Is *s equivalent to s[i]?

Question

On page 106 of K&R C, in strcmp function, it takes pointers *s and *t as arguments but in the for loop, it specifies

s[i]==t[i]

as a condition.

Just want to confirm, for arrays, as

*(s+i) and s[i]

are synonymous, hence, can the function statement use s[i] instead of *s?

I think you're misrepresenting the function: It doesn't take "pointers `*s` and `*t`", rather, it takes pointers `s` and `t`. — Kerrek SB, Jan 29 '17 at 12:59
A simple search for the `[]` poerator would have answered this. Before asking a question, try to solve to yourself. And how should `*s` be the same as `s[i]` (unless `i` is zero)? — too honest for this site, Jan 29 '17 at 13:00
Yes it's true that for any valid pointer *or array* `s` and index `i` the expression `*(s + i)` is equal to `s[i]`. That means that `*s` is equal to `s[i]` ***only*** if `i` is zero (`*s` is the same as `*(s + 0)` which is the same as `s[0]`). — Some programmer dude, Jan 29 '17 at 13:00
This is the reason why `3["abcde"]` is a valid expression which evalutes to `'d'`. — Gerhardh, Jan 29 '17 at 13:06
To expand on the comment by @Gerhardh, now that we know that `s[i]` is equal to `*(s + i)` we need to remember that addition is [*commutative*](https://en.wikipedia.org/wiki/Commutative_property) which means that `*(s + i)` is equal to `*(i + s)` which then is equal to `i[s]`. — Some programmer dude, Jan 29 '17 at 13:22
Possible duplicate of [Pointers - Difference between Array and Pointer](http://stackoverflow.com/questions/7725008/pointers-difference-between-array-and-pointer) — Claudio Cortese, Jan 29 '17 at 13:38

Vlad from Moscow · Accepted Answer · 2017-01-29T13:29:43.900

From the C Standard (6.5.2.1 Array subscripting)

2 A postﬁx expression followed by an expression in square brackets [] is a subscripted designation of an element of an array object. The deﬁnition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))). Because of the conversion rules that apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the initial element of an array object) and E2 is an integer, E1[E2] designates the E2-th element of E1 (counting from zero).

Thus

a[i] is equivalent to *( a + i ) and in turn is equivalent to i[a]

For example

int a[1] = { 10 };

printf( "a[0] == *( a + 0 ) is %s\n", a[0] == *( a + 0 ) ? "true" : "false" );
printf( "a[0] == 0[a] is %s\n", a[0] == 0[a] ? "true" : "false" );

Is *s equivalent to s[i]?

1 Answers1