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Why does the subscript of the array start from 0 instead of 1?

2022-04-23 06:07:00 【New ape and horse】

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One Array addressing formula

Two Array model representation

3、 ... and Array addressing calculation

One Array addressing formula 

a[i]_address = base_address + i * data_type_size

data_type_size Is the size of each element in the array .

Two Array model representation

With int[] a = new int[10] For example . Computers give arrays a[10], Allocated a contiguous memory space 2000～2039, The first address of the memory block is base_address = 2000.

3、 ... and Array addressing calculation

Suppose the array is from 0 Start , Computation first 9 The first element formula is ：a[9]_address = 2000 + 9 * 4 = 2036;

Suppose the array is from 1 Start , Computation first 9 The first element formula is ：a[9]_address = 2000 + （10-1） * 4 = 2036

contrast ： Array from 1 At first, it is equivalent to one more subtraction operation for each random access to array elements , about CPU Come on , Just one more subtraction instruction . Arrays are very basic data structures , Random access to array elements through subscripts is a very basic programming operation , The optimization of efficiency should be as extreme as possible .

Conclusion ： In order to reduce one subtraction operation , Array selected from 0 Numbered starting , Not from 1 Start .

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