C++ tutorial: if-then

Make simple decisions using 'if...'
The simplest branching operation in C/C++ is the 'if' statement. This allows the program to execute a particular set of statements if some condition is true. The general form of an 'if' statement is:

if (condition)
     {
     statement1;
     statement2;
     // ...more statements
     }

The statements to be executed if the condition is true are grouped together in the curly braces. If there is only one statement, you don't need the curly braces; the general form of the branch is then very simple:

if (condition)
     statement;

Note that there is no need for the word 'then' as there is in some other languages.

The 'condition' is any test that might be true or false. Here are some examples:

if ( x < 0 )
     {
     //Do this section if 'x' is less than zero
     }

if ( MenuSelection == Exit )
     {
     //User selected 'Exit' from the menu
     }

if ( cin.good() )
     {
     //Last input operation was a success...
     }

For more information about the types of comparison that can be made, see 'Operators' in the reference section.

In C/C++, '==' means 'is equal to?'
The symbol '=' (equals) is used to assign values to a variable, as descibed previously. To represent the notion of testing whether one thing is equal to another, a different symbol is used: '=='. This always struck me as a bit odd; after all, the designers of C did not shy away from using the same symbol for different jobs anywhere else. The asterisk (*), for example, has about half a dozen different meanings. In any event, this is something you will have to get used to. I have been programming in C and C++ for thirteen years, and I still regularly get it wrong. Surprisingly, if you want to set 'x' equal to, say, '100', and you write:

     x == 100;

this is not an error! In fact, this line of program does absolutely nothing!

In C/C++, '!=' means 'is not equal to?'

if (x != y) // ...

means: execute the following statement if 'x' is not equal to 'y'.

In C/C++, '0' is the same as 'false'
C/C++ has only a loose idea of 'true' and 'false'. In general, any number other than zero is equivalent to 'true'. This is why you will often see lines like this:

if (x) // ...

meaning: exectute the following statement if x is not zero. Personally, I would not write a line like this; I would always state explicity what is meant:

if (x != 0) // ...

Boolean variables store the results of tests
C++ has a 'boolean' variable that can (ideally) only store the values 'true' or 'false'. The notation for this is shown in the example below:

     bool Ready = false;
     // ...
     Ready = true;
     // ...
     if (Ready == true)
          {
          // ...
          }

In the first line the variable 'Ready' is set to 'false'. Later in the program the variable is used in an 'if' statement. It would equally correct to write

     if (Ready)
          {
          // ...
          }

In practice boolean variables are not that widely used, as integer variables can be used in exactly the same way, and many programmers got used to doing this in C, which does not have a boolean data type.