CODEWITHSUNDEEP
cpointers
user3463334
user3463334

Reputation:

About pointer to multidimension array

I read this from a book: for double beans[3][4]

if we want to make a pointer called it pbeans to point to it we need to declare it as

double (*pbeans) [4] =beans;

My question is:

  1. Why it is:

    double (*pbeans) [4] =beans;

    but not:

    double[4] *pbeans =beans; ?

  2. And how should I read double (*pbeans) [4] ? (say, const int * const pi we can read from right to left const pointer to integer constant).

Upvotes: 0

Views: 57

Answers (3)

haccks
haccks

Reputation: 106122

why it is double (*pbeans) [4] =beans; but not double[4] *pbeans =beans; ?

This is because you need pointer to the first element of array beans and the first element of it is an 1D array of 4 elements.
Why not double[4] *pbeans =beans;?

Because this is not a valid C syntax.

And how should I read double (*pbeans) [4]? (say, const int * const pi we can read from right to left const pointer to integer constant).

You can read this as *pbeans is a pointer to an array of 4 doubless. You can use spiral rule to parse such declarations.

Upvotes: 0

John Bode
John Bode

Reputation: 123598

C declaration syntax is based on the types of expressions, not objects. This is also phrased as "declaration mimics use."

You have a pointer to a 4-element array of double named pbeans. To access an element from the pointed-to array, you must first dereference pbeans using the unary * operator, and then apply the subscript to the result of that dereference:

double x = (*pbeans)[i];

The parentheses are necessary because the subscript operator [] has higher precedence than unary * operator; if you wrote *pbeans[i], it would be parsed as *(pbeans[i]), which would attempt to dereference the result of pbeans[i], which is not what we want.

The type of the expression (*pbeans)[i] is double, so it follows that the declaration of pbeans is

double (*pbeans)[4];

The decalration is read as

         pbeans         -- pbeans
       (*pbeans)        -- is a pointer to
       (*pbeans)[4]     -- a 4-element array
double (*pbeans)[4];    -- of double.

Upvotes: 1

LearningC
LearningC

Reputation: 3162

1) It is the format in C to declare arrays using,

datatype name[][];

format. here

double (*pbeans) [4] =beans;

means pbeans is a array of 4 items and where each item is of type double * i.e. a pointer to double.

2)you read 

const int * const pi

from right to left as pi is a constant pointer to a integer constant

Upvotes: 0

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