Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 16 | Low Level
16.10 Write a function called my2DAlloc which allocates a two dimensional array. Minimize
the number of calls to malloc and make sure that the memory is accessible by the
notation arr[i][j].
SOLUTION
We will use one call to malloc.
pg 82
Allocate one block of memory to hold the row vector and the array data. The row vector will
reside in rows * sizeof(int*) bytes. The integers in the array will take up another rows * cols *
sizeof(int) bytes.
Constructing the array in a single malloc has the added benefit of allowing disposal of the
array with a single free call rather than using a special function to free the subsidiary data
blocks.
1 #include
2
3 int** My2DAlloc(int rows, int cols) {
4 int header = rows * sizeof(int*);
5 int data = rows * cols * sizeof(int);
6 int** rowptr = (int**)malloc(header + data);
7 int* buf = (int*)(rowptr + rows);
8 int k;
9 for (k = 0; k < rows; ++k) {
10 rowptr[k] = buf + k*cols;
11 }
12 return rowptr;
13 }
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