can somebody explain the value of array numbers[3] and numbers[6] in following c code? [closed]

Question

Sample Program Using Arrays here we need to find final output of code but I can not get the same output as the compiler gets. I am getting wrong output in numbers[3] and numbers[6].

#include <stdlib.h>
#include <stdio.h>

int main(void)
{
    int numbers[10];
    int i, index = 2;

    for (i = 0; i < 10; i++)
        numbers[i] = i * 10;
    numbers[8] = 25;
    numbers[5] = numbers[9] / 3;
    numbers[4] += numbers[2] / numbers[1];
    numbers[index] = 5;
    ++numbers[index];
    numbers[numbers[index++]] = 100;
    numbers[index] = numbers[numbers[index + 1] / 7]--;

    for (index = 0; index < 10; index++)
        printf("numbers[ %d ] = %d\n", index, numbers[index]);
    return 0;
}

Answer 1

1. The easiest thing is to display the value of index and numbers in your debugger then execute each line till you are surprised. If your debugger supports it, you can also run your program in reverse.
2. You could modify your program with a function that prints out index and numbers then run that function after every significant change.
3. If you want to do it manually you could reason about the program in reverse by hand. From the output of your program:

numbers[ 3 ] = 100
numbers[ 6 ] = 99

so let's start with the last assignment:

numbers[ index ] = numbers[ numbers[ index + 1 ] / 7 ]-- =>
numbers[3] = numbers[numbers[4] / 7]-- =>
 = numbers[(40 + 20 / 10) / 7 ]-- =>
 = numbers[6]--

As index = 3 from the post increment in numbers[ numbers[ index++ ] ] = 100; of the initial value of 2 set with int i, index = 2. numbers[ 4 ] += numbers[ 2 ] / numbers[ 1 ]; which just uses the initial values set in the loop numbers[ i ] = i * 10.

We know, from the output, that numbers[3] the assigned the value 100 and numbers[6] is decremented to 99. The only place we assign the value 100 is the previous line numbers[ numbers[ index++ ] ] = 100 so we just need to convince ourselves that the left hand side is numbers[6]:

numbers[ numbers[ index++ ] ]  =>
numbers[numbers[2]] =>
numbers[6]

As index = 2 from the initial assignment discussed above, and numbers[2] = 6 from ++numbers[ index ] from the previous value of 5 set with numbers[ index ] = 5;.

The moral of the story is to prefer single assignment :-)

Answer 2

All the magic has happened in the last two lines:

numbers[numbers[index++]] = 100;
numbers[index] = numbers[numbers[index + 1] / 7]--;

I would suggest you to use a debugger and add numbers[3] and numbers[6] to the watcher. And then set a breakpoint at this numbers[numbers[index++]] = 100; line.

number[6]

1. index was 2, so numbers[index++] will give 6 due to these two lines

numbers[index] = 5; // numbers[2] = 5
++numbers[index]; // numbers[2] = 5 + 1 = 6

2. Now, numbers[numbers[index++]] = 100; will become numbers[6] = 100;
3. Now, index++ will execute making it 3
4. Hence, numbers[6] is now 100

numbers[3]

1. Now decode this line:

numbers[index] = numbers[numbers[index + 1] / 7]--;

2. index is now 3, so numbers[index + 1] is equal to numbers[4] which is equal to 42 which when divided by 7 becomes 6
3. Now, numbers[3] = numbers[6] = 100
4. After numbers[3] becomes 100, then numbers[6] will be decresed by 1 due to post-decrement operator --
5. Hence, numbers[3] = 100 and numbers[6] = 99