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Control structures selection
The document discusses control structures in C++ programs. It covers selection structures like if, if/else, and switch that allow a program to conditionally execute code. Relational and logical operators are used to form expressions that evaluate to true or false and are used in selection conditions. Logical expressions with operators like && and || allow combining multiple conditions.
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Control structures selection
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- 2. Objectives In this chapter,you will: • Learn about control structures • Examine relational and logical operators • Explore how to form and evaluate logical (Boolean) expressions • Discover how to use the selection control structures if, if...else, and switch in a program 2
- 3. Control Structures • Acomputer can proceed: – In sequence – Selectively (branch) - making a choice – Repetitively (iteratively) - looping • Some statements are executed only if certain conditions are met • A condition is met if it evaluates to true 3
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- 5. Relational Operators • Acondition is represented by a logical (Boolean) expression that can be true or false • Relational operators: – Allow comparisons – Require two operands (binary) – Evaluate to true or false 5
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- 7. Relational Operators (cont.) •You can use the relational operators with all three simple data types: – 8 < 15 evaluates to true – 6 != 6 evaluates to false – 2.5 > 5.8 evaluates to false – 5.9 <= 7.5 evaluates to true 7
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- 9. Logical (Boolean) Operatorsand Logical Expressions • Logical (Boolean) operators enable you to combine logical expressions unary binary binary 9
- 10. Logical (Boolean) Operatorsand Logical Expressions (cont.) 10
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- 13. Order of Precedence •Relational and logical operators are evaluated from left to right • The associativity is left to right • Parentheses can override precedence 13
- 14. Order of Precedence(cont.) 14
- 15. Order of Precedence(cont.) 15
- 16. Order of Precedence(cont.) 16
- 17. Order of Precedence(cont.) 17
- 18. Selection: if andif...else • One-Way Selection • Two-Way Selection • Compound (Block of) Statements • Multiple Selections: Nested if • Comparing if...else Statements with a Series of if Statements 18
- 19. Selection: if andif...else (cont.) • The syntax of one-way selection is: • The statement is executed if the value of the expression is true • The statement is bypassed if the value is false; program goes to the next statement • if is a reserved word 19
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- 21. Two-Way Selection • Two-wayselection takes the form: • If expression is true, statement1 is executed; otherwise, statement2 is executed – statement1 and statement2 are any C++ statements • else is a reserved word 21
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- 24. Compound (Block of)Statement • Compound statement (block of statements): • A compound statement is a single statement 24
- 25. Compound (Block of)Statement (cont.) if (age > 18) { cout << "Eligible to vote." << endl; cout << "No longer a minor." << endl; } else { cout << "Not eligible to vote." << endl; cout << "Still a minor." << endl; } 25
- 26. Multiple Selections: Nestedif • Nesting: one control statement in another • An else is associated with the most recent if that has not been paired with an else 26
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- 28. Comparing if…else Statementswith a Series of if Statements 28
- 29. Confusion Between ==and = • C++ allows you to use any expression that can be evaluated to either true or false as an expression in the if statement: if (x = 5) cout << "The value is five." << endl; • The appearance of = in place of == resembles a silent killer – It is not a syntax error – It is a logical error 29
- 30. switch Structures • switchstructure: alternate to if-else • switch (integral) expression is evaluated first • Value of the expression determines which corresponding action is taken • Expression is sometimes called the selector 30
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- 32. switch Structures (cont.) •One or more statements may follow a case label • Braces are not needed to turn multiple statements into a single compound statement • The break statement may or may not appear after each statement • switch, case, break, and default are reserved words 32
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- 34. Summary • Control structuresalter normal control flow • Most common control structures are selection and repetition • Relational operators: ==, <, <=, >, >=, != • Logical expressions evaluate to 1 (true) or 0 (false) • Logical operators: ! (not), && (and), || (or) • Two selection structures: one-way selection and two-way selection • The expression in an if or if...else structure is usually a logical expression 34
- 35. Summary (continued) • Nostand-alone else statement in C++ – Every else has a related if • A sequence of statements enclosed between braces, { and }, is called a compound statement or block of statements • Using assignment in place of the equality operator creates a semantic error • switch structure handles multiway selection • break statement ends switch statement Source: C++ Programming: From Problem Analysis to Program Design, Fourth Edition 35