Need help, Basic question

[atman]

Hello.,
Im very new to C++ but im trying to resolve a fragment of the following code:

#include <iostream>
 using namespace std;
 const int NO_STEPS = 4;

 void s(int* a, int i);

 int main ( ) {
     int i, a[NO_STEPS] = {2, 4, 1, 3};

     for ( i = 0; i < NO_STEPS; i++ )
         s( a, i );

     for ( i = NO_STEPS - 1; i >= 0; i-- )
         cout << a[i] << ' ';
     cout << "Boom!\n";

     return 0;
 }

 void s(int* a, int i) {
     int* p;
     int  x, n = NO_STEPS / 2;

     if (i != n) {
         p = a + i; // be careful - pointer addition here
         x = *p;
         *p = a[n];
         a[n] = x;
     }
 }

to understand it better i split the code into 2 parts and simplified it abit so im working only on the first loop having the following code:

#include<iostream>
using namespace std;

void s(int* a, int i);

main()
{
int i, a[4] = {2,4,1,3};

        for(i = 0; i < 4; i++)
       {
         s(a,i);
         printf("%d", a[i]);
}}

void s(int *a, int i)
{
  int *p; int x, n=2;

        if(i != 2)
        {
        p = a + i;
        x = *p;
        *p = a[2];
        a[2] = x;
}}

going through it on paper i dont fully understand the last line of function a[2] = x; we already know what the x is and we already stored it in a[2], why to restore the same value to x??? and my result was very different on paper then compilers, here are the arrays values i got after the loop is complete 2114, when the compiler got 1244, could anybody please let me know where my problem is? its driving me nuts.
Thank you very much

[Rashakil Fol]

The code doesn't store the value of x in a[2] before that line. What makes you think it does?

[atman]

The code doesn't store the value of x in a[2] before that line. What makes you think it does?

when in the function p=a+i; does it mean p=0+0? since a points to the 1st element which is a 0 and i is also 0 in a first pass of the loop??? or it would be a=0 + 2(since its the first element's value)?
thank you

[Rashakil Fol]

when in the function p=a+i; does it mean p=0+0?

Only if a and i are both zero. But a is not zero. since a points to the 1st element which is a 0 and i is also 0 in a first pass of the loop???

No, the first element is a 2. (The array whose memory location is passed as the parameter 'a' has the elements 2, 4, 1, 3.) But you're not adding the first element -- you're adding the memory address of the first element to an integer i. The sum (a+i) will be the memory address of the element i units over from the element pointed at by 'a'. For example, (a+2) will be the memory address of the element 1, and *(a+2) will be the value 1. or it would be a=0 + 2(since its the first element's value)?
thank you

What the ####? How could the line "p=a+i" end up assigning a value to a?

[atman]

Thank you so much for fast and detailed responce., so after statement p=a+i, does it mean that p equals 2 in the first pass? and does it mean that with statement x=*p means that x=2? i'm abit confused with these 3 lines...

thank you

[Rashakil Fol]

Thank you so much for fast and detailed responce., so after statement p=a+i, does it mean that p equals 2 in the first pass?

No, the statement p=a+i changes the variable p so that it contains a memory address i units after a. There is no way p will equal 2, because 2 is an int, and p is an int*. and does it mean that with statement x=*p means that x=2? i'm abit confused with these 3 lines...

If *p evaluates to 2, then that means x will contain 2. Here is a simple example:

int array[] = {2, 3, 5, 7, 11};
int* p = array;  // p contains the memory address of array[0]
int x = *p;  // *p returns whatever's at the memory address stored in p.  So it returns the value that's in array[0].  So it return 2.  Thus, the value 2 gets assigned to the variable x.
int* q = p + 2;  // q contains the memory address 2 units after p.
int y = *q;  // *q is 5, so y gets assigned 5.