Declaring Variables in C++

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Programming - C/C++

Thursday, 09 July 2009 00:07

Declaring Variables in C++

To declare a variable you use the syntax "type ;". 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.

Case Sensitivity

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.

Using Variables

Ok, so you now know how to tell the compiler about variables, but what about using them?

Here is a sample program demonstrating the use of a variable:

 
#include 

using namespace std;

int main()
{
  int thisisanumber;

  cout<<"Please enter a number: ";
  cin>> thisisanumber;
  cin.ignore();
  cout<<"You entered: "<< thisisanumber <<"\n";
  cin.get();
}

Let's break apart this program and examine it line by line. The keyword int declares thisisanumber to be an integer. The function cin>> reads a value into thisisanumber; the user must press enter before the number is read by the program. cin.ignore() is another function that reads and discards a character. Remember that when you type input into a program, it takes the enter key too. We don't need this, so we throw it away. Keep in mind that the variable was declared an integer; if the user attempts to type in a decimal number, it will be truncated (that is, the decimal component of the number will be ignored). Try typing in a sequence of characters or a decimal number when you run the example program; the response will vary from input to input, but in no case is it particularly pretty. Notice that when printing out a variable quotation marks are not used. Were there quotation marks, the output would be "You Entered: thisisanumber." The lack of quotation marks informs the compiler that there is a variable, and therefore that the program should check the value of the variable in order to replace the variable name with the variable when executing the output function. Do not be confused by the inclusion of two separate insertion operators on one line. Including multiple insertion operators on one line is perfectly acceptable and all of the output will go to the same place. In fact, you must separate string literals (strings enclosed in quotation marks) and variables by giving each its own insertion operators (<<). Trying to put two variables together with only one << will give you an error message, do not try it. Do not forget to end functions and declarations with a semicolon. If you forget the semicolon, the compiler will give you an error message when you attempt to compile the program.

Changing and Comparing Variables

Of course, no matter what type you use, variables are uninteresting without the ability to modify them. Several operators used with variables include the following: *, -, +, /, =, ==, >, <. The * multiplies, the - subtracts, and the + adds. It is of course important to realize that to modify the value of a variable inside the program it is rather important to use the equal sign. In some languages, the equal sign compares the value of the left and right values, but in C++ == is used for that task. The equal sign is still extremely useful. It sets the left input to the equal sign, which must be one, and only one, variable equal to the value on the right side of the equal sign. The operators that perform mathematical functions should be used on the right side of an equal sign in order to assign the result to a variable on the left side.

Here are a few examples:

 
a = 4 * 6; // (Note use of comments and of semicolon) a is 24
a = a + 5; // a equals the original value of a with five added to it
a == 5     // Does NOT assign five to a. Rather, it checks to see if a equals 5.

The other form of equal, ==, is not a way to assign a value to a variable. Rather, it checks to see if the variables are equal. It is useful in other areas of C++; for example, you will often use == in such constructions as conditional statements and loops. You can probably guess how function. They are greater than and less than operators.

For example:

 
a < 5  // Checks to see if a is less than five
a > 5  // Checks to see if a is greater than five
a == 5 // Checks to see if a equals five, for good measure

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Last Updated on Thursday, 09 July 2009 07:48

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