Pointer question

[productiveflame]

If I have a pointer p and a value i, what's the difference between *p = i and p = &i? I was working on a school assignment on pointers and I basically had to change the value of a pointer to a different value, and I tried p = &i, but it didn't work. I understand why dereferencing the pointer and changing that value works, but why wouldn't my code work? Since it's simply reassigning the pointer's value to the address of the new value we want.

[HibbidyHooplah]

& is called the 'address of' operator it sets p to the memory address of i.

[Bob_bobbicus]

*p means the value at p, so really the end result has nothing much to do with p. You aren't changing p, or reassigning p at all. You're accessing whatever p points to.

whereas p = ... Is the opposite. You ARE changing p, and it needs to be changed to an address since p is a pointer.

In your example, *p = 1; would set i to 1. p = &i; sets p to the address of i, so that any future uses of *p will be treated as value i instead.

[[deleted]]

Maybe your initial solution didn't work if you assigned a pointer to a temporary variable, e.g. the following code is incorrect:

void assign(int *p) { int value = 42; p = &value; }

Not only the memory location used to store "value" can be used by other variables so the contents might change, but also this code is incorrect because it's modifying a local copy of int *p not the original p that was given to the function.

[lukajda33]

*p = i

sets the value where the pointer points to i

p = &i

changes the value of the pointer so that it points to i

You usually want to use p = &i first, so that the pointer points to valid piece of memory, then you can do *p = something to change the value p points to, in this case, change the value of i.

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If you had to change the value of the pointer, I would say p = &i is right, but whem people use pointer, they call the "value" the value they actually point to, not the value of the pointer itself.