Chapter 7: A + B = C 79

Yes! You’re very observant. This type of int is the same one used to declare the main() function in every program you have written in this book — if you have been reading the chapters in order. Without getting too far ahead, you should now recognize that main() is known as an “integer function.” It also ties into the 0 value in the return statement, but I tell you more about that in a few chapters.

Computer geeks worldwide want you to know that an integer ranges from –32,768 up to 0 and then up to 32,767 only on personal computers. If, per­ chance, you ever program on a large, antique computer — doomed to ever-dwindling possibilities of employment, like those losers who pro­ gram them — you may discover that the range for integers on those computers is somewhat different. Yeah, this information is completely optional; no need cluttering your head with it. But they would whine if I didn’t put it in here.

Using an integer variable in the Methuselah program

If you need only small, whole-number values in a C program, you should use integer variables. As an example, the following program uses the variable age to keep track of someone’s age. Other examples of using integer variables are to store the number of times something happens (as long as it doesn’t happen more than 32,000-odd times), planets in the solar system (still 9), corrupt congressmen (always less than 524), and number of people who have seen Lenin in person (getting smaller every day). Think “whole numbers, not big.”

The following program displays the age of the Biblical patriarch Methuselah, an ancestor of Noah, who supposedly lived to be 969 years old — well beyond geezerhood. The program is METHUS1.C, from Methus, which was his nickname:

#include <stdio.h>

int main()

{

int age;

age=969;

printf(“Methuselah was %d years old.\n”,age); return(0);

}

Enter the text from METHUS1.C into your editor. Save the file to disk as METHUS1.C.

80	Part II: Run and Scream from Variables and Math

Compile the program. If you get any errors, reedit the source code and make sure that everything matches the preceding listing. Recompile.

Run the program and you see the following:

Methuselah was 969 years old.

The variable age was assigned the value 969. Then, the printf() statement was used, along with the %d placeholder, to display that value in a string.

The fifth line creates the age variable, used to store an integer value.

The seventh line assigns the value 969 to the age variable by using the equal sign (=). The variable age comes first, and then the equal sign, and then the value (969) to be placed in the age variable.

In the eighth line, the printf function is used to display the value of the age variable. In printf()’s formatting string, the %d conversion charac­ ter is used as a placeholder for an integer value. %d works for integers, just as %s is a placeholder for a string.

Assigning values to numeric variables

One thing worth noting in the METHUS1 program is that numeric variables are assigned values by using the equal sign (=). The variable goes on the left, and then the equal sign, and then the “thing” that produces the value on the right. That’s the way it is, was, and shall be in the C language:

var=value;

var is the name of the numeric variable. value is the value assigned to that variable. Read it as “The value of the variable var is equal to the value value.” (I know, too many values in that sentence. So shoot me.)

What could value be? It can be a number, a mathematical equation, a C lan­ guage function that generates a value, or another variable, in which case var has that variable’s same value. Anything that pops out a value — an integer value, in this case — is acceptable.

In METHUS1.C, the value for the variable age is assigned directly:

age=969;

Lo, the value 969 is safely stuffed into the age variable.

Chapter 7: A + B = C 81

The equal sign is used to assign a non-string value to a variable. The variable goes on the left side of the equal sign and gets its value from whatever’s on the right side.

String variables cannot be defined in this way, by using an equal sign. You cannot say

kitty=”Koshka”;

It just doesn’t work! Strings can be read into variables from the keyboard by using the scanf(), gets(), or other C language keyboard-reading functions. String variables can also be preset, but you cannot use an equal sign with them, like you can with numeric variables!

Entering numeric values from the keyboard

Keep the METHUS1.C program warm in your editor’s oven for a few seconds. What does it really do? Nothing. Because the value 969 is already in the pro­ gram, there’s no surprise. The real fun with numbers comes when they’re entered from the keyboard. Who knows what wacky value the user may enter? (That’s another reason for a variable.)

A small problem arises in reading a value from the keyboard: Only strings are read from the keyboard; the scanf() and gets() functions you’re familiar with have been used to read string variables. And, there’s most definitely a dif­ ference between the characters “969” and the number 969. One is a value, and the other is a string. (I leave it up to you to figure out which is which.) The object is to covertly transform the string “969” into a value — nay, an integer value — of 969. The secret command to do it is atoi, the A-to-I function.

The atoi() function

The atoi() (pronounced “A-to-I”) function converts numbers at the begin­ ning of a string into an integer value. The A comes from the acronym ASCII, which is a coding scheme that assigns secret code numbers to characters. So atoi means “convert an ASCII (text) string into an integer value.” That’s how you can read integers from the keyboard. Here’s the format:

var=atoi(string);

var is the name of a numeric variable, an integer variable created by the int keyword. That’s followed by an equal sign, which is how you assign a value to a variable.

82	Part II: Run and Scream from Variables and Math

On the difference between numbers and strings, if you dare to care

You have to know when a number in C is a value and when it’s a string. A numeric value is what you find lurking in a numeric variable. This book calls those things values, and not numbers. A value is 5 apples, 3.141 (for example), the national debt, and the number of pounds you can lose on celebrity diets featured in this week’s Star. Those are values.

Numbers are what appear in strings of text. When you type 255, for example, you’re enter­ ing a string. Those are the characters 2, 5, and 5, as found on your keyboard. The string “255” is not a value. I call it a number. By using the

atoi() function in the C language, you can translate it into a value, suitable for storage in a numeric variable.

There are numbers and there are values. Which is which? It depends on how you’re going to use it. Obviously, if someone is entering a phone number, house number, or zip code, it’s probably a string. (My zip code is 94402, but that doesn’t mean that it’s the 94-thousandth-something post office in the United States.) If some­ one enters a dollar amount, percentage, size, or measurement — anything you work with mathematically — it’s probably a value.

The atoi() function follows the equal sign. Then comes the string to con­ vert, hugged by atoi()’s parentheses. The string can be a string variable or a string “constant” enclosed in double quotes. Most often, the string to convert is the name of a string variable, one created by the char keyword and read from the keyboard by using gets() or scanf() or some other keyboardreading function.

The line ends in a semicolon because it’s a complete C language statement.

The atoi function also requires a second number-sign thingy at the begin­ ning of your source code:

#include <stdlib.h>

This line is usually placed below the traditional #include <stdio.h> thing — both of them look the same, in fact, but it’s stdlib.h in the angle pinchers that’s required here. The line does not end with a semicolon.

atoi is not pronounced “a toy.” It’s “A-to-I,” like what you see on the spine of Volume One of a 3-volume encyclopedia.

Numbers are values; strings are composed of characters.

If the string that atoi() converts does not begin with a number, or if the number is too large or too weird to be an integer, atoi spits back the value 0 (zero).