​

n-D arrays & Pointers

• int a[3][4][2] = {{{3,4,9},{12,17}}, {{21,29,20,24},{36,33,38}}, {{49,42,43,41},{56,53,52,55}}}; //
• //let the starting address of the first element in the multi-dim array be 1000.
• What is the value of a[1]?
• What is the value of a[1]+1?
• What is the value of a[3][0]?

​

​

n-D arrays & Pointers

• int a[3][4][2] = {{{3,4,9},{12,17}}, {{21,29,20,24},{36,33,38}}, {{49,42,43,41},{56,53,52,55}}}; //
• //let the starting address of the first element in the multi-dim array be 1000.
• What is the value of a[1]? (a[1] treated as address/pointer) 1032
• What is the value of a[1]+1? (a[1] treated as pointer) 1040
• What is the value of a[3][0]? (treated as address/pointer) 1096

​

​

n-D Arrays & Functions

• Multi-dimensional Arrays can be passed a arguments to functions as
1. A multi-dimensional array:

​

• As a “pointer to an array”

​

• As a pointer (with a type cast)

​

​

​

Multi-dimensional arrays as function arguments

• All except the first dimension are compulsory to mention like in the declaration
• If the array dimensions are constants, then they can be mentioned in the function declaration as follows

Ex: Function declaration: int max(int a[2][3]);

Function call: max(a);

Ex: incorrect function declaration: int max(int a[][]); //second dim missing

• If the array dimensions are variables, then these variables can be declared in the function declaration too
• Ex: Function declaration: int max(int x, int y, int a[x][y]); // here the dimensions of the 2D matrix are allowed to be variables. Note that the dimension variables x,y are declared before a.
• Function Call: int x,y; scanf(“%d %d”,&x,&y); int a[x][y] max(x,y,a);

​

​

Multi-dimensional arrays as function arguments

• Finding the max of elements of a 2Dmatrix

​

Passing as a pointer to a single element

• Multi-dimensional Arrays can be passed as arguments to functions as a data_type pointer (with a type cast)

​

​

Determinant Program

int det(int *a, int n);

int main()

{ int arr[4][4]={1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16};

printf("%d", det((int *)arr,4)); return 0;}

int det(int *a, int n)

{ if (n==1) {return *a;}

else {int sum =0;

for(int i=0;i<n;i++)

{ int b[n-1][n-1];

for(int j=0;j<n-1;j++)

{ for(int k=0;k<n-1;k++)

{int k1 = (k<i?k:k+1), j1 =j+1;

b[j][k] = *(a+j1*n+k1); } }

sum += *(a+i)* (i%2?-1:1) * det((int*)b,n-1); }

return sum; }

}

​

Array Bounds Checking

• In C there is no check to see if the subscript used for an array index is within the the array size;
• Therefore, programs have to be written carefully to ensure that the index is within the size.
• What does the following snippet print?

​

• int a[10]={0}, *p=a;
• printf("%d %d", a[-1], *(p+10));

​

Passing as a Pointer to an Array

Pointer Arrays or Array of Pointers

• Pointers are variables themselves, they can be stored in arrays just as other variables
• Declaration Syntax: int *pa[4]; // declares an array of four pointers
• Initialization and declarations rules are same as that of arrays.
• What is the output of the following code? Ans:1 2 3 4

​

Returning multiple values from function

• This is an indirect method of returning values
• The calling function can create variables to store the values that it desires to be returned
• It can then pass these variables by reference to the called function as an argument
• The called function can then assign return values as dereference of these pointers

Returning multiple values from function

Returning Arrays From Function

• Arrays can be returned as pointers from functions
• If the array is a local variable of the function warning is issued
• The array may be declared as static array to avoid this.