C Programming - Declarations and Initializations - Discussion

I tried out this

#include<stdio.h>
int main()
{
union a
{
int i;
char ch[2];
};
union a u;
u.ch[0] = 3;
u.ch[1] = 2;
u.ch[2] = 7;
printf("%d, %d, %d, %d\n", u.ch[0], u.ch[1], u.ch[2], u.i);
return 0;
}

I got the ans as 3,2,7,459267 and I also verified manually

But can u explain me why when char[2] declaration changed to char[3] gives result as 3,2,7,134676995

I mean I get the above result for the following program

#include<stdio.h>
int main()
{
union a
{
int i;
char ch[3];
};
union a u;
u.ch[0] = 3;
u.ch[1] = 2;
u.ch[2] = 7;
printf("%d, %d, %d, %d\n", u.ch[0], u.ch[1], u.ch[2], u.i);
return 0;
}

What does char[2] and char[3] declarations actually do to the program?
Pls explain me.

%d is a format specifier for integer. So when u use %d in printf() statement for a character it will print the ascii value of that character? Explain in detail how could the solution gain those output value.

union has two variables indeed. the one is an integer and a character sequence of 2.

So memory is allocated either for int variable or for character sequence. but could you access the all the variables of the union at a time?

I have a doubt that structure and union is almost same then why do we need union in first place if structure is already there? I know the difference between them but don't know the purpose of using union over structure? please anyone?

union a
{
int i;
char ch[2];
};
union a u;
u.ch[0] = 3;
u.ch[1] = 2;
printf("%d, %d, %d\n", u.ch[0], u.ch[1], u.i);

I got these output...not the mentioned in the explanation.

3, 2, 1075053059.......why?

Actually union size is the biggest size of its variable. But by using the instance of union if we access u.i it will indicate the value of union member/variable. But why it is giving the answer as 515?

@Suman sagar.

Consider it has a memory cell.

These are 2 memory cell(1byte =8 bits) ,size of the union is 2memory cells u.ch[0] occupies 1st memory cell ie 1-128, u.ch[1] occupies 2nd memory cell ie 256-65536. value assigned to u.ch[0] is 3 indicate the in 1st memory cell, u.ch[1] is assigned 2 indicate that is 2nd memory cell look for u.ch[1] it is like starting 1-128 but 1-65536 is one memory cell as a whole for the union.

u.ch[2] view   128 64 32   ....  2   1    |  128  ..  4   2   1   

u[i] view     65536  .. 1024   512    256 |  128   64..  2    1  

                               1     0                   1    1   

So according to u.ch[1] its value is 2.
But according to u[i] its value is 512 + 2+ 1.
Int occupies two memory cells whereas char occupies 1 memory cell.

Why we are assigning the 515 to the int i?

Why 'i' is not having any other value(like 0 or 1)?

Please give an example and give a description? Why we use struct and union ? where can we use it (with an example) ?

I confused that why we have to calculate the size of the union for i. Why it is not 0 like in structure?

Please explain in detail why is it giving 515 as the size of the union?