changing the index of array

Question

hi, so far, i m working on the array with 0th location but m in need to change it from 0 to 1 such that if earlier it started for 0 to n-1 then now it should start form 1 to n. is there any way out to resolve this problem?

Answer 1

C arrays are zero-based and always will be. I strongly suggest sticking with that convention. If you really need to treat the first element as having index 1 instead of 0, you can wrap accesses to that array in a function that does the translation for you.

Why do you need to do this? What problem are you trying to solve?

Answer 2

Array indexing starts at zero in C; you cannot change that.

If you've specific requirements/design scenarios that makes sense to start indexes at one, declare the array to be of length n + 1, and just don't use the zeroth position.

Answer 3

You can't.

Well in fact you can, but you have to tweak a bit. Define an array, and then use a pointer to before the first element. Then you can use indexes 1 to n from this pointer.

int array[12];
int *array_starts_at_one = &array[-1]; // Don't use index 0 on this one
array_starts_at_one[1] = 1;
array_starts_at_one[12] = 12;

But I would advise against doing this.

Answer 4

Subtract 1 from the index every time you access the array to achieve "fake 1-based" indexing.

If you want to change the numbering while the program is running, you're asking for something more than just a regular array. If things only ever shift by one position, then allocate (n+1) slots and use a pointer into the array.

enum { array_size = 1000 };

int padded_array[ array_size + 1 ];
int *shiftable_array = padded_array; /* define pointer */

shiftable_array[3] = 5; /* pointer can be used as array */
some_function( shiftable_array );

/* now we want to renumber so element 1 is the new element 0 */
++ shiftable_array; /* accomplished by altering the pointer */

some_function( shiftable_array ); /* function uses new numbering */

If the shift-by-one operation is repeated indefinitely, you might need to implement a circular buffer.

Answer 5

Some more arguments why arrays are zero based can be found here. Infact its one of the very important and good features of the C programming language. However you can implement a array and start indexing from 1, but that will really take a lot of effort to keep track off.

Say you declare a integer array

int a[10];
for(i=1;i<10;i++)
a[i]=i*i;

You need to access all arrays with the index 1. Ofcourse you need to declare with the size (REQUIRED_SIZE_NORMALLY+1). You should also note here that you can still access the a[0] element but you have to ignore it from your head and your code to achieve what you want to.

Another problem would be for the person reading your code. He would go nuts trying to figure out why did the numbering start from 1 and was the 0th index used for some hidden purpose which unfortunately he is unaware of.