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Pointer arithmetic and array variables in C++

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• Pointer Arithmetic
  • In C++, you can add an integer to or subtract an integer from a pointer variable:

    int *p, *q; q = p + 3; q = p - 5;

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  • The effect (result) is adding/subtracting the constant × data size to the address in the reference variable:

    Example 1:

    int *p, *q; q = p + 3; // q = p + 3*4 (int = 4 bytes) q = p - 5; // q = p - 5*4 (int = 4 bytes)

    Example 2:

    double *p, *q; q = p + 3; // q = p + 3*8 (doube = 8 bytes) q = p - 5; // q = p - 5*8 (doube = 8 bytes)

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• Pointer arithmetic and arrays
  • The reason to multiply the constant by the size of the data size is to use pointer arithmetic to access array elements

    Suppose we define an array a[10] and make a reference variable p point to the first element in the array:

    int a[10]; // Array of 10 elements int *p; // Pointer variable (points to an integer) a[0] // First element of the array &a[0] // Address of the first element of the array p = &a[0]; // p points to the first element of array a

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  • Then we can access the elements of array a using pointer variable p as follows:

    *p is the array element a[0] *(p+1) is the array element a[1] *(p+2) is the array element a[2] ... and so on

    Graphically:

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  • Example: printing an array with pointer arithmetic:

    int a[10]; int *p; a[0] = ...; a[1] = ...; ... etc // Print array using array variable for ( i = 0; i < 10; i++ ) cout << a[i] << endl; // print array // Print array using pointer variable: (1) p = &a[0]; for ( i = 0; i < 10; i++ ) cout << *(p + i) << endl; // print array // Print array using pointer variable: (2) p = &a[0]; for ( i = 0; i < 10; i++ ) { cout << *p << endl; // print array p++; } // Print array using pointer variable: (3) p = &a[0]; for ( i = 0; i < 10; i++ ) { cout << *p++ << endl; // print array }

    Note:

    The expression: *p++ (post increment) is a short-hand for: *p p++; (increment p AFTER using the value in p)

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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• Address of an array
  • The name (identifier) of an array in C++ evualates to the address of the first element of the array

    This location is known as the address of the array

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  • Hence, the above program can also be written as follows:

    int a[10]; int *p; a[0] = ...; a[1] = ...; ... etc // Print array using array variable for ( i = 0; i < 10; i++ ) cout << a[i] << endl; // print array // Print array using pointer variable: (1) p = a; // a is same as &a[0] for ( i = 0; i < 10; i++ ) cout << *(p + i) << endl; // print array // Print array using pointer variable: (2) p = a; // a is same as &a[0] for ( i = 0; i < 10; i++ ) { cout << *p << endl; // print array p++; } // Print array using pointer variable: (3) p = a; // a is same as &a[0] for ( i = 0; i < 10; i++ ) { cout << *p++ << endl; // print array }

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• Well-known loop constructs with pointer-arithmetics
  • Pointer-arithmetics is commonly used to copy arrays:

    int a[10], b[10]; int *p, *q; a[0] = ...; a[1] = ..., etc // Copy elements in array a to array b p = a; q = b; for ( i = 0; i < 10; i++ ) *q++ = *p++;

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• Array notations and pointer arithmetic
  • Array notation (a[i]) in C++ can be used with pointer variables

    Example:

    int a[10]; int *p; p = a; p[4] // array notation used with a pointer variable

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  • The meaning of array notation used with pointer variables is as follows:

    int a[10]; int *p; p = a; p[4] means: *(p + 4)

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  • Example: print an array with offset

    int a[20]; int i, *p; a[0] = 12; a[1] = 23; a[2] = 34; a[3] = 45; a[4] = 56; a[5] = 67; a[6] = 78; a[7] = 89; a[8] = 90; a[9] = 111; a[10] = 1012; a[11] = 1023; a[12] = 1034; a[13] = 1045; a[14] = 1056; a[15] = 1067; a[16] = 1078; a[17] = 1089; a[18] = 1090; a[19] = 1111; p = &a[4]; for ( i = 0; i < 10; i++ ) { cout << p[i] << " "; }

    The program will print elements a[4], a[5], ..., a[13]

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  • Example Program: (Demo above code)                                                
    • Prog file: click here