# 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?

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`// 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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