C For Dummies

Lesson 9-1: Asking for Arrays

Array is one of those words that loses its meaning the more you say it. Array. A ray. Air ray. Hairy. Hooray!

An array is nothing more than an organized collection of stuff. Suppose that you're decorating cookies, and, because you're obsessively neat, you have arranged them into two lines: one ready for decorating and another already decorated. Nerdy folk would say that you have two arrays of cookies. C programmers would say that you have an initialized array (decorated cookies) and an uninitialized array (undecorated cookies). Me? I would just eat the cookies and suck the raw frosting from the decorating tube.

What does all this mean? Nothing more than identifying a collection of stuff with the word array.

• An array is a collection of stuff, like a row of cookies or a list of items in a column or a line of cars backed up seven miles on the freeway.
• You can cheerfully ignore the terms initialized and uninitialized for now.
• Arrays open up the heretofore unexplored region of database programming in C. Because an array is essentially a collection of stuff in a database, you can do databasey things with arrays: List them, sort them, unsort them, create bogus databases for the IRS, and so on.
• It's the database manipulation that makes arrays so handy.

The whole deal with arrays

In the C language, an array is a collection of variables, all of the same type. You can have an array of integers, single-character variables, floats, doubles, and so on.

Alas, you can't have an array of strings -- not yet, anyway. You have enough to do with integer arrays to keep you busy for this lesson.

Suppose that a bunch of football jocks are standing in line at the school café. Since you're standing behind them, they'll probably hoard all the good food before you get there. But that's not the point. What you should notice instead are the numbers printed across the back of their jerseys.

Unbeknownst to the football players, integers are printed on their jerseys. (They know that they have numbers on their jerseys, but not all of them know that the numbers are integers.) Standing there, they make up, in a way, an integer array, which you could write like this:

16, 88, 45, 99, 6, 33

Hike!

You could also make an array from their names -- a string array:

Sherman, Fernan, Hastings, Harrison, Best, Hayden

Then you could make an array from the first letter of their girlfriend's names:

S, E, K, K, K, K

(Kelly is very popular.)

Each of these three examples is an array. The examples are a collection of items all of the same type: integers, strings, and characters.

Now for the nerdy terms:

Element: Each item in an array is called an element. The array can have as many elements as it needs (although in your computer the true size is limited by how much memory your PC has). In the preceding example, each of the arrays contains six elements: six jersey numbers, six names, six girlfriend's initials.

(Yes, even though Kelly appears four times in the last array, the K still counts as four elements. Not all the elements in an array have to be different.)

Here's something weird:

Zero: The first element in an array is numbered zero. Normally, if C were the least bit normal, the first element would be one. After all, all humans begin counting things as "One, two, three. . . ." Because C has to be different, though, the first element is zero.

• C says, "Zero, one, two. . . ." Get used to it.
• Please don't run away screaming just yet.
• Arrays contain all the same type of variable: all integers, all characters, all floats -- all the same.
• Each item in an array is an element.
• Not every element has to be different. You can have an array of 52 items that are all the number 12, and it's still an array.
• The first item in an array is number zero. This requirement is very weird and will toss you for a loop more than once during your C programming career.
• Quite a few things in your PC begin numbering at zero. For example, the first memory address in your computer is address zero (written as 0x00000). The first argument typed at the DOS prompt is referred to as argv[0], which is also an array, as described in Lesson 6-3 in C For Dummies®, Volume I. Also gander at the PRINTME1.C program in Lesson 8-4. See how the first printer port in your PC (called LPT1) is really zero, or 0x00? LPT2, the second printer port, is value 0x01. See? I'm not making this stuff up either.

A simple program before you get too bored

An array can hold any series of numbers, such as the numbers in a safe's combination. The following program, SAFE.C, is the same program the manager at my local bank uses to remind herself how to open the Big Safe. (I'm reprinting this program with her permission; as long as you don't know that it's the Bank of America on First Street, she said that it would be okay.)

Carefully type this source code into your editor. The integer array, which is discussed in the following sections, is the only new item in there. For now, just type carefully, and you'll be okay:

Name: SAFE.C

#include <stdio.h>

void main()
{

int numbers[] = { 36, 24, 12 };

printf("The combination for the safe: ");
printf("1. Turn to %i left. ",numbers[0]);
printf("2. Turn to %i right. ",numbers[1]);
printf("3. Turn to %i left, open. ",numbers[2]);

Compile and run. Here's what you should see:

The combination for the safe:
1. Turn to 36 left.
2. Turn to 24 right.
3. Turn to 12 left, open.

