Until now, we have used IF statements to implement decisions
involving one or two alternatives. In this section, you will see how the
IF statement can be used to implement decisions involving more
than two alternatives.
Example 4.5
The IF statement below has three alternatives. It causes
one of three variables (NumPos, NumNeg, or
NumZero) to be increased by 1 depending on whether X
is greater than 0, less than 0, or equal to 0, respectively. This statement
might be part of a program to keep track of the number of positive, negative,
and zero values in a set of values. It assumes that all the variables have been
properly initialized.
-- Increment NumPos, NumNeg, or NumZero depending on X
IF X > 0 THEN
NumPos := NumPos + 1;
ELSIF X < 0 THEN
NumNeg := NumNeg + 1;
ELSE -- X = 0
NumZero := NumZero + 1;
END IF;
The execution of this IF statement proceeds as
follows: The first condition (X > 0) is tested; if it is true,
the statement NumPos := NumPos+1 increments NumPos by
1 and the rest of the IF statement is skipped. If the first
condition is false, the second condition (X < 0) is tested; if
it is true, NumNeg is incremented; otherwise, NumZero
is incremented. It is important to realize that the second condition is tested
only when the first condition is false.
Figure
4.6 diagrams the execution of this statement. Each condition is shown in a
diamond-shaped box. If a condition is true, its arrow labeled True is followed.
If a condition is false, its arrow labeled False is followed. This diagram
shows that one and only one of the statement sequences in a rectangular box
will be executed. A trace of the IF statement for X =
-7 is shown in Table 4.4.
Figure 4.6
Flow Chart of the IF Statement in Example 4.5
Table 4.4
Trace of IF Statement in Example 4.5 for X = -7
Statement Part X Effect
-7 IF X > 0 THEN -7 > 0 is false ELSIF X < 0 THEN -7 < 0 is true NumNeg := NumNeg + 1; Add 1 to NumNeg
SYNTAX DISPLAY
Multiple Alternative IF Statement
IF condition1 THEN
statement sequence1
ELSIF condition2 THEN
statement sequence2
...
ELSIF conditionk THEN
statement sequencek
ELSE
statement sequencen
END IF;
IF N >= 0 THEN
Ada.Text_IO.Put(Item=>"Positive");
ELSIF N = 0 THEN
Ada.Text_IO.Put(Item=>"Zero");
ELSE
Ada.Text_IO.Put(Item=>"Negative");
END IF;
IF statement are
evaluated from top to bottom until a true value is obtained. The statement
sequence following the first true condition is executed and the rest of the
IF statement is skipped. If every condition is false, statement
sequencen (between ELSE and END) is executed.
ELSE and
statement sequencen are present, exactly one statement sequence is
always executed. If ELSE and statement sequencen are
omitted, no statement sequence is executed when every expression is false.
Also note the spelling required by Ada: ELSIF is spelled
without a second E or space; END IF must have a space
between END and IF.
PROGRAM STYLE
Writing a Multiple-Alternative IF Statement
IF statement, align
the reserved words IF, ELSE, ELSIF, and
END IF and indent each statement sequence consistently. This is
done to make the IF statement more readable; indentation is
ignored by the compiler. Very often the conditions in a multiple-alternative decision are not mutually exclusive. This means that it may be possible for more than one condition to be true for a given data value. If this is the case, the order of the conditions becomes very important because only the statement sequence following the first true condition is executed.
Example 4.6
The table below describes the assignment of grades based on an exam score.
Exam Score Grade Assigned 90 and above A 80-89 B 70-79 C 60-69 D below 60 F
The multiple alternative IF statement below displays the letter
grade assigned according to this table. The last three conditions are true for
an exam score of 85; however, a grade of B is assigned because the first true
condition is Score >= 80.
-- correct grade assignment
IF Score >= 90 THEN
Ada.Text_IO.Put (Item=>'A');
ELSIF Score >= 80 THEN
Ada.Text_IO.Put (Item=>B');
ELSIF Score >= 70 THEN
Ada.Text_IO.Put (Item=>'C');
ELSIF Score >= 60 THEN
Ada.Text_IO.Put (Item=>'D');
ELSE
Ada.Text_IO.Put (Item=>'F');
END IF;
It would be wrong to write the decision as shown next. All
passing exam scores (60 or above) would be incorrectly categorized as a grade
of D because the first condition would be true and the rest would be skipped.
Writing the IF this way would be a mistranslation of the table
into code.
-- incorrect grade assignment
IF Score >= 60 THEN
Ada.Text_IO.Put (Item=>'D');
ELSIF Score >= 70 THEN
Ada.Text_IO.Put (Item=>'C');
ELSIF Score >= 80 THEN
Ada.Text_IO.Put (Item=>'B');
ELSIF Score >= 90 THEN
Ada.Text_IO.Put (Item=>'A');
ELSE
Ada.Text_IO.Put (Item=>'F');
END IF;
Example 4.7
You can use a multiple alternative IF statement to
implement a decision table that describes several alternatives. Let's
say you are an accountant setting up a payroll system for a small firm. Each
line of Table 4.5 indicates an employee's salary range and a corresponding base
tax amount and tax percentage. Given a salary amount, the tax is calculated by
adding the base taxfor that salary range and the product of the
percentage of excessand the amount of salary over the minimum salary
for that range.
Table 4.5
Tax Table for Example 4.7
Salary Range Base Tax Percentage of Excess 1 0.00-1499.99 0.00 15% 2 1500.00-2999.99 225.00 16% 3 3000.00-4999.99 465.00 18% 4 5000.00-7999.99 825.00 20% 5 8000.00-14999.99 1425.00 25%For example, the second line of the table specifies that the tax due on a salary of $2000.00 is $225.00 plus 16% of the excess salary over $1500.00 (i.e., 16% of $500.00). Therefore, the total tax due is $225.00 plus $80.00, or $305.00.
