Lab: Multi-Dimensional Arrays

Summary: In today's laboratory, we explore issues pertaining to mutli-dimensional arrays.

Contents:

• Preparation
• Exercises
  • Exercise 1: Initializing Multi-Dimensional Arrays
  • Exercise 2: Multi-Dimensional Arrays as Single-Dimensional Arrays
  • Exercise 3: Bounds Violations in Multi-Dimensional Arrays
  • Exercise 4: Printing Cells, Revisited
  • Exercise 5: Declarations, Revisited
  • Exercise 6: Partial Specifications
  • Exercise 7: Three-Dimensional Arrays
• For Those with Extra Time
  • Extra 1: Printing Arrays
  • Extra 2: Printing Arrays, Revisited

Preparation

a. Create a directory for this lab.

b. Add the standard Makefile to that directory. (Note that the -Wall flag will give you warnings for some of the code we provide. You can safely ignore some of the warnings.) Now that you know how to use the debugger, you might want to add -g to CFLAGS.

Exercises

Exercise 1: Initializing Multi-Dimensional Arrays

Here is a sample initialization of a one-dimensional array of integers.

int ant[5] = { 5, 2, 7, 3, 4 };

Figure out how to initialize the two-dimensional array bat so that the row zero contains 8, 16, 32, and 64; row one contains 5, 7, 9, and 11; and row two contains 0, 1, 2, 3.

int bat[3][4] = figure-this-out;

You can print out your array with

  printf ("Row 0: %d, %d, %d, %d\n", 
          bat[0][0], bat[0][1], bat[0][2], bat[0][3]);
  printf ("Row 1: %d, %d, %d, %d\n", 
          bat[1][0], bat[1][1], bat[1][2], bat[1][3]);
  printf ("Row 2: %d, %d, %d, %d\n", 
          bat[2][0], bat[2][1], bat[2][2], bat[2][3]);

Exercise 2: Multi-Dimensional Arrays as Single-Dimensional Arrays

Suppose we've declared bat as above and cow and i as follows:

int *cow;
int i;

a. What do you expect the effect of the following code to be?

  cow = (int *) bat;
  for (i = 0; i < 12; i++) {
    printf("cow[%d]: %d\n", i, cow[i]);
  } // for 

b. Verify your anwer experimentally.

c. Consider why we might have gotten these results.

Exercise 3: Bounds Violations in Multi-Dimensional Arrays

Consider again the declaration of bat above.

a. What values do you expect to get for the following?

  printf ("bat[0][4] = %d\n", bat[0][4]);
  printf ("bat[0][7] = %d\n", bat[0][7]);
  printf ("bat[1][7] = %d\n", bat[1][7]);
  printf ("bat[2][4] = %d\n", bat[2][4]);
  printf ("bat[2][-1] = %d\n", bat[2][-1]);

b. Check your answer experimentally.

c. Consider why you got those results.

Exercise 4: Printing Cells, Revisited

You may have noted a bit of repetition in the code for the previous problem. As you might expect, that repetition is dangerous, because we might mistype something, as in

  printf ("bat[2][4] = %d\n", bat[2][5]);

How do we avoid that problem? Ideally, we'd use a function. However, functions in C can't treat something as both a string (e.g., "bat[2][4]") and a value (e.g., bat[2][4]). Fortunately, macros can.

Consider the following macro

#define PRINT_INT_CELL(CELL) printf ("%s: %d\n", #CELL, CELL);

a. Add that macro to your program from the previous exercise.

b. Replace the first call to printf with a call to PRINT_INT_CELL.

  PRINT_INT_CELL (bat[0][4]);

c. Using cc -E (run the prEprocessor), find out what PRINT_INT_CELL expands to.

d. Replace all of the calls to printf with calls to PRINT_INT_CELL. Then compile your program and make sure that it works in the same way.

Exercise 5: Declarations, Revisited

Consider the following code.

  int rabbit[2][3] = { 1, 2, 3, 4, 5, 6 };
  int r, c;

  for (r = 0; r < 2; r++)
    for (c = 0; c < 3; c++)
      printf ("rabbit[%d,%d] = %d\n", r, c, rabbit[r][c]);

a. What do you expect to happen when you try to compile this code?

b. Check your answer experimentally.

c. What do you expect to happen when you try to run this code?

d. Check your answer experimentally.

Exercise 6: Partial Specifications

We have been specifying our two-dimensional arrays by giving both the number of rows and the number of columns. However, K&R suggest that one can elide the number of rows. For example, consider the following example from section 5.9.

char aname[][15] = { "Illegal month", "Jan", "Feb", "Mar" };

a. Verify that our compiler permits the declaration with neither errors nor warnings.

b. What do you expect the results of the following to be?

#define PRINT_CHAR_CELL(CELL) printf ("%s = '%c' (%d)\n", #CELL, CELL, CELL);
// ...
  PRINT_CHAR_CELL(aname[0][0]);
  PRINT_CHAR_CELL(aname[0][1]);
  PRINT_CHAR_CELL(aname[0][2]);
  PRINT_CHAR_CELL(aname[1][0]);
  PRINT_CHAR_CELL(aname[2][0]);

c. Check your answer experimentally.

d. What do you expect the size of aname to be?

e. Check your answer experimentally.

f. What do you expect the results of the following to be?

  PRINT_CHAR_CELL(aname[1][0]);
  PRINT_CHAR_CELL(aname[1][1]);
  PRINT_CHAR_CELL(aname[1][2]);
  PRINT_CHAR_CELL(aname[1][3]);

g. Check your answer experimentally.

h. What do your results suggest about what has happened in the declaration?

Exercise 7: Three-Dimensional Arrays

Consider the following declaration of a three-dimensional array:

int chinchilla[2][3][4];

a. How many elements does chinchilla have?

b. Check your answer with sizeof.

c. Can one initialize chincilla while declaring it?

d. Check your answer experimentally.

e. Where in memory is chinchilla[i][j][k] relative to chinchilla[0][0][0]?

f. How would you verify your previous answer?

For Those with Extra Time

Extra 1: Printing Arrays

a. Suppose a is a NxM array. Write instructions for printing a as a grid. For example, for rabbit above, we might print

  1  2  3
  4  5  6

Note: You are simply writing code, not a function. (Functions are hard to write for this because you need to specify the size in the parameter.)

b. Rewrite your code to work with rabbit.

c. Rewrite your code to work with bat.

Extra 2: Printing Arrays, Revisited

Write a function, printIntMatrix (int rows, int cols, int matrix[rows][cols]), that prints a matrix as in the previous exercise.