The array stored the three numbers, and then various printf statements reached into the array to grab the numbers back out.

• Like any other variable, an array must be declared before you can use it. This subject is covered in the very next section.
• Arrays are declared with the same words as any other variable: int for integer arrays, float for floating-point number arrays, and so on.
• Square brackets are big in Array Land.
• Elements in an array are referred to by the array name and then the element number. This subject is covered later in this lesson, in the section "Referring to elements in an array."

Properly declaring arrays in C

Like everything else in C, you can't pluck an array from thin air. No, as with other types of variables, you must declare it and type it in a proper and cryptic manner along with any other variables you plan to use in your program.

An array in C can be declared in two ways, depending on whether the array is empty or full.

A full array is one that already has information in it, like the numbers array in the SAFE.C program. It's defined like this:

int jock_array[] = {16, 88, 45, 99, 6, 33};

Table 9-1 tells you how each item fits into the big picture. Basically, there are three things: the type of array, int (integer); the array name, jock_array; and the values assigned to that array. The whole thing ends with a semicolon, just as any variable declaration does.

Table 9-1 Items You Need for a Full (Initialized) Array

The other type of array in C is empty, one you create to be filled in later. It's defined this way:

float coworker_IQ[20];

Rather than have numbers or other data assigned to this array, it merely has a size specified; 20 elements, in this example. It means that the coworker_IQ array has room for as many as 20 floating-point numbers.

The following check marks should answer most of the questions buzzing around in your head right now:

• In both cases, the array is declared similarly to any other variable. First comes the variable type and then the variable name. The first array in the preceding example is an integer array named jock_array. The second array is a float array named coworker_IQ.
• A full array is also referred to as an initialized array. It means that all the data -- each element in the array -- is already there.
• An empty array is referred to as an uninitialized array.
• For an initialized array, the values in the array are listed when the array is defined -- just as you can define any variable with a preset value, as explained in Lesson 3-3 in Volume I. Each element appears in curly brackets separated by a comma.
• Love them curly brackets.
• Commas appear after each number in the array, except for the last one. You will forget this statement often, and the compiler may or may not warn you, but your program will definitely screw up.
• The last item in the array does not have a comma by it.
• As always, a variable declaration must end in a semicolon, no matter which type of array you define.
• For an uninitialized (or empty) array, you simply state how many elements you want in the square brackets. In the preceding example, the number of elements in the coworker_IQ array is set equal to 20; that array can have as many as 20 elements in it.
• Technically speaking, when you declare an array like this:

int chairs[12];

you're telling the compiler to set aside space for an array that contains 12 elements of integer number size. It's the same as though you were throwing a dinner party and invited 12 people -- you would need a long table with 12 chairs. (And then pray that no more than 12 people show up and each of them needs only one chair on which to sit.)

• No, you cannot readily change an array's size; you must set the value when you write the program and hope that you set it big enough. This challenge is something C programmers wrestle with every day.
• The square brackets are empty when you declare an initialized array. Fortunately, the compiler is smart enough to figure out the array's size and set it for you automatically. In the following declaration:

char cards[] = {'A', 'J', 'Q', 'K'};

the compiler creates a single-character array named cards that can hold four elements. Then the characters A, J, Q, and K are stuffed into the array.

Referring to elements in an array

Each element in an array is its own variable. It has to be. Otherwise, you would never be able to pluck out any individual item in the array, and then, well, what would be the point?

To refer to any item in an array, you have to know the array's name and the element's number. The array's name is like any other variable name in C. The element number, however, is weird because the first element is zero, not one. When you know those two things, you just write down the array name, stick the element number between the square brackets, and -- presto! -- you have a variable just like any other.

In the SAFE.C program, for example, the first item in the array is referred to as numbers[0]. Even though a funky square bracket is in there, the compiler sees the thing as a single integer variable.

