CS256 - Principles of Structured Design Fall 2021

Reading:

• Reading: Chapter 15.1: Multi-dimensional Arrays

• Reading: Chapter 16: Pointers

2 Dimensional Arrays:

A text file can be thought of as a 2D grid of characters (lines x columns) If a single array is linear, i.e. 1 dimensional, then an array of arrays is 2 dimensional (i.e. 2D). An array of array is defined by adding a second [] subscript:

char data[rows][cols];

data = {
  {'.','.',... cols ...,'.'},   ┐
  {'.','.',... cols ...,'.'},   ├── rows
    ...                         │
  {'.','.',... cols ...,'.'}    ┘
}

rows and cols should normally be constant values, however C allows dynamic arrays:

char data[LINES][COLS];

Note: You may not attempt to initialize a dynamic sized array however.

A single character can then be accessed in almost the same way that screen coordinate would be:

data[y][x] = '#'; // places '#' at row y, column x in the 2D data array.

The amount of space required for the array is rows * cols * sizeof(char).

If the last subscript ([x]) is omitted, then data[y] is the string of characters at row y. i.e.:

printf("%s", data[y]);

Reading a text file into a 2D array:

// Define outside of a function:
#define MAXLINES    200
#define MAXCOLS     120

// Inside of a function:
// Open the file (for reading):
FILE *fp = fopen("file.txt", "r");

// Define space for the rows and columns of data:
int lines[MAXLINES][MAXCOLS];
int line = 0;

// Read a line at at time from the file:
while(fgets(lines[line], MAXCOLS, fp) != NULL) {
  line++;
  if (line >= MAXLINES) break; // Stop when there is no more room in the array.
}

Pointers:

A pointer is a variable that points to a storage area rather than defining a storage area itself. Pointers can be incremented to move to the next available storage area in memory as well.

A pointer variable is defined with a leading *:

char *str;       // A string (character) pointer
int *nums;       // An integer pointer
char *strs[10];  // An array of 10 string pointers.
int **x;         // A pointer to a pointer to an integer.

A pointer can be thought of an used like an array, however it does not allocate space for any data by itself:

char s[10];     \ These are not the same, although they can be used in the
char *s;        / same way.
char s1[] = "text";
char *s2  = "text";

In this case s1 allocates space for "text\0" which is copied into it, data in s1 can be modified. s2 points to a read-only version of "text\0", which can be read, but not modified.

To access the second character in s1 or s2, you can use the array index method:

s1[1] == 'e'
s2[1] == 'e'

For s2 however you can use the * operator to "dereference" the pointer and get the data that it points to:

*s2 == 't'         same as *(s2+0) == 't'
*(s2+1) == 'e'
*(s2+2) == 'x'
...

int strlen(char *s)
{
  int len = 0;
  while (*s != '\0') {
    len++;
    s++;          // Move pointer to the next character
  }
  return len;
}