In C, an index is an integer position used to identify an array element. For an array with n elements, indexing starts at 0 and ends at n−1; the index can be understood as the element’s distance from the array’s start. TAKEAWAY: The first element has index 0, and the last element has index n−1. section 2.3
Designated initialization
A designated initializer makes the index explicit: double A[5] = { [0] = 9.0, [1] = 2.9, [4] = 3.E+25, [3] = .00007, }; Here, [4] initializes the last element. The omitted index [2] receives 0.0. Indexing identifies positions; designated initialization selects those positions during array creation. section 2.3
Indexing is also tied to pointer expressions. If a points to the first element of an array, a+i points to its ith element, while *(a+i) accesses that element’s value. C therefore provides equivalent-looking routes through an array, but a pointer is not itself an array and does not reveal the array length. section 11.1.2 section 11.3
Indexing through a pointer
This summation uses pointer arithmetic rather than bracket notation: double sum0(size_t len, double const* a) { double ret = 0.0; for (size_t i = 0; i < len; ++i) ret += *(a + i); return ret; } The length must be passed separately because a pointer cannot reconstruct the array’s length. section 11.1.2
8 double A[5] = {
9 [0] = 9.0,
10 [1] = 2.9,
11 [4] = 3.E+25,
12 [3] = .00007,
13 };
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Inspect the indexed initializer and see how each bracketed integer selects an array position. section 2.3
Function parameters add another rule: an array parameter rewrites to a pointer, so array notation and pointer notation can describe equivalent interfaces. With multidimensional parameters, only the innermost dimension rewrites; conventional indexing remains available inside the function. section 11.3.2
