So far e know how to write a simple program to display information typed in by you, the programmer, and how to describe your program with comments. That’s great, but what about interacting with your user? Fortunately, it is also possible for your program to accept input. The function you use is known as cin, and is followed by the insertion operator >>.
Of course, before you try to receive input, you must have a place to store that input. In programming, input and data are stored in variables. There are several different types of variables which store different kinds of information (e.g. numbers versus letters); when you tell the compiler you are declaring a variable, you must include the data type along with the name of the variable. Several basic types include char, int, and float.
A variable of type char stores a single character, variables of type int store integers (numbers without decimal places), and variables of type float store numbers with decimal places. Each of these variable types – char, int, and float – is each a keyword that you use when you declare a variable.
What are variable types?
Sometimes it can be confusing to have multiple variable types when it seems like some variable types are redundant (why have integer numbers when you have floats?). Using the right variable type can be important for making your code readable and for efficiency–some variables require more memory than others. Moreover, because of the way the numbers are actually stored in memory, a float is “inexact”, and should not be used when you need to store an “exact” integer value.
Declaring Variables in C++
To declare a variable you use the syntax “type <name>;”. Here are some variable declaration examples:
int x; char letter; float the_float;
It is permissible to declare multiple variables of the same type on the same line; each one should be separated by a comma.
int a, b, c, d;
If you were watching closely, you might have seen that declaration of a variable is always followed by a semicolon (note that this is the same procedure used when you call a function).
Common Errors when Declaring Variables in C++
If you attempt to use a variable that you have not declared, your program will not be compiled or run, and you will receive an error message informing you that you have made a mistake. Usually, this is called an undeclared variable
Now is a good time to talk about an important concept that can easily throw you off: case sensitivity. Basically, in C++, whether you use uppercase or lowercase letters matters. The words Cat and cat mean different things to the compiler. In C++, all language keywords, all functions and all variables are case sensitive. A difference in case between your variable declaration and the use of the variable is one reason you might get an undeclared variable error.