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Chapter 7 Selection Dept of Computer Engineering Khon Kaen University.

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1 Chapter 7 Selection Dept of Computer Engineering Khon Kaen University

2 178110: Computer Programming (II/2546) 2 Major Concepts The if statement The if...else… statement Statement blocks Compound conditions Short-circuiting Boolean expressions Nested selection statements The switch statement

3 178110: Computer Programming (II/2546) 3 Introduction The programs in the previous chapters have sequential execution Each statement in the program executes once, and they are executed in the same order that they are listed What if we want some statements to be executed only if some condition is true?

4 178110: Computer Programming (II/2546) 4 The if Statement The if statement allows conditional execution Its syntax is if (condition) a block of statements condition is an integral expression a block of statements consists of one or more executable statements The statement will be executed only if the value of the integral expression is nonzero

5 178110: Computer Programming (II/2546) 5 Flow Chart of “if” Statement condition statement true false

6 178110: Computer Programming (II/2546) 6 An “if” Example int main() { int n, d; cout << “Enter two positive integers: “; cin >> n >> d; if (n%d) cout << n << “ is not divisible by “ << d << endl; return 0; }

7 178110: Computer Programming (II/2546) 7 An “if” Example (Cont.) if (n%d) cout << n << “ is not divisible by “ … In which condition, the above cout statement is executed? n = 66, d = 7 n = 6, d = 3 In C++, whenever an integral expression is used as a condition, The value 0 means “false” All other values mean “true”

8 178110: Computer Programming (II/2546) 8 The if…else… Statement What if we want to have the cout statement when n is divisible by n? We need to have “if …else…” statement The if…else… statement causes one of two alternative statements to execute depending upon whether the condition is true

9 178110: Computer Programming (II/2546) 9 If…else… Statement (Cont.) Its syntax is if (condition) statement1; else statement2; If the value of the condition is nonzero then statement1 will execute; otherwise statement2 will execute

10 178110: Computer Programming (II/2546) 10 Flow Chart of “if…else…” condition statement 1 true false statement 2

11 178110: Computer Programming (II/2546) 11 An “if…else…” Example int main() { int n, d; cout << “Enter two positive integers: “; cin >> n >> d; if (n%d) cout << n << “ is not divisible by “ << d << endl; else cout << n “ << is divisible by “ << d << endl; return 0; }

12 178110: Computer Programming (II/2546) 12 An “if…else” Example (Cont.) if (n%d) cout << n << “ is not divisible by “ … else cout << n << “ is divisible by “ … In these conditions, which cout statement is executed? n = 66, d = 7 n = 6, d = 3 In “if…else…” clause, how many statement is there? One

13 178110: Computer Programming (II/2546) 13 Keywords in C++ A keyword is a word that is already defined and is reserved for a unique purpose in programs written in that language Standard C++ now has 74 keywords Examples of C++ keywords if else int bool float double

14 178110: Computer Programming (II/2546) 14 Keywords in C++ There are two kinds of keywords Reserved words Standard identifiers A reserved word is a keyword that serves as a structure marker, used to define the syntax of the language Examples: if else A standard identifier is a keyword that names a specific element of the language Examples: int bool

15 178110: Computer Programming (II/2546) 15 Comparison Operators The six comparison operators are x < y // x is less than y x > y // x is greater than y x <= y // x is less than or equal to y x >= y// x is greater than or equal to y x == y // x is equal to y x != y // x is not equal to y These can be used to compare the values of expressions of any ordinal type

16 178110: Computer Programming (II/2546) 16 Comparison Example1 int main() { int m, n; cout << “Enter two integers: “; cin >> m >> n; if (m < n) cout << m << “ is the minimum “ << endl; else cout << n << “ is the minimum “ << endl; }

17 178110: Computer Programming (II/2546) 17 Comparison Example2 int a = 4; float b = 3.4; char c = 'c'; if (a < b) cout << "a is less than b" << endl; else cout << "a is greater than or equal to b" << endl; if (a < c) cout << "a is less than c" << endl; else cout << "a is greater than or equal to b" << endl;

18 178110: Computer Programming (II/2546) 18 What is Wrong? int main() { int n; cout << “Enter an integer:”; cin >> n; if (n = 2) cout << n << “ = 2 “ << endl; else cout << n << “!= 2 “ << endl; } (n = 2) assigns the value 2 to n (n == 2) compares the values of n and 2

19 178110: Computer Programming (II/2546) 19 Different Kinds of Errors Compile-time error: syntax error, e.g., forgetting a semicolon at the end of the statement Caught by a compiler Run-time error: error occurs when running the program, e.g., diving a number by zero Caught by an operating system Logical error: error in which the program does not work as the programmer wishes, e.g., typing ‘=‘ instead of ‘==‘ when comparing two numbers Cannot be caught by anything

