Why output of firstvalue is 10 not 20?

Question

int firstvalue = 5, secondvalue = 15;
int * p1, * p2;
p1 = &firstvalue;
p2 = &secondvalue;
*p1 = 10;
*p2 = *p1;
p1 = p2;
*p1 = 20;

Answer 1

Let's trace to see what's going on

Q: Why output of firstvalue is 10 not 20?

int firstvalue = 5, secondvalue = 15;

int * p1, * p2;

// p1 <- firstVal address
p1 = &firstvalue;

// p2 <- secondVal address
p2 = &secondvalue;

// indirect assignment
// firstVal <- *p1 <- 10
*p1 = 10;

// dereference and assignment (PAY ATTENTION)
// secondVal <- *p2 <- firstVal <- *p1
*p2 = *p1;

// pointer assignment (PAY ATTENTION)
// address of firstVal = addrest of secondVal
p1 = p2;

// Indirect assignment
// secondVal <- *p1 <- 20
*p1 = 20;

firstVal = 10
secondVal = 20

Answer 2

Check this out.

https://code.sololearn.com/c40AiP26AbWG

Answer 3

dears thank you very much for useful feedback. It is clear now, just have a feeling of something "unusual". need to get used to it :)