5.11.4. Passing Arrays to Functions

To pass an array as a parameter to a function, pass it as a pointer (since it is a pointer). For example, the following procedure sets the first n cells of array A to 0.

  void zero(int* A, int n)
  {
    for(int k = 0; k < n; k++)
    {
      A[k] = 0;
    }
  }

  int B[100];
  zero(B, 100);

No implicit copying

Since an array is passed as a pointer, the array's memory is not copied. The function uses the memory of the same array that is passed to it, and can change what is in that memory.

Passing an array by reference

Because arrays are already pointers, there is usually no reason to pass an array explicitly by reference. For example, parameter A of procedure zero above has type int*, not int*&. The only reason for passing an array explicitly by reference is so that you can change the pointer to point to a different array.

Constant arrays

If a function only looks at the contents of an array, and does not change what is in the array, you usually indicate that by adding const to the parameter. For example, the following function firstPositive does not change what is in array A. // firstPositive(A,n) returns the first of // A[0], ..., A[n-1] that is positive. If // none of them is positive, then it returns 0. int firstPositive(const int* A, int n) { for(int i = 0; i < n; i++) { if(A[i] > 0) { return A[i]; } } return 0; }

Passing the array size

Recall that you cannot ask an array how big it is. An array is only a pointer to the beginning of the array, and information about how many items are in the array is not available from that. Typically, you pass the size of an array along with the array. Functions zero and firstPositive, above, pass array A along with its size n. When you do pass the size, it is important to understand whether you are passing the logical size or the physical size of the array. Most often, you find that you want to pass the logical size. Sometimes you want to pass both sizes, as separate parameters.

A notational issue

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In order to accommodate programmers who understand arrays but do not understand pointers, C++ has a notation for array parameters. Function heading

  void zero(int A[], int n)

  void zero(int* A, int n)

Exercises

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1. Write a definition of function sum(A,n) which takes an array if ints A and its logical size n and returns the sum of all of the integers in the array. Include a contract. Answer

2. Write a definition of procedure cpy(A, B, n) that copies B[0, ..., n−1] into A[0, ..., n−1]. Make A and B each be arrays of ints. Include a contract. Answer

3. Write a definition of function copyPositives(A, B, n), which copies all of the positive numbers in array B into array A, where B has logical size n.

   The values put in A should be in the same order as those in B, but if there are k positive numbers in array B then they must be put into A[0,...,k−1]. copyPositives should return the number k of values stored in array A.

   Make arrays A and B be arrays of ints. Include a contract.

   Answer

4. Write a definition of procedure reverse(A, n), which reverses the order of the values in A[0, ..., n−1]. Make A be an array of longs. For example, if n = 3 and A contains

     A[0] = 10
     A[1] = 20
     A[2] = 30
   

   before calling reverse(A,3), then A contains

     A[0] = 30
     A[1] = 20
     A[2] = 10
   

   after calling reverse(A,3). Answer

5. You are given a collection of rectangular buildings as seen from a particular vantage point. Your goal is to construct the skyline as seen from that vantage point.

   There are 101 horizontal positions numbered 0 to 100 that can be horizontal building boundaries. Each building profile has a three features: the position of its left edge, the position of its right edge and its height.

   The goal is to write out the height of the skyline for each length 1 horizontal position. The first number gives the skyline height from position 0 to position 1, the second is for the segment from position 1 to position 2, etc. The last height is for the skyline from 99 to 100. If there is no building at a given position then the skyline height is 0. Write one number per line.

   The input is a sequence of lines of the form

   L R H
   

   where L, R and H are integers representing the left-side position, the right-side position and the height of a building. The input ends on a line that only contains 0.

   Hints.

   1. Store an array, skyline, where skyline[i] holds the height of the skyline from position i to position i+1.

   2. Write a function to install a single building given by features L, R and H into the skyline. Think this out. The skyline height at a particular place is the height of the tallest building that occupies that place.

   3. Write a function to read all f the buildings and install them into the skyline.

   4. Write a function to write out the skyline.

   Answer