The IF statement in Figure 4.7 implements the tax table. If the
value of Salary is within the table range (0.00 to 14999.99),
exactly one of the statements assigning a value to TAX will be
executed. A trace of the IF statement for Salary =
$2000.00 is shown in Table 4.6. The value assigned to Tax
is $305.00, as desired.
Figure 4.7
IF statement for Table 4.5
IF Salary < 0.0 THEN
Ada.Text_IO.Put (Item=>"Error! Negative salary $");
Ada.Float.Text_IO.Put (Item=>Salary, Fore=>1, Aft=>2,
Exp=0);
Ada.Text_IO.New_Line;
ELSIF Salary < 1500.00 THEN -- first range
Tax := 0.15 * Salary;
ELSIF Salary < 3000.00 THEN -- second range
Tax := (Salary - 1500.00) * 0.16 + 225.00;
ELSIF Salary < 5000.00 THEN -- third range
Tax := (Salary - 3000.00) * 0.18 + 465.00;
ELSIF Salary < 8000.00 THEN -- fourth range
Tax := (Salary - 5000.00) * 0.20 + 825.00;
ELSIF Salary < 15000.00 THEN -- fifth range
Tax := (Salary - 8000.00) * 0.25 + 1425.00;
ELSE
Ada.Text_IO.Put (Item=>"Error! Too large salary $");
Ada.Float.Text_IO.Put (Item=>Salary, Fore=>1, Aft=>2, Exp=0);
Ada.Text_IO.New_Line;
END IF;
Table 4.6
Trace of IF Statement in Fig. 4.7 for Salary = $2000.00
Statement Part Salary Tax Effect
2000.00 ?
IF Salary < 0.0 2000.0 < 0.0 is false
ELSIF Salary < 1500.00 2000.0 < 1500.0 is false
ELSIF Salary < 3000.00 2000.0 < 3000.0 is true
Tax := (Salary - 1500.00) difference is 500.00
* 0.16 product is 80.00
+ 225.00 305.00 sum is 305.00
PROGRAM
STYLE
Validating the Value of Variables
IF statement in Fig. 4.7 displays an
error message if the value of Salary is outside the range covered
by the table (0.0 to 14999.99). The first condition is used to detect negative
salaries, and an error message is displayed if Salary is less than
zero. All conditions evaluate to False if Salary is greater than
14999.99, and the alternative following ELSE displays an error
message.The statement sequence inside a control statement can contain another
control statement. For example, an IF statement can contain
another IF. The second control statement is said to be
nested inside the first control statement. The inner control statement
can itself contain a control statement; in fact, there is no theoretical limit
on the depth to which control statements can be nested.
The ability to nest control statements allows us to write very sophisticated
programs. In Chapters 5 and 6
we will introduce many examples of
IF statements nested inside loops and vice versa. For the time
being, consider the following example.
Depending on a student's Grade Point Average (GPA), the following
fragment displays one of three messages. If the GPA is less than or equal to
1.5, the painful message following the second ELSE is displayed.
If GPA is greater than 1.5, the inner IF statement is
executed, and a more pleasant message is displayed.
IF GPA > 1.5 THEN
IF GPA < 3.0 THEN
Ada.TextIO.Put(Item => "Progressing satisfactorily");
ELSE
Ada.Text_IO.Put (Item => "Made the Dean's List - send money");
END IF;
ELSE
Ada.Text_IO.Put (Item => "Flunked out");
END IF;
The nested statements below have the same effect as the ones
above. Again, the inner IF statement is executed when GPA exceeds
1.5.
IF GPA <= 1.5 THEN
Ada.Text_IO.Put (Item => "Flunked out");
ELSE
IF GPA < 3.0 THEN
Ada.Text_IO.Put (Item => "Progressing satisfactorily");
ELSE
Ada.Text_IO.Put (Item => "Made the Dean's List - send money");
END IF;
END IF;
Nested IF statements can sometimes be confusing to
write and to read. Often, a single multiple-alternative IF
statement can replace nested IF statements, resulting in a more
readable program. Verify for yourself that the IF statement below
has the same effect as the earlier nested IF statements.
IF GPA <= 1.5 THEN
Ada.Text_IO.Put (Item => "Flunked out");
ELSIF GPA < 3.0 THEN
Ada.Text_IO.Put (Item => "Progressing satisfactorily");
ELSE
Ada.Text_IO.Put (Item => "Made the Dean's List - send money");
END IF;
PROGRAM STYLE
Indentation Conventions for Nested Control Structures
IF is indented the same amount as the Put following
the ELSE.
Developing a consistent indentation style is one way of making your programs clear and easy to read. Many companies have adopted companywide or projectwide programming style standards that include indentation rules. This makes it easy for programmers to read each other's source code. There is no one "best" indentation rule; the most important principle is consistency.
In this book, we indent each structure several spaces deeper than the structure within which it is nested. The complete program examples use a consistent indentation of two spaces, and the code fragments in the text are usually indented a bit more for added clarity. We recommend an indentation convention similar to that used in the programs. If your teacher states different rules, follow them consistently.
IF statement in Fig. 4.7 for
Salary = 13500.00.
IF statement of Fig. 4.7?
IF statement for Example 4.8 using only the
relational operator < in all conditions.
IF statement. Assume that the grade point average is within the
range 0.0 through 4.0.
Grade Point Average Transcript Message
0.0-0.99 Failed semester -- registration suspended
1.0-1.99 On probation for next semester
2.0-2.99 (no message)
3.0-3.49 Deans list for semester
3.5-4.0 Highest honors for semester
Copyright © 1996 by Addison-Wesley Publishing Company, Inc.