How to categorize elements of array in C - c

Question

I have a problem regarding arrays.
So, I have an array with the size of 255 (let's call it array(255)). I have an integer named block also.
I want to categorize this arrays (I hope the term is right).
What I want is:
If the chosen array element is
From (0-63) , block = 0
From (64-127) , block = 1
From (128-191), block = 2
From (192-255), block = 3
I know with this case I can just use if statement. But what if I scale up my array size into thousands?
I can't use rows of If statement.
Is there any simpler and more efficient solution for this?

Answer 1

You can simply do it with an integer division:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <stdint.h>
#include <inttypes.h>
int main(void)
{
uint8_t array[255] = {0};
srand(time(NULL));
for(size_t i=0; i<sizeof(array)/sizeof(array[0]); i++)
{
array[i] = rand() % 256;
printf("Array[%zu]= %"PRIu8"\tblock= %"PRIu8"\n", i, array[i], array[i] / 64 );
}
}

Answer 2

You should do something like:
int array[255];
...
int getArrayItemByCateg(int category, int index) {
return array[category * 64 + index];
}

Answer 3

If you have only an pointer to your element in your code you could use something like the following function:
int getBlock(void* array, void* arrayElement, size_t size)
{
return (int)((arrayElement - array) / (size * 64));
}
This function can be uses for different array types but the origin array has to be known when you are calling this function. See the following examples:
uint32_t array_u32[256];
uint16_t array_u16[256];
printf("array u32 index 32 is block %d\n", getBlock(array_u32, &array_u32[32], sizeof(array_u32[0])));
printf("array u32 index 129 is block %d\n", getBlock(array_u32, &array_u32[129], sizeof(array_u32[0])));
printf("array u16 index 32 is block %d\n", getBlock(array_u16, &array_u16[32], sizeof(array_u16[0])));
printf("array u16 index 129 is block %d\n", getBlock(array_u16, &array_u16[129], sizeof(array_u16[0])));
The following result will be printed:
array u32 index 32 is block 0
array u32 index 129 is block 2
array u16 index 32 is block 0
array u16 index 129 is block 2
Also see the code at IDEONE.

Answer 4

How about a multidimensional array? Information: Tutorials Point - Multidimensional Arrays
This may not be an elegant solution but it is pretty simple. This may not be ideal for larger scale data however larger scale data may be best using a binary or quad tree anyhoo.
Multidimensional Array:
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
const int COLUMN_SIZE = 64;
const int ROW_SIZE = 4;
// Access index of row using the labels below
const int BLOCK_ONE = 0;
const int BLOCK_TWO = 1;
const int BLOCK_THREE = 2;
const int BLOCK_FOUR = 3;
int main(int argc, char* argv[]) {
int array[ROW_SIZE][COLUMN_SIZE] = {0};
srand(time(NULL));
/* Initialise array */
for(int x = 0; x < ROW_SIZE; x++) {
for(int i = 0; i < COLUMN_SIZE; i++) {
array[x][i] = rand() % (ROW_SIZE * COLUMN_SIZE);
}
}
/* Test */
for(int x = 0; x < ROW_SIZE; x++) {
for(int i = 0; i < COLUMN_SIZE; i++) {
printf("Array[%d][%d]: %d\n", x, i, array[x][i]);
}
printf("\n");
}
return 0;
}