• You can use array items throughout your program just as you would use any variable.
• Assign a value to an array element just as you would assign any variable:

  orbit[1] = 53.449;

  or

  election[2000] = 78123;

  or

  input[7] = getch();

• Even the element number can be a variable:

  loop[x] = counter();

  Here, variable x is used to indicate an element in the loop array, an element that has the value returned by the counter function.

Sticking stuff into an array (initializing it)

Here's a truly meaningless program for you. It creates an empty (uninitialized) array that can hold 20 elements (numbered 0 through 19, natch). Then the program fills those elements with the numbers 1 through 20, then 20 through 1, and then random numbers.

Carefully type this program into your editor. It uses the random-number-generation routines from the program RANDOM4.C in Lesson 5-7 in Volume I. You can cut and paste that chunk of text (the rnd and seedrnd functions). Although the source code seems rather long, it's really the same for loop repeated three times. Cut and paste!

Name: HURRAY.C

#include <stdio.h>
#include <stdlib.h>
#include <time.h> //for seeding randomizer

#define ELEMENTS 20

int rnd(int range);
void seedrnd(void);

void main()

{

int blorf[ELEMENTS];

int x;

/* First pass, fill array with values 1 through 20 */

printf("Initialize array from 1 to 20: ");

for(x=0;x<ELEMENTS;x++)

{

blorf[x] = x+1;

printf("%i\x20",blorf[x]);

}

/* Second pass, fill array with values 20 through 1 */

printf(" And now from 20 to 1: ");

for(x=0;x<ELEMENTS;x++)

{

blorf[x] = 20-x;

printf("%i\x20",blorf[x]);

}

/* Final pass, fill array with random values */

printf(" And now random numbers: ");

for(x=0;x<ELEMENTS;x++)

{

blorf[x] = rnd(20)+1;

printf("%i\x20",blorf[x]);

}

}

int rnd(int range)

{

int r;

r=rand()%range; //spit up random number

return(r);

}

void seedrnd(void)

{

srand((unsigned)time(NULL));

}

Double-check your work. An error is bound to be in there somewhere -- a missing semicolon or comment character.

Compile and run.

Here's the result:

Initialize array from 1 to 20:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
And now from 20 to 1:
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
And now random numbers:
7 11 3 11 17 18 16 16 9 7 5 19 12 20 13 1 13 2 4 8

The array named blorf is filled with three different sets of numbers, each of which can be changed like any other variable. See the previous sidebar, "Soothing words for the harried HURRAY.C program," for more detailed information about what's going on.

• Values can also be placed into an array by reading the information from the keyboard. You can use the scanf or gets functions for that; refer to Lesson 3-1 in Volume I for information about reading numerical values from the keyboard.
• Use gets to read information from the keyboard. More and more, I'm growing to hate the scanf function.
• Keep in mind that values read in from the keyboard must match the type of variable the array holds: ints for integer arrays, floats for floating-point arrays, and so on.
• The #define directive in this program sets the size of the blorf array to 20 elements. That's done so that the array size can be adjusted easily by editing that one line. Refer to Lesson 3-4 in Volume I for information about how handy the #define directive can be.
• The \x20 escape sequence in the various printf statements inserts a blank (a space) between the array numbers. Refer to Table 2-1 in Volume I for more information about escape sequences.

An old but useful chestnut

The following program is a lottery-number picker, which is listed at the end of Volume I. Now that you're familiarizing yourself with arrays, the program should make a little more sense to you. It's listed next, minus the extensive comments from Volume I so that you can peruse the array aspects of the program:

Name: LOTTO.C

#include <stdio.h>
#include <stdlib.h>
#include <time.h> //for the seedrnd() function

#define RANGE 50 //number of numbers
#define BALLS 6 //number of balls to draw
#define DELAY 1000000 //delay interval between picks

int rnd(int range);
void seedrnd(void);

void main()

{

int numbers[RANGE]; //array that holds the balls

int i,b;

unsigned long d; //delay variable

printf("L O T T O P I C K E R ");

seedrnd(); //seed the randomizer

/* initialize the array */

for(i=0;i<RANGE;i++) //initialize the array

numbers[i]=0;

printf("Press Enter to pick this week's numbers:");

getchar();

/* draw the numbers */

printf(" Here they come: ");

for(i=0;i<BALLS;i++)

{

for(d=0;d<=DELAY;d++); //pause here