Initializing Numeric Variables

When you declare a variable, you instruct the compiler to set aside storage space for the variable. However, the value stored in that space--the value of the variable--isn't defined. It might be zero, or it might be some random "garbage" value. Before using a variable, you should always initialize it to a known value. You can do this independently of the variable declaration by using an assignment statement, as in this example:

int count;   /* Set aside storage space for count */
count = 0;   /* Store 0 in count */

Note that this statement uses the equal sign (=), which is C's assignment operator and is discussed further on Day 4, "Statements, Expressions, and Operators." For now, you need to be aware that the equal sign in programming is not the same as the equal sign in algebra. If you write

x = 12

in an algebraic statement, you are stating a fact: "x equals 12." In C, however, it means something quite different: "Assign the value 12 to the variable named x."
You can also initialize a variable when it's declared. To do so, follow the variable name in the declaration statement with an equal sign and the desired initial value:

int count = 0;
double percent = 0.01, taxrate = 28.5;

Be careful not to initialize a variable with a value outside the allowed range. Here are two examples of out-of-range initializations:

int weight = 100000;
unsigned int value = -2500;

The C compiler doesn't catch such errors. Your program might compile and link, but you might get unexpected results when the program is run.

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DO understand the number of bytes that variable types take for your computer.
DO use typedef to make your programs more readable.

DO initialize variables when you declare them whenever possible.

DON'T use a variable that hasn't been initialized. Results can be unpredictable.

DON'T use a float or double variable if you're only storing integers. Although they will work, using them is inefficient.

DON'T try to put numbers into variable types that are too small to hold them.

DON'T put negative numbers into variables with an unsigned type.