With arrays, why is it the case that a[5] == 5[a]?

As Joel points out in [Stack Overflow podcast #34](

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), in [C Programming Language](

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) (aka: K & R), there is mention of this property of arrays in C: `a[5] == 5[a]`

Joel says that it's because of pointer arithmetic but I still don't understand. **Why does `a[5] == 5[a]`**?

One thing no-one seems to have mentioned about Dinah's problem with `sizeof`:

You can only add an integer to a pointer, you can't add two pointers together. That way when adding a pointer to an integer, or an integer to a pointer, the compiler always knows which bit has a size that needs to be taken into account.

Because array access is defined in terms of pointers. `a[i]` is defined to mean `*(a + i)`, which is commutative.

To answer the question literally. It is not always true that `x == x`

double zero = 0.0;
double a[] = { 0,0,0,0,0, zero/zero}; // NaN
cout << (a[5] == 5[a] ? "true" : "false") << endl;

prints

false

For pointers in C, we have

a[5] == *(a + 5)

and also

5[a] == *(5 + a)

Hence it is true that `a[5] == 5[a].`

in c compiler

a[i]
i[a]
*(a+i)

are different ways to refer to an element in an array ! (NOT AT ALL WEIRD)