C language array element subscript starting from 0

Many classmates may have this question when learning the array, why does the subscript not start from 1? Isn't it more in line with People's daily habits starting from 1? In life we usually say a 1th, not a No. 0. Indeed, some computer languages, such as the early Pascal language, are the subscript for array elements starting from 1. is C language deliberately to be different? To figure this out, first look at how the bottom of the computer handles array elements. We first wrote a small program and then disassembled it in Visual Studio. The source and disassembly sections of the code are as follows:

SOURCE program:

int arr[5]; A global array

int Main ()

{

int i;

for (I =0;i < 5;i++)

Arr[i] = 9;

return 0;

}

Post-disassembly part of the code:

int i;
for (i = 0; i < 5; i++)
0101168E	C7 F8 00 00 00 00	mov dword ptr [ebp-8], 0
01011695	EB 09	JMP 010116a0
01011697	8B F8	mov eax, DWORD ptr [Ebp-8]
0101169A	C0 01	Add eax,1
0101169D	F8	mov DWORD ptr [Ebp-8],eax
010116a0	7D F8 05	CMP DWORD ptr [ebp-8],5
010116a4	7D 10	Jge 010116b6
Arr[i] = 9;
010116a6	8B F8	mov Eax,dword ptr [ebp-8]
010116a9	C7 04 85 80 95 01 01 09 00 00 00	mov dword ptr [eax*4+01019580h], 9
010116b4	EB E1	JMP 01011697
return 0;
010116b6	C0	XOR Eax,eax

In the above table in addition to the program source code, the other part, the leftmost column is the address of the instruction, the middle is the machine code, that is, the last code executed by the machine, the right column is the corresponding assembly language code. It is not possible for beginners to understand these assembly codes. Let's take a look at this sentence:mov dword ptr [eax*4 + 01019580h], 9its function is toArr[i] = 9, whereeaxthe variables are stored in theIthe value,4indicates that each element occupies4bytes,01019580his an arrayarrto confirm that you can type in the Watch Windowarror&arr[0], we can see that their values are equal to0x01019580has shown in the following:

In other words, when assigning a value to the element I, first calculate its address, namely: First address +i*4, in this case, the first address is01019580h, so the first0elements are stored in a01019580hstarting at four bytes, the first1elements are stored in a01019584hin the first four bytes,... .., the first4elements are stored in a01019590hthe start of the four bytes. After calculating the address, use themovdirective will9passed to the address, beginning with the4bytes are stored in a. After executing forafter the loop, view01019580hstart the memory situation as shown in:

As you can see, there are 5 9 bytes in a row starting at 0x01019580h .

as seen from the above, when the array element subscript from the 0 at the beginning, the address of each element is calculated as follows:

Section 0 element Address: First address ( First address + 0*4)

Section 1 element Address: First address + 1*4

Section 2 element Address: First address + 2*4

...

Section I element Address: First address + i*4

when an array element is subscript from the 1 at the beginning, the address of each element is calculated as follows:

Section 1 element Address: First address

                              2 + ( 2-1 ) *4

Section 3 element Address: First address + ( 3-1 ) *

...

Section I element Address: First address + ( i-1 ) *

Obviously, if the array element subscript starts at 1, you will need to do a subtraction operation each time the address is computed. Therefore, in order to improve efficiency, the C language array element subscript starts from 0. C Language of high efficiency is reflected in these points drip, need to learn slowly experience!

C language array element subscript starting from 0