Control Structures Selection or Decision Branching

Decision Two types: simple alternative if Which the statements are executed Two types: simple alternative if compound alternative if...else

Single Alternative Decision An action is taken if the condition is true, otherwise the control goes to the next statement.

Single Alternative Decision Syntax if (expression) statement If expression is true, statement is executed; otherwise statement is skipped. no ; note: 2 = signs Example: if (stomach == empty) eat a Snickers bar; * *

Single Alternative Decision An expression is any combination of variables, constants, or function calls that evaluate to a value. ex. 5 x + y a = 3 + j N n++ f(12.3, a, “Yvonne”) Syntax if (expression) statement * *

Single Alternative Decision Syntax if (expression) statement If expression is true, statement is executed; otherwise statement is skipped. no ; note: 2 = signs Example: if (stomach == empty) eat a Snickers bar; eat a marshmallow sunday; * *

Single Alternative Decision Example: if (grade >= 90) cout << “Congratulations!\n”; cout << “Your grade is “ << grade << “.\n";

The Compound Statement Example: if (u > v) { a = 1; b = 2; if ( u > z) x =11; y = 12; } Syntax if (expression) { statement; statement; { statement; statement; } The compound statement is itself a statement. * * * * *

if Examples Valid: if (y != 0.0) z = x/y; if (a < b && b < c) { d = a + b + c; cout << "All OK\n"; } Not Valid: if b == a area = a * a; if (a < b) && (b < c) if (a < b) ; Valid But... if (a < b) ; * *

if Problems Using = in place of == What is the difference between these two? if (toss == 7) cout << “You win the bet.”; if (toss = 7) cout << “You win the bet.”;

Compounding 2 if Statements if (j < k) { min = j; cout << “the smaller number is “ << min; } if (j < k) cout << "j is smaller than k\n"; More Efficient: if (j < k) { min = j; cout << “the smaller number is “ << min; cout << "j is smaller than k\n"; } * *

Double Alternative Decision An action (or set of actions) is taken if the condition is true, another action (or set of actions) is taken if the condition is false, then the control goes to the next statement. if ... else is the typical double alternative. * *

The if-else Statement Syntax if (expression) statement1 else If expression is nonzero then statement1 is executed and statement2 is skipped. If expression is zero statement1 is skipped and statement2 is executed.

if ... else Examples if (stomach == empty) { eat a pizza; eat a Snickers bar; } else eat a salad; if ( j < k ) { min = j; k = k * 3; } else { min = k; j = j * 3; * *

Interactive Program Finding the minimum of three values

Finding the Minimum of Three Values int x, y, z, min; cout << “Input three integers: “; cin >> x >> y >> z; if (x < y) min = x; else min = y; if (z < min) min = z; cout << “The minimum value is “ << min << ‘\n’; * * *

Interactive Program Finding the minimum of three values Output: Input three integers: 9 5 -12 The minimum value is -12 _ * *

The if-else Statement Syntax if (expression) if (a > b) statement1 max = a; else max = b; expression1 ? expression2 : expression3 max = (a > b) ? a : b; * * *

Nested if Statements A nested if statement is an if statement that is included within another if statement. Syntax if (expression1) { if (expression2) statement }

Nested if Example if (number == secretnumber) cout << “You guessed it!”; if (number != secretnumber) { cout << “Sorry, that’s not the number.\n”; if (number > secretnumber) cout << “You guessed too high.\n”; else cout << “You guessed too low.\n”; } * *

Chained if...else Example 2 statement1 3 else if (expression2) 4 statement2 5 . . . 6 else if (expressionN) 7 statementN 8 else 9 last statement 10 next statement Syntax 1 if (expression1) *

Chained if...else Example if (total >=90) grade = ‘A’; else if (total >= 80) grade = ‘B’; else if (total >= 70) grade = ‘C’; else if (total >= 60) grade = ‘D’; else grade = ‘E’; next statement * *

The Dangling else if (avg >= 60.0) if (avg < 70.0) cout << “Passing, but marginal”; else cout << “Failing”; if (avg >= 60.0) { if (avg < 70.0) cout << “Passing, but marginal”; } else cout << “Failing”; * *

