i have 5 digits in 1 pointer
int* reversevec(int* ptr, unsigned int Ne){
int* ReverResult;
unsigned int rever=Ne, z;
ReverResult=(int*)malloc(Ne*sizeof(int));
for (int i = 0; i < Ne; ++i)
{
z=*(ptr+rever);
printf("%i ",z);//to be sure z starts from the last number on ptr to the fisrt
rever--;
}
return ReverResult;
}
example
Number of elements(Ne)=5
int* ptr have 5 numbers {1 2 3 4 5}
every time when z is printed i got {5 4 3 2 1}
but i cant save z into *ReverResult
ReverResult=(z+rever);
this line is what i tried to put into cicle for to save z and position into int pointer ReverResult but i cant convert int to int*;
There are many problems here
z is a local variable int.
its address will not be useful to return, because it will be out of scope.
returning an offset from its address is even worse, since that is a totally unrelated place in memory.
you also have an off-by-one error. imagine Number elements is one. You will then try to view ptr+1 instead of ptr+0.
you've also tagged this c++ but are writing c style code.
to answer your primary question, rather than writing ReverResult=(z+rever) one could write *(ReverResult + rever - 1) = *(ptr + i)
The other way round, you need to dereference your pointer to be able to assign. After all, you are easier off with pure pointer arithmetics:
int* result = malloc(sizeof(*ptr) * ne);
// let's just have to pointers running through the two arrays:
for(int* p = ptr + ne, *r = result; p-- != ptr; ++r)
{
*r = *p;
// ^ ^ this is the important part: dereference both pointers
}
If you still prefer indices you can use operator[]:
--ne; // so that we don't have to subtract within the loop again and again
for(size_t index = 0; index <= ne; ++index)
{
result[index] = ptr[ne - index];
// ^^^^^^^ again: dereferencing, this time with offset
}
array[index] is entirely equivalent to *(array + index) – and actually it, too, to index[array] (e. g. 7[array]), which is often used to fool the inexperienced...
Related
I have read online that memmove is expected to perform no action if the number of bytes to copy is 0. However what I want to know is if it is expected that the source and destination pointers will not be read in that case
Below is a simplified version of some of my code, the section I am interested in is shiftLeft:
#include <array>
#include <cstring>
#include <iostream>
class Foo final {
unsigned just = 0;
unsigned some = 0;
unsigned primitives = 0;
};
template <unsigned Len>
class Bar final {
unsigned depth = 0;
std::array<Foo, Len> arr;
public:
Bar() = default;
// Just an example
void addFoo() {
arr[depth] = Foo();
depth++;
}
void shiftLeft(unsigned index) {
// This is what my question focuses on
// If depth is 10 and index is 9 then index + 1 is out of bounds
// However depth - index - 1 would be 0 then
std::memmove(
&arr[index],
&arr[index + 1],
(depth - index - 1) * sizeof(Foo)
);
depth--;
}
};
int main() {
Bar<10> bar;
for (unsigned i = 0; i < 10; ++i)
bar.addFoo();
bar.shiftLeft(9);
return 0;
}
When Len is 10, depth is 10, and index is 9 then index + 1 would read out of bounds. However also in that case depth - index - 1 is 0 which should mean memmove would perform no action. Is this code safe or not?
The memmove function will copy n bytes. If n is zero, it will do nothing.
The only possible issue is with this, where index is already at the maximum value for array elements:
&arr[index + 1]
However, you are permitted to refer to array elements (in terms of having a pointer point to them) within the array or the hypothetical element just beyond the end of the array.
You may not dereference the latter but you're not doing that here. In other words, while arr[index + 1] on its own would attempt a dereference and therefore be invalid, evaluating the address of it is fine.
This is covered, albeit tangentially, in C++20 [expr.add]:
When an expression that has integral type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the expression P points to element x[i] of an array object x with n elements, the expressions P + J and J + P (where J has the value j) point to the (possibly-hypothetical) element x[i + j] if 0 ≤ i + j ≤ n; otherwise, the behavior is undefined.
Note the if 0 ≤ i + j ≤ n clause, particularly the final ≤. For an array int x[10], the expression &(x[10]) is valid.
It's also covered in [basic.compound] (my emphasis):
A value of a pointer type that is a pointer to or past the end of an object represents the address of the first byte in memory occupied by the object or the first byte in memory after the end of the storage occupied by the object, respectively.