20 178110: Computer Programming (II/2546) 20 The Minimum of Three Integers #include Using namespace std; int main() { // finds the minimum of three input integers: int n1, n2, n3; cout << “Enter three integers:”; cin >> n1 >> n2 >> n3; int min=n1; // now min = n1 if (n2 < min) min = n2; // now min <= n1 and min = n2 if (n3 < min) min = n3; // now min <= n1, min <= n2, and min = n3 cout << “Their minimum is “ << min << endl; return 0; }

21 178110: Computer Programming (II/2546) 21 The Minimum of Three Integers (Cont.) int min=n1; // now min = n1 if (n2 < min) min = n2; // now min <= n1 and min = n2 if (n3 < min) min = n3; // now min <= n1, min <= n2, and // min = n3 After the first if statement executes, min is the minimum of the set {n1, n2} After the last if statement executes, min is the minimum of the set {n1, n2, n3}

22 178110: Computer Programming (II/2546) 22 Statement Blocks A statement block is a sequence of statements enclosed by { } A statement block can be used anywhere that a single statement can be used Example: { int temp = x; x = y; y = temp; }

23 178110: Computer Programming (II/2546) 23 Example: Statement Blocks #include using namespace std; int main() { int x, y; cout << "Enter two integers: "; cin >> x >> y; if (x > y) { int temp=x; x = y; y = temp; } // swap x and y cout << x << " <= " << y << endl; return 0; }

24 178110: Computer Programming (II/2546) 24 Example: Statement Blocks (Cont.) if (x > y) { int temp=x; x = y; y = temp; } // swap x and y The three statements are within the same statement block Temp is declared inside the block. That makes it local to the block, i.e. it only exists during the execution of the block If the condition is false, then does temp exist? No

25 178110: Computer Programming (II/2546) 25 Using Blocks to Limit Scope 1. int n = 44; 2. cout << “n = “ << n << endl; 3. { 4. int n = 2;cout << “n = “ << n << endl; 5. } 6. { 7. cout << “ n = “ << n << endl; 8. } 9. { 10. int n;cout << “n = “ << n << endl; 11. } 12. cout << “n = “ << n << endl; What are the values of n at line 2, line 4, line 7, line 10, and line 12?

26 178110: Computer Programming (II/2546) 26 Compound Conditions Conditions such as (n % d) and (x >= y) can be combined to form compound conditions This is done using the logical operators && (and) || (or) ! (not)

27 178110: Computer Programming (II/2546) 27 Compound Conditions (Cont.) p && q evaluates to true If and only if both p and q are evaluated to true p || q evaluates to true If and only if either p or q is true !p evaluates to true If and only if p evaluates to false (2 2) evaluates to? true

28 178110: Computer Programming (II/2546) 28 Three Logic Operators pqp && qp || q FFFF FTFT TFFT TTTT Let T stands for True and F stands for False p!p FT TF

29 178110: Computer Programming (II/2546) 29 Using Compound Conditions int main() { int n1 = 1, n2 = 2, n3 = 3; if (n1 <= n2 && n1 <= n3) cout << “n1 is less than n2 and n3”; if (n1 <= n2 || n2 <= n3) cout << “n1 is not the greatest number”; if (!(n1 – 1)) cout << “n1 is ” << n1;

30 178110: Computer Programming (II/2546) 30 Short-Circuiting Compound conditions that use && and || will not even evaluate the second operand of the condition unless necessary. Short-circuiting: can evaluate the condition without evaluating every terms For p && q, what is the value of p that makes us not need to evaluate q? p is false

31 178110: Computer Programming (II/2546) 31 Short-Circuiting (Cont.) For p || q, what is the value of p that makes us not need to evaluate q? p is true How do we use the short-circuiting to solve the problem of dividing a number by zero? How do we prevent the program not to evaluate n%d when d = 0?

32 178110: Computer Programming (II/2546) 32 Short-Circuit (Cont.) int main() { int n, d; cout << “Enter two positive integers: “; cin >> n >> d; if ((d != 0) && (n%d == 0)) cout << “n is divisible by d” << endl; else cout << “n is not divisible by d” << endl; return 0; } When d is zero the expression, n%d will not be evaluated

33 178110: Computer Programming (II/2546) 33 Boolean Expressions A boolean expression is a condition that is either true or false Boolean expressions evaluate to integer values The value 0 means “false” and every nonzero value means “true” Since all nonzero integer values are interpreted as meaning “true”, boolean expressions are often disguised

34 178110: Computer Programming (II/2546) 34 Boolean Expressions (Cont.) For example, the statement if (n) cout << “n is not zero”; will print “n is not zero” precisely when n is not zero because this is when the boolean expression (n) is interpreted as “true” Since boolean expressions have integer values, it can lead to some surprising anomalies in C++

35 178110: Computer Programming (II/2546) 35 Another Logical Error int main() { int n1, n2, n3; cout < “Enter three integers: “; cin >> n1 >> n2 >> n3; if (n1 >= n2 >= n3) cout << “max = ” << n1; } What is the output when n1 = 0, n2 = 0, n3 =1? max = 0

36 178110: Computer Programming (II/2546) 36 The Source of the Logical Error 1. n1 = 0, n2 = 0, n3 = 1 2. if (n1 >= n2 >= n3) 3. cout << “max = “ << n1; The source of this error is the fact that boolean expressions have numeric values The expression (n1 >= n2 >= n3) is evaluated from left to right The first part n1 >= n2 evaluates to true since 0 >= 0

37 178110: Computer Programming (II/2546) 37 Source of Logical Error (Cont.) The expression (n1 >= n2 >= n3) is evaluated from left to right (n1 >= n2)  “true” “true” is stored as the numeric value 1 This value is then compared to the value of n3 which is also 1 (1 >= n3)  (1 >= 1)  true The complete expression evaluates to “true” even though it is really false!

