IPC144 - Introduction to Programming Using C  

Lecture 8 - Arrays

References:

Chapter 7. Arrays

• an array is a variable that can store multiple values of the same type
• each value in the array is accessed by specifying its position, the first element of the array being at position 0, the second element has position 1, etc.
• when an array is declared, the number of elements must be specified with a constant as follows:

        int x[10];

• defines the array x with 10 elements, x[0] to x[9], and this is how they are referenced
• examples of use:

        x[0] = 6;
        x[7] = 23;
        x[2] = x[0] + x[7];

• the position of an element within an array, or its index, is an int value which can be a variable
• sample program (read 5 numbers and print in reverse order):

        #include <stdio.h>
        main() {
           int nums[5], i;
           printf("Enter %d numbers: ", 5);
           for (i = 0; i < 5; i++)
              scanf("%d", &nums[i]);
           printf("Thank you.\n");
           printf("In reverse order, those numbers are:\n");
           for (i = 5 - 1; i >= 0; i--)
              printf("%d", nums[i]);
           printf("\n");
        }

• sample program (same program, using a constant to define the array size):

        #include <stdio.h>
        #define SIZE 5
        main() {
           int nums[SIZE], i;
           printf("Enter %d numbers: ", SIZE);
           for (i = 0; i < SIZE; i++)
              scanf("%d", &nums[i]);
           printf("Thank you.\n");
           printf("In reverse order, those numbers are:\n");
           for (i = SIZE - 1; i >= 0; i--)
              printf("%d", nums[i]);
           printf("\n");
        }

• don't let an index go "out of range", such as <0 or >4 for the array "nums" above, or you will be addressing memory locations that are not within the array itself

Passing arrays to functions

• when passing an array as an argument to a function, the array name is used without the square brackets
• the array name without the square brackets is actually a pointer to the array, so the function can change the original array
• a function declares an array received as an argument to be an array, but doesn't specify the size in the parameter list
• sample program (modular version of previous program):

        #include <stdio.h>
        #define SIZE 5
        void fill(int x[]);
        void reverse(int x[]);
        main() {
           int nums[SIZE];
           printf("Enter %d numbers: ", SIZE);
           fill(nums);
           printf("Thank you.\n");
           printf("In reverse order, those numbers are:\n");
           reverse(nums);
        }
        void fill(int x[]) {
           int i;
           for (i = 0; i < SIZE; i++)
              scanf("%d", &x[i]);
        }
        void reverse(int x[]) {
           int i;
           for (i = SIZE - 1; i >= 0; i--)
              printf("%d", x[i]);
           printf("\n");
        }

• sample program (read a line of characters, print in reverse order):

        #include <stdio.h>
        #define MAX 80
        void backwards(char line[], int size);
        int getline(char line[]);
        main() {
           char input[80];
           int numchars;
           printf("Enter a line to be reversed\n");
           numchars = getline(input);
           printf("See if you can say this:\n");
           backwards(input, numchars);
        }
        /* displays "size" characters from "line" in reverse order */
        void backwards(char line[], int size) {
           int i;
           for (i = size - 1; i >= 0; i--)
              printf("%c", line[i]);
           printf("\n");
        }
        /* Gets up to MAX characters from the user and stores them
        * in "line", returning the actual number of characters entered.
        * The newline at the end of the line is discarded, as are
        * any characters in excess of MAX on the input line.
        */
        int getline(char line[]) {
           int n = 0;
           char c;
           while ('\n' != (c = getchar()))
              if (n < MAX)
                 line[n++] = c;
           return n;
        }

• note that the above program specifies an "input" size of 80, which would cause problems if MAX were set larger than 80
• this is a common error, MAX should be used as the array size

Initializing arrays

• when declared, multiple elements of an array can be initialized
• for example:

        int x[5] = { 10, -2, 3, 45, 6 };

• is equivalent to:

        int x[5];
        x[0] = 10;
        x[1] = -2;
        x[2] = 3;
        x[3] = 45;
        x[4] = 6;

• if there are not enough values specified for the entire array, remaining elements are set to zero
• to initialize an entire array to zero:

        int x[5] = { 0 };

• if there are too many values specified, excess values are ignored
• note that if an array is not initialized, array elements are undefined; they are not automatically initialized to zero

Character strings

• a character string is a character array with a null character ('\0') placed after the last character of data
• functions in <string.h>, and some functions in <stdio.h> are designed to work with strings
• if the null character may be a valid data element, then character strings won't work, so a separate counter must be maintained as in our previous examples - the string functions can not be used in this case
• an array must be declared with one element more than the largest string desired, since the terminating null character will occupy one element
• for example, the string "Hello" is stored as the character array 'H', 'e', 'l', 'l', 'o', '\0'