#include <iostream>
using namespace std;
int main ()
{
int numbers[5];
int * p;
p = numbers; *p = 10;
p++; *p = 20;
p = &numbers[2]; *p = 30;
p = numbers + 3; *p = 40;
p = numbers; *(p+4) = 50;
for (int n=0; n<5; n++)
cout << numbers[n] << ", ";
return 0;
}
Fairly new coder here. This is an example taken from the pointers page on cplusplus.com. They talk about dereferencing, the address of operator, and then they talk about the relationship between arrays and pointers. I noticed if I simply printed out a declared array it spits out the address of the array. I'm really trying to understand exactly what some of these lines of code are doing and am having a hard time.
Everything makes sense up until *p=10. I read this as, "the value being pointed to by pointer p is equal to 10" but an array isn't just one value. In this case it's 5... So, whats the deal? Will the compiler just assume we are talking about the first element of the array?
Even more confusing for me is p++;. p right now is the address of the array. How do you do p=p+1 when p is an address?
I understand p= &numbers[2];. As well as the following *p = 30;.
p = numbers + 3; confuses me. Why are we able to take an address and just add 3 to it? And, what does that even mean?
Everything within the for loop makes sense. It's just printing out the array correct?
Thanks for your help in advance!
As Pointed out this due to pointer arithmetic.
So both array numbers and pointer p are int, so after assigning numbers's base address to pointer p and the *p=10 does numbers[0]=10
Now when you do p++ the address of p is incremented but not to the next numeric value as in normal arithmetic operation of ++ but to the address of the next index of numbers.
Let's assume a int is 4byte of memory and numbers's initial address is 1000.
So p++ makes address it to be incremented to 1004 which is numbers[1]
Basically it points to the next int variable position in memory.
So when *p = 20 is encountered the value 20 will be assigned to the next adjacent memory i.e. 1004 or numbers[1]=20.
After that the p directly assigned address of 3rd index numbers[2] i.e 1008 (next int var in memory). Now p is pointing to the 3rd index of numbers so *p=30 is again numbers[2]=30
p = numbers + 3; *p = 40; does similar thing that is assigns base address of numbers incremented by 3 indexes (1000 + 3*sizeof(int) =1012 ==> 1000 + 3*4 =1012) to p and assigns 40 to that. so it becomes equivalent to numbers[3]=40.
Then p = numbers; *(p+4) = 50; is assigning 1000 to p and the adding 4 to the address of p by using the () and assigns 50 to the value by using *() which is value of the address(1000+ 4*4 = 1016) inside the braces.
Using a pointer like this in C++: p = numbers makes p point to the first element of the array. So when you do p++ you are making p point to the second element.
Now, doing *p = 10 assigns 10 to whatever p is pointing to, as in your case p is pointing to the first element of the array, then *p = 10 is the same as numbers[0] = 10.
In case you see this later: *(p + i) = 20, this is pointer arithmetic and is the same as p[i] = 20. That's why p[0] = 10 is equivalent to *(p + 0) = 10 and equivalent to *p = 10.
And about your question: How do you do p=p+1 when p is an address?: as arrays in C++ are stored sequentially in memory, if the element p is pointing at memory address 6400010 then p++ points at 6400014 assuming each element pointed by p occupies 4 bytes. If you wonder why p++ is 6400014 instead of 6400011, the reason is a little magic C++ does: when increasing a pointer, it doesn't increase by 1 byte but by 1 element so if you are pointing to integers, each one occupying 4 bytes, then p will be increased by 4 instead of 1.
Let's break it down:
int numbers[5];
int * p;
Assigning the array variable to a pointer means that the pointer will be pointing to the first element:
p = numbers; // p points to the 1st element
*p = 10; // 1st element will be 10
p++; // go to next location i.e. 2nd element
*p = 20; // 2nd element will be 20
You can do pointer arithmetic and it works according to the type. Incrementing an int pointer on a 32-bit machine will do a jump of 4 bytes in memory. On a 64-bit machine, that will be 8 bytes. Keep in mind that you cannot do this with a void pointer.
p = &numbers[2]; // assign the address of 3rd element
*p = 30; // 3rd element will be 30
p = numbers + 3; // numbers + 3 means the address of 4th element
*p = 40; // 4th element will be 40
p = numbers; // p points to the 1st element
*(p + 4) = 50; // p + 4 means the address of 5th element
// dereference 5th location and assign 50
// 5th element will be 50
There are a lot of pages on the web on how pointers work. But there are a couple things to quickly point out.
int* p;
The above line mean p is a pointer to an int. Assigning something to it doesn't change what it is. It is a pointer to an int, not an array of int.
int number[5];
p = number;
This assignment takes the address where the array is stored, and assigns it to p. p now points to the beginning of that array.
p++;
p now points to the 2nd int in the array. Pointers are incremented by the size of the object they point to. So the actual address may increase by 4 or 8, but it goes to the next address of an int.