38 178110: Computer Programming (II/2546) 38 Nested Selection Statements Selection statements can be used wherever any other statement can be used Thus, a selection statement can be used within another selection statement This is called nesting statements

39 178110: Computer Programming (II/2546) 39 Nesting Selection (Cont.) if (d != 0) if (n%d == 0) cout << n << “ is divisible by “ << d; else cout << n << “ is not divisible by “ << d; else cout << “d is zero”;

40 178110: Computer Programming (II/2546) 40 Coding Style Code fragment 1 if (a > 0) if (b > 0) ++a; else c--; Code fragment 2 if (a > 0) { if (b > 0) ++a; else c--; } Which one is better? Which one is more readable?

41 178110: Computer Programming (II/2546) 41 Using Nesting Selection How do we find the minimum of (n1, n2, n3) by using “if … else” statements? if (n1 < n2) if (n1 < n3) cout << “Their minimum is “ << n1; else cout << “Their minimum is “ << n3; else if (n2 < n3) cout << “Their minimum is “ << n2; else cout << “Their minimum is “ << n3 ;

42 178110: Computer Programming (II/2546) 42 Multiple Conditions if (score >= 90) cout << “Your grade is A” << endl; else if (score >= 80) cout << “Your grade is B” << endl; else if (score >= 70) cout << “Your grade is C” << endl; else cout << “Your grade is D” << endl;

43 178110: Computer Programming (II/2546) 43 The else if Construct It is better to use “else if” to test a sequence of parallel alternatives if (score >= 90) cout << “Your grade is A” << endl; else if (score >= 80) cout << “Your grade is B” << endl; else if (score >= 70) cout << “Your grade is C” << endl; else cout << “Your grade is D” << endl;

44 178110: Computer Programming (II/2546) 44 The switch Statement The switch statement can be used instead of the else if construct to implement a sequence of parallel alternatives Its syntax is switch (expression) { case constant1: statement1; case constant2: statement2; … default: statement0; }

45 178110: Computer Programming (II/2546) 45 The Switch Statement (Cont.) This evaluates the expression Then looks for its value among the case constants If the value is found among the constants listed Then the statements in the corresponding statementList are executed If there is a default (which is optional), then the program branches to its statementList

46 178110: Computer Programming (II/2546) 46 Example1: Switch Statement switch (score) { case 5: cout << “your score is 5” << endl; break; case 4: cout << “your score is 4” << endl; break; case 3: case 2: case 1: cout << “you need to improve!” << endl; break; default: cout << “Error: score is out of range” << endl; }

47 178110: Computer Programming (II/2546) 47 Example 2: Switch Statement switch (score) { case 5: cout << “your score is 5” << endl; break; case 4: cout << “your score is 4” << endl; break; case 3: case 2: case 1: cout << “you need to improve!” << endl; break; } What is an output when score = 0?

48 178110: Computer Programming (II/2546) 48 Example 3: Switch Statement switch (score) { case 5: cout << “your score is 5” << endl; case 4: cout << “your score is 4” << endl; case 3: case 2: case 1: cout << “you need to improve!” << endl; } What happen when score = 5, score = 4, score = 1?

49 178110: Computer Programming (II/2546) 49 Conditional Expression Operator C++ provides a special operator that often can be used in place of the “if…else” statement It is called the conditional expression operator It uses the ? and the : symbols in this syntax: condition ? expression1: expression2

50 178110: Computer Programming (II/2546) 50 Conditional Expression Operator (Cont.) It is a ternary operator; i.e., it combines three operands to produce a value That resulting value is either the value of expression1 or the value of expression2, depending upon the boolean value of the condition For example, the assignment min = (x < y ? x : y); assign the minimum of x and y to min

51 178110: Computer Programming (II/2546) 51 Conditional Expression Operator (Cont.) If the condition x<y is true The expression (x<y? x: y) evaluates to x Otherwise, it evaluates to y Conditional expression statements should be used sparingly Only when the condition and both expressions are very simple

52 178110: Computer Programming (II/2546) 52 Example: Finding the Minimum int main() { int m, n; cout << “Enter two integers:”; cin >> m >> n; cout << (m<n? m: n) << “ is the minimum “ << endl; } (m<n ? m : n) evaluates to m if m < n

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