different representation of array element

Question

consider following C code

int a[]={1,2,3,4};
printf("%d",2[a]);

this prints "3".How is it possible? I know in a[2] a is the base address of array.But in 2[a] what is 2? and how it accessses array a?I am totally confused with this representation of array.

Nice catch on the duplicate, @devnull ! I have to admit I did not know the answer. — Floris, Nov 20 '13 at 06:39
@Floris Looking at [this](http://stackoverflow.com/a/14547307/2235132), I must say that I don't believe you. — devnull, Nov 20 '13 at 06:41
@devnull it's true: I am a self-taught hacker; as such I have occasional areas of real depth, and other complete blind spots. Which is why I hang around on this site a lot... But yeah - that was an answer I still feel pretty good about :-) — Floris, Nov 20 '13 at 06:43

score 2 · Answer 1 · answered Nov 20 '13 at 06:41

There are two things to remember here:

The first is that array access is basically just a fancy way of using pointer arithmetic. For example, if you have the array
```
int a[10];
```
then
```
a[3] = 5;
```
is equal to
```
*(a + 3) = 5;
```
The second thing to remember is that addition (like in a + 3 above) is commutative, so a + 3 is the same as 3 + a. This leads to e.g.
```
*(3 + a) = 5;
```
which can be interpreted as
```
3[a] = 5;
```

score 1 · Answer 2 · answered Nov 20 '13 at 07:00

int a[]={1,2,3,4}; is an integer array containing 4 elements and a is the Base Address, Let the Base Address be denoted by X . Now a[1] means element at address X + sizeOf(int) * 2 = Y (suppose) i.e. element at address Y, likewise 2[a] means element at adsress sizeOf(int) * 2 * X = Y.

Thus even if you write a[2] or 2[a] eventually complier recognizes it as Y and refres to the element at address Y which is 3 in our case.

Hope it addresses the problem right.

score 0 · Answer 3 · answered Nov 20 '13 at 06:37

0

*(expr1+expr2) is equivalent to expr1[expr2] or expr2[expr1].

*(expr2+expr1) is equivalent to expr2[expr1] or expr1[expr2].

answered Nov 20 '13 at 06:37

Sadique

22,572
7
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score 0 · Answer 4 · answered Nov 20 '13 at 06:42

It is just another way to write an element of an array.

In int a[]={1,2,3,4};, element at index 3 can be referenced by many methods:

As an array element: a[3]
Using pointer: *(a + 3) or *(3 + a) [Addition is commutative in arithmetics and in C]
Now you can write the second way of representation using pointer as 3[a], i.e. a[3] is equal to *(a + 3) is equal to *(3 + a) is equal to 3[a].

different representation of array element

4 Answers4