Finding value in array using pointer

Question

For a computer science class i have the following assignment.

Given a 2-dimensional array of doubles containing 128 rows, and 32 columns. The array is stored by placing the columns after eachother in the memory. Complete the following function, which returns the correct element given a pointer to the first element of the array, and some column and row value.

#define N_COLUMNS 32
#define N_ROWS 128
static inline double get_element(const double *A, const int row, const int column)
{...
}

This is what I have so far, but I don't think it's correct.

return *A + column * sizeof(double) + row * sizeof(double);

Can someone help me out?

Are you sure your teacher is not teaching C? (And even that would be taught poorly it seems.) — Baum mit Augen
– Baum mit Augen ♦, Commented Jun 24, 2016 at 13:46
How is A a two-dimensional array if its type is double* A? — Codor
– Codor, Commented Jun 24, 2016 at 13:46
Adding something after dereferencing A doesn't seem good, either. — MikeCAT
– MikeCAT, Commented Jun 24, 2016 at 13:46

Codor · Accepted Answer · 2016-06-24 13:58:01Z

6

To my understanding, the desired result can be obtained as follows by using pointer arithmetic, assuming zero-based indexing.

return *( A  + ( column * N_ROWS ) + row );

Here sizeof(double) does not need to be used, as A is suitably typed. Adding to it will advance sizeof(double) times the summand.

edited Jun 24, 2016 at 13:58

answered Jun 24, 2016 at 13:49

Codor

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phil · Accepted Answer · 2016-06-24 13:49:27Z

1

try A + column_number * column_size + row_number

remember arrays start at column 0 row 0 for ease of this calculation

answered Jun 24, 2016 at 13:49

phil

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Comments

Vlad from Moscow · Accepted Answer · 2016-06-24 13:59:32Z

0

The return statement will look like

return *( A + column * N_ROWS  + row );

In fact you have a one-dimensional array that stores N_COLUMNS * N_ROWS elements that is split in groups of N_ROWS elements.

Take into account that there is no need to use the qualifier const with the parameters row and column

These function declarations

static inline double get_element(const double *A, const int row, const int column);

and

static inline double get_element(const double *A, int row, int column);

are equivalent and declare the same one function.

edited Jun 24, 2016 at 13:59

answered Jun 24, 2016 at 13:53

Vlad from Moscow

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