Getting an array from pointer in a structure

Question

I have a structure like this

struct quantum_reg_struct
{
  int width;    /* number of qubits in the qureg */
  int size;     /* number of non-zero vectors */
  int hashw;    /* width of the hash array */
  COMPLEX_FLOAT *amplitude;
};

typedef struct quantum_reg_struct quantum_reg;

quantum_reg reg;

If *amplitude is pointing to the start of array of type COMPLEX_FLOAT then I want to be able to store this in a an array of type Complex_float. Is it possible for me?

Secondly I need to understand the usage of -> operator. What happens if something like this is written .

reg->amplitude[1] *= -1;

Is COMPLEX_FLOAT a structure? If so then allocate memory to it -> operator us used to access structure members using a pointer variable. — Gopi
– Gopi, Commented Nov 12, 2014 at 10:51

VoidStar · Accepted Answer · 2014-11-12 10:57:41Z

1

As is, your code would crash if you accessed amplitude without assigning it to point to memory.

quantum_reg reg;
reg.amplitude = malloc(5 * sizeof (COMPLEX_FLOAT));
reg.amplitude[0] = 1.2f;  // I'm guessing COMPLEX_FLOAT is just a typedef'd float for this to work 
reg.amplitude[1] = 1.5f;
// reg->amplitude won't compile because reg is not a pointer.
// if COMPLEX_FLOAT is a struct, then reg.amplitude->fieldname could work, where fieldname is something in COMPLEX_FLOAT.
free(reg.amplitude); // free the allocated memory.

If amplitude has a fixed number of elements, consider redefining it as an array, e.g. COMPLEX_FLOAT amplitude[5];. This is easier to use because you don't have to worry about malloc and free. The advantage of malloc and free is that you can pick the size (number of elements) at runtime, but if you don't need this, then stick with an array.

answered Nov 12, 2014 at 10:57

VoidStar

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1 Comment

jav321 Over a year ago

Yes,COMPLEX_FLOAT is a typedef for float. Thanks for pointing me to the right direction. In this line. reg.amplitude = malloc(5 * sizeof (COMPLEX_FLOAT)) is 5 here for some reason or is it just an arbitrary example

Swann · Accepted Answer · 2014-11-12 10:54:51Z

0

Yes you could doing something like this :

quantum_reg.amplitude = malloc(sizeof(*quantum_reg.amplitude) * n);

Now the -> operator is just a shortcut for

(*reg).amplitude

answered Nov 12, 2014 at 10:54

Swann

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3 Comments

Lightness Races in Orbit Over a year ago

You're proposing a memory leak... maybe. Depends what the OP started off with. The question is unclear.

Swann Over a year ago

I just did not mention the free part as it was not the purpose of the question. We are not doing a course on how/where/when to free the memory

Lightness Races in Orbit Over a year ago

Is that supposed to be a joke?!

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