c how to make a dynamic array pointer in a struct

Question

I am taking in input from the user for the number of elemenets inside an array. The array is inside my struct 'Polymer'

struct Polymer
{
    int length;

    struct Monomer *monomer;

}polymer;

In main, I am creating a new monomer array pointer and setting the mononomer pointer in "Polymer" to it

struct Monomer *monomers[size];
polymer.monomer = momomers;

I am getting the error "Assignment from incompatible pointer type" which I assume is because we are converting a monomer array pointer to a monomer. How do I declare it as a monomer array pointer in the struct?

Also, you should use size_t for array sizes, string lengths, and anything else with a size. int is signed (you don't want an array of -1 length, do you?) and is not guaranteed to be large enough to properly store a size (or, in the pathological case, it might be too large). — Chris Lutz
– Chris Lutz, Commented Oct 7, 2011 at 19:19

cnicutar · Accepted Answer · 2011-10-07 18:52:17Z

5

You are declaring an array of monomer pointers when you probably want an array of monomers. Drop the *:

struct Monomer monomers[size];
polymer.monomer = momomers;

answered Oct 7, 2011 at 18:52

cnicutar

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Mahesh · Accepted Answer · 2011-10-07 18:53:30Z

2

struct Monomer *monomers[size];
polymer.monomer = momomers;

monomers is an array of pointers. They aren't pointing to any valid locations and has garbage values. While Polymer::monomer is a pointer. Array of pointers isn't type compatible to just a pointer.

Instead try -

struct Monomer monomers[size];
polymer.monomer = momomers;  // 2

Now this statement 2 is valid because array decays to a pointer.

answered Oct 7, 2011 at 18:53

Mahesh

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2 Comments

BlackBear Over a year ago

This isn't C++, shouldn't it be Polymer.monomer?

Mahesh Over a year ago

@BlackBear - Technically to say, . operator should be associated with an object. Here Polymer is the struct name. So, to say it's member I just used the operator ::.

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