Controlling Function Behavior
Sequence, Selection and Repetition
Review
We’ve seen that we can write a simple
function that encodes a formula:
double EllipseArea(double length, double width);
{
double halfLength = length/2.0,
halfWidth = width/2.0;
return PI * halfLength * halfWidth;
}
The statements in such a function are
executed in sequence (one after another),
which is called sequential execution.
Difficulty
There are other operations that are difficult to
implement, using just sequential execution.
Example: Let’s write a program to read in a
sequence of test scores, and display their
average and a corresponding pass/fail grade.
Behavior
Our program should prompt for, read, and
average a sequence of test scores. It
should then display the average. It should
then compute the pass-fail letter grade (P or
F) corresponding to that average. Finally, it
should display that letter grade.
Objects
Description
Type
sequence of
???
test scores
average
double
P/F grade
char
Kind
???
varying
varying
Name
???
average
grade
Operations
Description
prompt for, read
and average
a sequence of
doubles
display a double
compute grade
display char
Predefined? Library?
no
yes
no
yes
--
iostream
-iostream
Name
--
<<
-<<
Algorithm
0. Display via cout the purpose of the program.
1. Compute average by prompting for, reading and averaging a
sequence of test scores.
2. Display average.
3. Compute P/F letter grade corresponding to average.
4. Display grade.
Discussion
We have predefined operations for steps 0, 2
and 4 of our algorithm, but no predefined
operations for steps 1 or 3...
Solution: Build functions to perform those
operations.
Function 1 Behavior
Our function should read, sum, and count the
sequence of scores using the input-loop
pattern. If the number of scores was
positive, it should return sum/numScores.
Otherwise (numScores is not positive) it
should display an error message and
terminate the program.
Function 1 Objects
Description
Type
Kind
Name
number of
scores
sum of scores
a prompt
a score
error msg
int
varying
numScores
double
string
double
string
varying
constant
varying
constant
sum
-score
--
Function 1 Operations
Description
Predefined? Library?
display a string
read a double
test for sentinel
add score to sum
increment count
repeat above steps
divide doubles
return a double
select steps
terminate program
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
iostream
iostream
built-in
iostream
iostream
built-in
built-in
built-in
built-in
cstdlib
Name
<<
>>
<
+=
++
for
/
return
if
exit()
Function 1 Algorithm
0. Initialize sum to 0, count to 0.
1. Loop:
a. Read score.
b. If score < 0: terminate repetition.
c. Increment count.
d. Add score to sum.
End loop.
2. If count > 0
Return sum / count.
Else
Display an error message and terminate the program.
Function 1 Coding
#include <cstdlib>
// exit()
double ReadAndAverage()
{
double score, sum
= 0.0;
int count = 0;
for (;;)
{
cout << “Enter a test score (-1 to quit): “;
cin >> score;
if (score < 0) break;
// test for sentinel
count++;
sum += score;
}
}
if (count > 0)
return sum / count;
else
{
cerr << “\n* no scores to average!\n” << endl;
exit(1);
}
Function 2 Behavior
Our function should receive from its caller an
average. If that average is less than 60, it
should return a failing grade; otherwise, it
should return a passing grade.
Function 2 Objects
Description
Type
average
failing grade
passing grade
double
char
char
Kind
varying
constant
constant
Name
avg
‘F’
‘A’
Function 2 Operations
Description
Predefined? Library?
receive double
yes
return a char
yes
select from among
yes
different returns
built-in
built-in
built-in
Name
-return
if
Function 2 Algorithm
0. Receive avg from caller.
1. If avg < 60:
Return ‘F’.
Else
Return ‘P’.
Function 2 Coding
char PassFailGrade(double avg)
{
if (avg < 60)
return ‘F’;
else
return ‘P’;
}
Discussion
ReadAndAverage() and PassFailGrade() are
sufficiently useful that it is probably worth
storing them in a library (e.g., grading).
Once we have functions for each operation
that is not predefined, we can encode the
algorithm that solves our problem.
Coding
#include <iostream>
#include “grading.h”
// ReadAndAverage(), PassFailGrade()
int main()
{
cout << “\nThis program reads a sequence of test scores”
<< “\n and computes their pass-fail grade.\n”;
double average = ReadAndAverage();
char grade = PassFailGrade(average);
cout << “\nThe average is “ << average
<< “\n and the grade is “ << grade
<< endl;
}
Testing
This program reads a sequence of test scores
and computes their pass-fail grade.
Enter
Enter
Enter
Enter
a
a
a
a
score:
score:
score:
score:
65
55
75
-1
The average is 65 and the grade is P
.
.
.
Summary (i)
The C++ for loop lets you repeat a block of
statements a specified number of times.
Patterns:
for (int count = Start; count <= Stop; count++)
Statements
for (;;)
{
StatementList1
if (Condition) break;
StatementList2
}
These permit repetitive execution of statements.
Summary (ii)
The C++ if statement lets you select from
among two statements.
Pattern:
if (BooleanExpression)
Statement1
[else
Statement2]
This permits selective execution of statements.