CSO101: Computer Programming

Pointers & Arrays

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O.S.L. Bhavana

Passing Arrays via pointers: Recall

• Array name refers to the address of the first element in the array.
• For an array declaration ex: “int a[10];”, the value of a, &a[0], &a is same. The value of all these expressions is the address of the first element in the array.

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• Difference between array name and pointer: Pointers may be

re-assigned different values but array name should not be

re-assigned to a different address.

• Ex: int a,b, *pa=&b, arr[10]={0};
• pa = &a; // is valid
• arr = &b; // is invalid

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1-D Array and Pointers

int a[3]={2,5,6};

An array name has multiple interpretations

1. a as an array – is a collection of 3 elements

1-D Array and Pointers

int a[3]={2,5,6};

An array name has multiple interpretations

1. a as an array – is a collection of 3 elements
2. a as an address – is the address 1000

1-D Array and Pointers

int a[3]={2,5,6};

An array name has multiple interpretations

1. a as an array – is a collection of 3 elements
2. a as an address – is the address 1000
3. a as a pointer – points to a[0]

(not the entire array)

• sizeof operator treats a as an array. sizeof(a) = 12
• Pointer arithmetic treats a as a pointer:

a+1 = address of a + 1 (size of (a[0]))

= address of a + 4

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2-D Array Memory Map

int a[3][4]={{2,5,6,7}, {12,15,16,17}, {22,25,26,27}};

For simplicity we view 2D arrays as matrices

with the left dimension specifying no. of rows

right dimension specifies no. of columns

2-D Array Addressing

int a[3][4]={{2,5,6,7}, {12,15,16,17}, {22,25,26,27}};

Address of a[i][j] = address of a[0][0]

+( i * num_cols +j) sizeof_datatype

= address of a[0][0] + +( i * num_cols +j) 4

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3-D Array Memory Map

int a[2][3][2]={2,5,6,7,12,15,16,17,22,25,26,27};

3-D Array Memory Map

int a[2][3][2]={2,5,6,7,12,15,16,17,22,25,26,27};

2-D Array and Pointers

Interpretation of 2-D array:

1. a as a multi-dimensional array

– is a collection of 3*4 elements

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int a[3][4]={{2,5,6,7}, {12,15,16,17}, {22,25,26,27}};

2-D Array and Pointers

int a[3][4]={{2,5,6,7}, {12,15,16,17}, {22,25,26,27}};

Interpretation of 2-D array:

1. a as a multi-dimensional array
2. a as an address – is the address 1000

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2-D Array and Pointers

int a[3][4]={{2,5,6,7}, {12,15,16,17}, {22,25,26,27}};

Interpretation of 2-D array:

1. a as a multi-dimensional array
2. a as an address
3. a as a pointer – points to a[0]

of the array. In this case a[0] itself is an array.

Therefore a points to the entire array a[0]

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2-D Array and Pointers

int a[3][4]={{2,5,6,7}, {12,15,16,17}, {22,25,26,27}};

Interpretation of 2-D array:

1. a as a multi-dimensional array
2. a as an address
3. a as a pointer – points to a[0].

In this case a[0] itself is an array.

Therefore a points to the entire array a[0]

1. sizeof operator treats a as an array. sizeof(a) = 48
2. Pointer arithmetic treats a as a pointer:

a+1 = address of a + 1 (size of data pointed by a)

= address of a + sizeof(a[0])

= address of a + 16;

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2-D Array and Pointers

int a[3][4]={{2,5,6,7}, {12,15,16,17}, {22,25,26,27}};

Interpretation of 2-D array name:

1. a as a multi-dimensional array
2. a as an address
3. a as a pointer – points to a[0]

Recall that a+i points to an int array of 4 elements.

Therefore, the address given by a+0 is 1000,

a+1 is 1016, and a+2 is 1032

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3-D Array Addressing

int a[2][3][2]={2,5,6,7,12,15,16,17,22,25,26,27};

For int a[p] [q] [r];

Address of a[i][j] [k]= address of a[0][0]

+( i * q*r +j *r + k) sizeof_datatype

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Similar interpretations extend for any general

n-dimensional array