*p = 30;
This is an assignment, not a comparison. It doesn't say that what p points to is 30, it copies 30 into the address pointed to by p. Saying "equals" is generally confusing in programming terms, so use either "assign" or "compare" to be clear.
1> p=numbers;
pointer p now points to the start of the array, i.e first element.
2> *p=10;
first element now becomes 10, as p is pointing to the first element.
3> p++;
this moves the pointer to the next element of the array, i.e second element
So, again *p=20; will make second element=20
4> p = &numbers[2];
this statement means that p now points to 3rd element of the array.
So, *p = 30; will make 3rd element=30
5> p = numbers + 3;
Wen we do p=numbers+1; it points to second element, so p=numbers+3; will now point to 4th element of array. *p = 40; make the fourth element=40
6> p = numbers;
Again p points to first element of array
*(p+4) = 50; will now make 5th element=50
I am studying C++ and came across this Code
int a[] = {9,8,−5};
int∗ p = &a[2] ;
and was wondering what exactly that means? So a is an Array and p is a pointer. But what does the & before a[] mean?
and what does mean then?
std::cout << (p − a) << "\n";
edit:
so i have read some answeres but what i still dont understand is what the & is really for. Is there a difference between
int∗ p = &a[2] ;
and
int∗ p = a[2] ;
?
Thanks for your help.
This code
int∗ p = &a[2];
is equivalent to this:
int∗ p = a + 2;
so even mathematically you can see that p - a is equal to a + 2 - a so I think result should be obvious that it is equal to 2.
Note name of array can decay to a pointer to the first element but is not that pointer, as many may mistakenly say so. There is significant difference.
About your note, yes there is difference, first expression assigns address of third element to p, second is syntax error as you try to assign int to int * which are different types:
int a[] = { 1, 2, 3 };
int var = a[2]; // type of a[2] is int and value is 3
int *pointer = &a[2]; // type of &a[2] is int * and value is address of element that holds value 3
Subject of your question says that you think & means reference in this context. It only means reference when applied to types, when used in expression it means different things:
int i, j;
int &r = i; // refence when applied to types
&i; // address of in this context
i&j; // binary AND in this
In your case & is used to get the address to a value. So what will happen there is that p will contain the address of a[2], thus p will be a pointer to the third element of the array.
EDIT: Demo (with p-a included)
I am learning C++ and one of my practice exercises is to use pointers to set all the elements in an array to 0. I have no idea how to do this by incrementing the pointer to the next position in the array since my IDE log said that comparison between int and * is forbidden. I only need a small snippet as an example to help me better understand where i'm going wrong. The array I have created is of type int and has a single dimension with 5 elements consisting of 1,2,3,4 and 5.
int array[5] = {1, 2, 3, 4, 5};
for(int *i = &array[0], *end = &array[5]; i != end; i++)
*i = 0;
The code creates a pointer to the start &array[0] and a pointer to one position past the end &array[5]
Then it steps the pointer through the array, setting each element to zero.
A more advanced concept that is very similar is iterators.
You could use std::fill, http://en.cppreference.com/w/cpp/algorithm/fill, as follows.
const size_t dataSize = 10;
int data[dataSize];
std::fill(data, data + dataSize, 0);
I was looking through the forums and saw a question about counting the numbers of each letter in a string. I am teaching myself and have done some research and am now starting to do projects. Here I have printed the elements of the array. But I do so without pointers. I know I can use a pointer to an array and have it increment for each value, but I need some help doing so.
Here is my code without the pointer:
code main() {
char alph [] = {'a', 'b', 'c'};
int i, o;
o = 0;
for(i=0; i < 3; i++)
{ cout << alph[i] << ' '; };
};
Here is my bad code that doesn't work trying to get the pointer to work.
main() {
char alph [] = {'a', 'b', 'c'};
char *p;
p = alph;
for (; p<=3; p++);
cout << *p;
return 0;
};
I hope that it's not too obvious of an answer; I don't mean to waste anyone's time. Also this is my first post so if anyone wants to give me advice, thank you.
Very good try. There's just one tiny thing wrong, which is this:
p <= 3
Pointers are just some number which represents a memory address. When you do p = alph, you're not setting p to 0, you're setting it to point to the same address as alph. When looping over an array with pointers, you have to compare the current pointer with a pointer that is one past the end of the array. To get a pointer to one element past the end of the array, you add the number of elements to the array:
alph + 3 // is a pointer to one past the end of the array
Then your loop becomes
for (; p < alph + 3; ++p)
You may think that getting a pointer to one past the end of the array is going out of bounds of the array. However, you're free to get a pointer to anywhere in memory, as long as you don't dereference it. Since the pointer alph + 3 is never dereferenced - you only use it as a marker for the end of the array - and everything is fine.
