22A. Arrays

Review

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A program needs to be able to use large chunks of memory, and arrays are a mechanism to support that.

We have seen the basics of arrays. An array is a numbered sequence of variables; variable numbers are called indices (singular: index). Expression A[i] indicates the variable in array A with index i.

Remember that the indices start at 0. If array A contains 4 variables, then they are A[0], A[1], A[2] and A[3]. The last index in an array of size n is n−1.

Arrays

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Now we begin to look at arrays in more detail. C++ has an unusual way of treating arrays. It separates the issue of allocating an array from the issue of using an array.

Chunks

It is helpful to have a term for a piece of memory, possibly small or possibly quite large. Let's call it a chunk. A chunk has a starting address and a size. A chunk of integers might contain memory for 1 integer (4 bytes, for 32-bit integers) or for 1000 integers (4000 bytes).

An array is a pointer

In C++, an array is not a chunk. The definition of an array is:

So, not only is an array not a chunk, it is not even a pointer to a chunk. It is a pointer to the first variable in a chunk. If A is an array, and you have allocated a chunk for it, then you get something like this:

For example, an array of integers is a pointer to the first integer variable in a chunk of some unspecified number of integers. Since a pointer to an integer has type int*, an array of integers must also have type int*. An array of real numbers has type double*, an array of characters has type char*, etc.

You can think of * as meaning array, as long as you remember that the "array" sometimes contains only one variable. There is no way for you to know just from type int* whether a pointer points to a single variable or to a large chunk. The presence or absence of other things after the first one in the memory is considered an allocation issue.

A pointer is an array

Since an array is a pointer, you can store that pointer into any pointer variable of the correct pointer type. An array of integers has type int*, so

  int  A[10];
  int* p = A;
  p[0] = 0;

Exercises

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1. How do you refer to the variable at index k in array W ? Answer

2. If array B is created by

     double B[4];
   

   what are the allowed indices for B? Answer

3. Write statements that create an array of 100 ints and store 0 into each variable in the array. Answer

4. Is the following allowed?

     long  B[20];
     long* p = B;
   

   Answer

5. Suppose that p has type int*. Is the following allowed?

     int k = p[4];
   

   Answer