Sunday, 3 March 2013

Looping Statements

The for loop
The statements in the for loop repeat continuously for aspecific number of times.  The while and do-while loops repeat until a certain condition is met.  The for loop repeats until a specific count is met.  Use a for loop when the number of repetition is know, or can be supplied by the user.  The coding format is:
for(startExpression; testExpression; countExpression)
{
    block of code;

}
The startExpression is evaluated before the loop begins.  It is acceptable to declare and assign in the startExpression(such as int x = 1;).  This startExpression is evaluated only once at the beginning of the loop.
The testExpression will evaluate to TRUE (nonzero) orFALSE (zero).  While TRUE, the body of the loop repeats.  When the testExpression becomes FALSE, the looping stops and the program continues with the statement immediately following the for loop body in the program code.
The countExpression executes after each trip through the loop.  The count may increase/decrease by an increment of 1 or of some other value. 
Braces are not required if the body of the for loop consists of only ONE statement.  Please indent the body of the loop for readability.
CAREFUL:  When a for loop terminates, the value stored in the computer's memory under the looping variable will be "beyond" the testExpression in the loop.  It must be sufficiently large (or small) to cause a false condition.  Consider:
for (x = 0; x <= 13; x++)
     cout<<"Melody";
When this loop is finished, the value 14 is stored in x.

Nested if and if-else statements


The if-else statement allows a choice to be made between two possible alternatives. Sometimes a choice must be made between more than two possibilities. For example the sign function in mathematics returns -1 if the argument is less than zero, returns +1 if the argument is greater than zero and returns zero if the argument is zero. The following C++ statement implements this function:
if (x < 0)
   sign = -1;
else
   if (x == 0)
      sign = 0;
   else
      sign = 1;
This is an if-else statement in which the statement following the else is itself an if-else statement. If x is less than zero then sign is set to -1, however if it is not less than zero the statement following the else is executed. In that case if x is equal to zero then sign is set to zero and otherwise it is set to 1.
Novice programmers often use a sequence of if statements rather than use a nested if-else statement. That is they write the above in the logically equivalent form:
if (x < 0)
   sign = -1;
if (x == 0)
   sign = 0;
if (x > 0)
   sign = 1;
This version is not recommended since it does not make it clear that only one of the assignment statements will be executed for a given value of x. Also it is inefficient since all three conditions are always tested.
If nesting is carried out to too deep a level and indenting is not consistent then deeply nested if or if-else statements can be confusing to read and interpret. It is important to note that an else always belongs to the closest if without an else.
When writing nested if-else statements to choose between several alternatives use some consistent layout such as the following:
if ( condition1 )
statement1 ;
else if ( condition2 )
statement2 ;
. . .
else if ( condition-n )
statement-n ;
else
statement-e ;
Assume that a real variable x is known to be greater than or equal to zero and less than one. The following multiple choice decision increments count1 if 0 $\leq$ x $<$0.25, increments count2 if 0.25 $\leq$ x $<$ 0.5, increments count3 if 0.5 $\leq$ x $<$ 0.75 and increments count4 if 0.75 $\leq$ x $<$ 1.
if (x < 0.25)
  count1++;
else if (x < 0.5)
  count2++;
else if (x < 0.75)
  count3++;
else
  count4++;
Note how the ordering of the tests here has allowed the simplification of the conditions. For example when checking that x lies between 0.25 and 0.50 the test x < 0.50 is only carried out if the test x < 0.25 has already failed hence x is greater than 0.25. This shows that if x is less than 0.50 then x must be between 0.25 and 0.5.
Compare the above with the following clumsy version using more complex conditions:
if (x < 0.25)
  count1++;
else if (x >= 0.25 && x < 0.5)
  count2++;
else if (x >= 0.5 && x < 0.75)
  count3++;
else
  count4++;