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Type Conversion

When dealing with assignment statements and calculations, data types can sometimes be converted to other data types through something that is called type conversion. Conversions that take place without the programmer specifically asking for it are called implicit conversions. Conversions that are requested by the programmer are called explicit conversions. Programmers run into problems during implicit conversions because they can be unaware of what is actually happening during execution of their program until they examine the code very carfully.

It's pretty clear that programmers can run into problems because of implicit conversions. However, if you fully understand data types and how they are defined for certain arithmetic operators, you should be able to avoid them. For instance, if y and z are float, and a is int, when the statement z = a + y is executed, the value of a will be stored in a temporary memory location as float for use in the rest of the statement. Then, the addition is done (in float form) and finally, the result stored in z. So what happened? [a was implicitly converted into float during the execution of the statement]

Another form of implicit conversion may take place during assignment, but not necessarily from a smaller type to a larger type.

Example:

   int a;
   float b = 3.5;
   float c = 5.0;
   int d = 2;
   a = b * c / d;
   cout << a;        --> 8

What conversion took place in the above example? First of all, d was implicitly converted to 2.0 during b * c / d. The result of that multiplication and division is 8.75, but a was declared int, so a was implicitly converted to 8 during assignment.

While programmers are unaware of implicit conversions, they are completely in control of explicit conversions. Explicit conversions, or typecasting, can come in handy as you will see in later problems. Typecasting can be easily accomplished and it has a easy to use form which is: type( variable ) where type is whatever data type you choose and variable is the variable you are converting. It simply takes the value of variable and converts in into the type specified.

Example 1:

   double d;
   int k;
   d = 12.78;
   k = int(d);        --> k = 12
   d = double(k);        --> d = 12.0

Example 2:

   int m = 10, n = 4, r;
   double q;
   q = m / n;        --> q = 2.0
   r = double(m) / n;        --> 2

   r = m / n;        --> 2
   q = double(m) / n        --> 2.5

Note: q = (double)m / n is equivalent to q = double(m) / n;

Ok, so we have convered assignment statements, arithmitic ops, precedence, associativity, type conversions, but we haven't covered how programmers compare expressions in C++. Read on for more about logical and relational expressions...

Logical Expressions

Logcial expressions, somtimes called boolean expressions, are expressions that return a value of true or false. A ( 1 ) is always associated with a true expression; a ( 0 ) is always associated with a false expression. There will be many times when you as a programmer will have to write code to handle certain (true/false) situations. If an expression is true, then you will issue some command. If an expression is false, then you will issue another command. Logical expressions make it possible to test values to see if something is true or false. A relational expression is a special type of logical expression in which some type of comparison is performed. The relational operators are >, <, <=, >=, ==, and !=.

Relational Operators

   > : greater than
   < : less than
   >= : greater than or equal to
   <= : less than or equal to
   == : is equal to
   != : is not equal to

One of the biggest problems many beginning programmers have is distinguishing between = and ==. Experienced programmers can even run into problems with these two operators because it is very easy to mistype them when programming. = is an operator that is only used for assignment purposes; it does not imply equality. == (is equal to) is an relational operator that is used to compare two values to see if they are equal. Be careful not to use = in relational expressions when == is needed.

Examples of relational expressions (also logical expressions):

1. a< 2 * b - 7
2. c != -1
3. b > c + 4 * 7

A relational expression is always a logical expression. Logical expressions are either relational expressions or a combination of multiple expressions joined together by the logical operators: &&, ||, and !.

Logical Operators

   && : logical and
   || : logical or
   ! : logical not

Example of a logical expression (not a relational expression):

(a < b) || ( b < c)

If a = 5, b = 3, and c = 10, the result of this expression is 1 (true).

A quick way to tell if an expression is logical but not relational is if one of the logical operators is being used.

Common Errors:

1. Using = (assignment operator) when == is needed.
2. Using an expression such as:
   
    midTerm || final == 'A' which needs to be changed to    midTerm == 'A' || final == 'A'
   
3. Using an expression such as:
   
      a < b < c which needs to be changed to    a < b && b < c
   
4. Using && when || is needed or vice versa:
   
   Suppose a variable x is being used and x = -1 and x = -99 have special meaning:
   
      if (x != -1 || x != -99) is wrong and needs to be changed to    if (x != -1 && x != -99)
   
   In general, the operators have precedence (highest to lowest):
   
      A - arithmetic
      R - relational
      L - logical
   
   - the one exception is logical not ( ! ), which is done before the arithmetic operators.

Specifically, the overall precedence for operators in this tutorial is:

   1 : !, +, - : (unary)
   2 : *, /, %
   3 : +, - : (binary)
   4 : <, <=, >, >=
   5 : ==, !=
   6 : &&
   7 : ||

Practical Use Example:

Suppose an insurance company offers a discount on auto insurance for students who have no prior accidents and a GPA of 3.0 or above. What expression could be used to ask this?

   numAccidents == 0 && gpa >= 3.0

Most compilers also have a capability called short-circuit evaluation when evaluating logical expressions. It makes logical operations faster. If a compound logical expression involves one section that is false and an AND ( && ) operation is used, the entire expression must be false. If one section is true and OR ( || ) is involved, the entire expression must be true.

With expressions out of the way, it's time to see how the flow of programs can be controlled with C++. Read on for more about the different control structures...