Here are some rough correlations for the different versions:
/-----------------------------------\
| Pointer Version | Index Version |
-------------------------------------
| p | i |
| p = alph | i = 0 |
| *p | alph[i] |
| alph + 3 | 3 |
| p < alph + 3 | i < 3 |
\-----------------------------------/
Also note that instead of doing alph + 3, you may want to use sizeof. sizeof gives you the number of bytes that an object occupies in memory. For arrays, it gives you the number of bytes the whole array takes up (but it doesn't work with pointers, so you can do it with alph but not with p, for example). The advantage of using sizeof to get the size of the array is that if you change the size later, you do not have to go and find all the places where you wrote alph + 3 and change them. You can do that like this:
for (; p < alph + sizeof(alph); ++p)
Additional note: because the size of char is defined to be 1 byte, this works. If you change the array to an array of int, for example (or any other type that is bigger than 1 byte) it will not work any more. To remedy this, you divide the total size in bytes of the array with the size of a single element:
for (; p < alph + sizeof(alph) / sizeof(*alph); ++p)
This may be a little complicated to understand, but all you're doing is taking the total number of bytes of the array and dividing it by the size of a single element to find the number of elements in the array. Note that you are adding the number of elements in the array, not the size in bytes of the array. This is a consequence of how pointer arithmetic works (C++ automatically multiplies the number you add to a pointer by the size of the type that the pointer points to).
For example, if you have
int alph[3];
Then sizeof(alph) == 12 because each int is 4 bytes big. sizeof(*alph) == 4 because *alph is the first element of the array. Then sizeof(alph) / sizeof(alph) is equal to 12 / 4 which is 3, which is the number of elements in the array. Then by doing
for (; p < alph + sizeof(alph) / sizeof(*alph); ++p)
that is equivalent to
for (; p < alph + 12 / 4; ++p)
which is the same as
for (; p < alph + 3; ++p)
Which is correct.
This has the good advantage that if you change the array size to 50 and change the type to short (or any other combination of type and array size), the code will still work correctly.
When you get more advanced (and hopefully understand arrays enough to stop using them...) then you will learn how to use std::end to do all this work for you:
for (; p < std::end(alph); ++p)
Or you can just use the range-based for loop introduced in C++11:
for (char c : alph)
Which is even easier. I recommend understanding pointers and pointer arithmetic well before reclining on the convenient tools of the Standard Library though.
Also good job SO, you properly syntax-highlighted my ASCII-Art-Chart.
The pointer loop should be:
for (char * p = alph; p != alph + 3; ++p)
{
std::cout << *p << std::endl;
}
You can get a bit more fancy by hoisting the end of the array out of the loop, and by inferring the array size automatically:
for (char * p = alph, * end = alph + sizeof(alph)/sizeof(alph[0]); p != end; ++p)
In C++11, you can do even better:
for (char c : alph) { std::cout << c << std::endl; }
Your problem (apart from compilation issues noted in my comment to the question) is that pointers are not as small as 3. You need:
for (p = alph; p < alph + sizeof(alph); p++)
for the loop. Note that sizeof() generates a compile-time constant. (In C99 or C2011, that is not always the case; in C++, it is always the case, AFAIK). In this context, sizeof() is a fancy way of writing 3, but if you add new characters to the array, it adjusts automatically, whereas if you write 3 and change things, you have to remember to change the 3 to the new value too.
Ruminations on the use of sizeof()
As Kerrek SB points out, sizeof() returns the size of an array in bytes. By definition, sizeof(char) == 1, so in this context, it was safe to use sizeof() on the array. There are also times when it is not safe - or you have to do some extra work. If you had an array of some other type, you can use:
SomeType array[] = { 2, 3, 5, 7, 11, 13 };
then the number of elements in the array is (sizeof(array)/sizeof(array[0])). That is, the number of elements is the total size of the array, in bytes, divided by the size of one element (also in bytes).
The other big gotcha is that if you 'pass an array' as a function argument, you can't use sizeof() on it to get the correct size - you get the size of a pointer instead. This is a good reason for not using C-style strings or C-style arrays: use std::string and std::vector<SomeType> instead, not least because you can find their actual size reliably with a member function call.
I recommand you the following revised code.
char alph [] = {'a', 'b', 'c', 0};
char *p;
p = alph;
while(*p)
{
std::cout << *p++;
}
0 marks the end of a string in both c and c++.