C – Why Are Both index[array] and array[index] Valid?

arrayclanguage-designpointersstandards

For example consider:

int index = 3;
int array[4] = {0, 1, 2, 3};

then both index[array] and array[index] are valid expressions, much like *(index + array) and *(array + index).

In C arrays why is this true? array[5] == 5[array] explains how it works. My question is: Why did the language designers choose to allow this? Why not just enforce that index[array] is invalid, for clarity's sake?

Best Answer

First of all, it would help to read dmr's Development of the C Language to get some insights into some of C's quirks, particularly when it comes to array semantics (basically, blame BCPL and B for most of it).

As for the question "[w]hy not just enforce that index[array] is invalid, for clarity's sake," what would such a check buy you in exchange for the cost of performing it? The form almost never appears outside of the IOCCC, so it's not like it's a major problem in production code (compared to the use of, say, gets, or unchecked array accesses (which disallowing i[a] won't help with), or <fill in the blank>). It's not a bug; it doesn't introduce any undefined behavior; it doesn't introduce any security holes not already present with a[i]; the only complaints against it are stylistic in nature.

It's like asking why both T *p and T* p are valid; there is no "why" beyond it being an accident of the language syntax. There's nothing deliberate behind allowing both, it's just a function of how the grammar works. Same with a[i] and i[a]. Professional programmers are (usually) grown-ups, and don't deliberately introduce confusion where it isn't warranted, so most will naturally use a[i].

You're basically trying to guard against a problem that doesn't really exist.

Related Solutions

Correct For Loop Design

You forgot to mention wheter your string variables in Felix start with index 0 or 1. Searching for that in the web, its additional job for readers. And affects the way your example is evaluated.

Anyway. Are you sure that:

for(i=0; predicate(i); increment(i))

In C: "The predicate is tested after the increment, but the terminating increment is not universally valid!"

Traslates to this:

i=0
continue:
  body
  increment(i)
  if not predicate(i) goto break
  goto continue
break:

Instead of this:

continue:
  i=0
  if not predicate(i) goto break
  body
  increment(i)
  goto continue
break:

Since your for loop its more specific like pascal, you may consider how should be translated and evaluated in case the index value is equal or lesser to the initial value.

Usually, if the initial value, and final value are the same, the loop is executed once, if the final value is greater that the initial value, the loop is not executed.

Why is x=x++ undefined

Maybe you should first answer the question why it should be defined? Is there any advantage in programming style, readability, maintainability or performance by allowing such expressions with additional side effects? Is

y = x++ + ++x;

Best Answer

Related Solutions

Correct For Loop Design

Why is x=x++ undefined

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