Performance of loops - Learnengineeringforu

Performance of loops (A caching question)

Consider below two C language functions to compute sum of elements in a 2D array. Ignoring the compiler optimizations, which of the two is better implementation of sum?

// Function 1 int fun1(int arr[R][C]) {     int sum = 0;     for (int i=0; i<R; i++)       for (int j=0; j<C; j++)           sum += arr[i][j]; }    // Function 2 int fun2(int arr[R][C]) {     int sum = 0;     for (int j=0; j<C; j++)       for (int i=0; i<R; i++)           sum += arr[i][j]; }

In C/C++, elements are stored in Row-Major order. So the first implementation has better spatial locality (nearby memory locations are referenced in successive iterations). Therefore, first implementation should always be preferred for iterating multidimensional arrays.

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