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Chapter 7 Selection

Chapter 7 Selection. Dept of Computer Engineering Khon Kaen University. Major Concepts. The if statement The if...else… statement Statement blocks Compound conditions Short-circuiting Boolean expressions Nested selection statements The switch statement. Introduction.

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Chapter 7 Selection

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  1. Chapter 7 Selection Dept of Computer Engineering Khon Kaen University

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

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

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

  20. The Minimum of Three Integers #include <iostream> 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; } 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} 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; } 178110: Computer Programming (II/2546)

  23. Example: Statement Blocks #include <iostream> 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; } 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 178110: Computer Programming (II/2546)

  25. Using Blocks to Limit Scope • int n = 44; • cout << “n = “ << n << endl; • { • int n = 2; cout << “n = “ << n << endl; • } • { • cout << “ n = “ << n << endl; • } • { • int n; cout << “n = “ << n << endl; • } • cout << “n = “ << n << endl; • What are the values of n at line 2, line 4, line 7, line 10, and line 12? 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) 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 < 0) || (3 > 2) evaluates to? • true 178110: Computer Programming (II/2546)

  28. Three Logic Operators Let T stands for True and F stands for False 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; 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 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? 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 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 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++ 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 178110: Computer Programming (II/2546)

  36. The Source of the Logical Error • n1 = 0, n2 = 0, n3 = 1 • if (n1 >= n2 >= n3) • 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 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! 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 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”; 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? 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; 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; 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; 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; } 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 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; } 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? 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? 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 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 178110: Computer Programming (II/2546)

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