3
Variables and Types

Continuing with the recipe metaphor from the last chapter, sometimes a chef will keep a small blackboard in the kitchen for storing data. For example, when unpacking a turkey, he notices a label that says “14.2 Pounds.” Before he throws the wrapper away, he will scribble “weight = 14.2” on the blackboard. Then, just before he puts the turkey in the oven, he will calculate the cooking time (15 minutes + 15 minutes per pound) by referring to the weight on the blackboard.

Figure 3.1  Keeping track of data with a blackboard

Keeping track of data with a blackboard

During execution, a program often needs places to store data that will be used later. A place where one piece of data can go is known as a variable. Each variable has a name (like cookingTime) and a type (like a number). In addition, when the program executes, the variable will have a value (like 228.0).

Types

In a program, you create a new variable by declaring its type and name. Here’s an example of a variable declaration: 

 ​ ​ ​ ​f​l​o​a​t​ ​w​e​i​g​h​t​;​

The type of this variable is float, and its name is weight. At this point, the variable doesn’t have a value.

In C, you must declare the type of each variable for two reasons:

  • The type lets the compiler check your work for you and alert you to possible mistakes or problems. For instance, say you have a variable of a type that holds text. If you ask for its logarithm, the compiler will tell you something like “It doesn’t make any sense to ask for this variable’s logarithm.”

  • The type tells the compiler how much space in memory (how many bytes) to reserve for that variable.

Here is an overview of the commonly used types. We will return in more detail to each type in later chapters.

short, int, long

These three types are whole numbers; they don’t require a decimal point. A short usually has fewer bytes of storage than a long, and int is in between. Thus, you can store a much larger number in a long than in a short.

float, double

A float is a floating point number – a number that can have a decimal point. In memory, a float is stored as a mantissa and an exponent. For example, 346.2 is represented as 3.462 x 102 A double is a double-precision number, which typically has more bits to hold a longer mantissa and larger exponents.

char

A char is a one-byte integer that we usually treat as a character, like the letter 'a'.

pointers

A pointer holds a memory address. It is declared using the asterisk character. For example, a variable declared as int * can hold a memory address where an int is stored. It doesn’t hold the actual number’s value, but if you know the address of the int then you can easily get to its value. Pointers are very useful, and there will be more on pointers later. Much more.

struct

A struct (or structure) is a type made up of other types. You can also create new struct definitions. For example, imagine that you wanted a GeoLocation type that contains two float members: latitude and longitude. In this case, you would define a struct type.

These are the types that a C programmer uses every day. It is quite astonishing what complex ideas can be captured in these five simple ideas.

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