Controlling Function Behavior Sequence Selection and Repetition Calvin

Controlling Function Behavior Sequence, Selection and Repetition Calvin College

Review We’ve seen that we can write a simple function that encodes a formula: double Ellipse. Area(double length, double width); { double half. Length = length/2. 0, half. Width = width/2. 0; return PI * half. Length * half. Width; } The statements in such a function are executed in sequence (one after another), which is called sequential execution. Calvin College

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. Calvin College

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. Calvin College

Objects Description Type sequence of ? ? ? test scores average double P/F grade char Calvin College Kind ? ? ? varying Name ? ? ? average grade

Operations Description prompt for, read and average a sequence of doubles display a double compute grade display char Calvin College Predefined? Library? 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. Calvin College

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. Calvin College

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/num. Scores. Otherwise (num. Scores is not positive) it should display an error message and terminate the program. Calvin College

Function 1 Objects Description Type number of scores sum of scores a prompt a score error msg int varying num. Scores double string varying constant sum -score -- Calvin College Kind Name

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 Calvin College yes yes yes iostream 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. Calvin College

Function 1 Coding #include <cstdlib> // exit() double Read. And. Average() { 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); } Calvin College

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. Calvin College

Function 2 Objects Description Type average failing grade passing grade double char Calvin College Kind varying constant Name avg ‘F’ ‘A’

Function 2 Operations Description Predefined? Library? receive double yes return a char yes select from among yes different returns Calvin College built-in Name -return if

Function 2 Algorithm 0. Receive avg from caller. 1. If avg < 60: Return ‘F’. Else Return ‘P’. Calvin College

Function 2 Coding char Pass. Fail. Grade(double avg) { if (avg < 60) return ‘F’; else return ‘P’; } Calvin College

Discussion Read. And. Average() and Pass. Fail. Grade() 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. Calvin College

Coding #include <iostream> #include “grading. h” // Read. And. Average(), Pass. Fail. Grade() int main() { cout << “n. This program reads a sequence of test scores” << “n and computes their pass-fail grade. n”; double average = Read. And. Average(); char grade = Pass. Fail. Grade(average); cout << “n. The average is “ << average << “n and the grade is “ << grade << endl; } Calvin College

Testing This program reads a sequence of test scores and computes their pass-fail grade. Enter a a score: 65 55 75 -1 The average is 65 and the grade is P. . . Calvin College

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 (; ; ) { Statement. List 1 if (Condition) break; Statement. List 2 } These permit repetitive execution of statements. Calvin College

Summary (ii) The C++ if statement lets you select from among two statements. Pattern: if (Boolean. Expression) Statement 1 [else Statement 2] This permits selective execution of statements. Calvin College