The Dangling else if (avg >= 60.0) { if (avg < 70.0) cout << “Passing, but marginal”; } else cout << “Failing”;

AND vs. OR if( (rel == 'S') && (rel == 'M') && (rel == 'F') ) cout << "\nImmediate family.\n"; if( (rel != 'S') && (rel != 'M') && (rel != 'F') ) { cout << "\nNot immediate family,\n"; cout << " but a close relation.\n"; }

Random Numbers #include<stdlib.h> // defines rand() & srand() #include<time.h> // defines time() in main(): srand(time(NULL)); l l l num1 = 1 + rand() % 3; num1 = 1 + rand() % 6; num1 = 6 + rand() % 5; how many numbers 1, 2, 3 1, 2, 3, 4, 5, 6 6, 7, 8, 9, 10 starting number * * *

The switch Statement Similar to if statements Can list any number of branches Used in place of nested if statements Avoids confusion of deeply nested ifs

The switch Statement Syntax switch (expression) no ; use : { case value1: statement1; break; case value2: statement2; · · · case valuen: statementn; default: statement; } no ; use : * *

The switch Statement Syntax switch (expression) no ; use : { case value1: statement1; break; case value2: statement2; · · · case valuen: statementn; default: statement; } no ; use :

The switch Statement switch (let_grd) { case ‘A’: cout << “Grade is between 90 & 100”; break; case ‘B’: cout << “Grade is between 80 & 89”; case ‘C’: cout << “Grade is between 70 & 79”; break; cont.

The switch Statement case ‘D’: cout << “Grade is between 60 & 69”; break; case ‘E’: cout << “Grade is between 0 & 59”; default: cout << “You entered an invalid grade.”; } next statement

The switch Statement switch (let_grd) { case ‘A’: cout << “Grade is between 90 & 100”; break; case ‘B’: cout << “Grade is between 80 & 89”; case ‘C’: cout << “Grade is between 70 & 79”; case ‘D’: cout << “Grade is between 60 & 69”; case ‘E’: cout << “Grade is between 0 & 59”; default: cout << “You entered an invalid grade.”; }

The break Statement switch (let_grd) { case ‘A’: cout << “Grade is between 90 & 100”; break; case ‘B’: cout << “Grade is between 80 & 89”; case ‘C’: cout << “Grade is between 70 & 79”; case ‘D’: cout << “Grade is between 60 & 69”; case ‘E’: cout << “Grade is between 0 & 59”; default: cout << “You entered an invalid grade.”; }

The break Statement switch (let_grd) { case ‘A’: case ‘B’: cout << “Good Work”; break; case ‘C’: cout << “Average Work”; break; case ‘D’: case ‘E’: cout << “Poor Work”; }

The break Statement switch (let_grd) { case ‘A’: case ‘a’: case ‘B’: case ‘b’: cout << “Good Work”; break; case ‘C’: case ‘c’: cout << “Average Work”; break; etc.

The switch Statement * * * * Menu * * * * 1. NY Yankees 2. Orioles 3. Dodgers Choose either 1, 2, or 3:

The switch Statement switch (choice) { case 1: cout << “World Champs”; case 2: cout << “Good Guys”; case 3: cout << “Da Bums”; } What will be the output?

The switch Statement What will be the output when the user enters 1 World ChampsGood GuysDa Bums 2 Good GuysDa Bums 3 Da Bums 4 skips the switch * * * * * * * *

The switch Statement switch (choice) switch (choice) { { case 1: cout << “World Champs”; case 2: cout << “Good Guys”; case 3: cout << “Da Bums”; } switch (choice) { case 1: cout << “World Champs”; break; case 2: cout << “Good Guys”; case 3: cout << “Da Bums”; default: cout << “Enter a 1, 2, or 3”; } *

Common Errors Using = in place of == Improper braces in nested ifs Too deeply nested ifs Missing break statements in switch the statement Copyright © 1999 by Freedom TLC, Inc.

Debugging Syntax errors vs. Logic error Prevention - plan first! Valuation tables Display values C++ Debugger Copyright © 1999 by Freedom TLC, Inc.

“I discovered I always have choices, and sometimes it’s only a choice of attitude” Judith M. Knowlton Copyright © 1999 by Freedom TLC, Inc.