Understanding Control Statements in Programming: Iteration and Conditional Execution

1. CSCI 125 & 161 Lecture 7 Martin van Bommel

2. Control Statements • Statements that affect the sequence of execution of other statements • Normal is sequential • May want to repeat statements - iteration • May or may not want particular statement executed at all - conditional

3. Control Statement Structure • Control line - first line (e.g. while) • Body - statements enclosed in { } that are repeated • Body typically indented to show range of control statement

4. Conditional Execution • May want to execute a statement only if some condition applies • May wish to choose between two alternative courses of actions depending on some test • Simplest form - if statement

5. if Statement if (conditional-test) { …statements executed if test true... } } else { …statements executed if test false... }

6. Relational & Logical Operators == Equal != Not equal > Greater < Less >= Greater or equal <= Less or equal • Logical operate on results of comparisons ! Not (true if operand false – one operand) && And (true if both true) || Or (true if either true)

7. Example 1 • “x is not equal to either 2 or 3” if (x != 2 || x != 3) • No, it should be if (!(x == 2 || x == 3)) • or if (x != 2 && x != 3)

8. Example 2 • “x is in the range from 0 to 10 exclusive” if (0 < x < 10) • No, it should be if (0 < x && x < 10)

9. Example • Leap year every fourth year, except centuries, then just every fourth century • year is divisible by 4 but not by 100, or • year is divisible by 400 • Try ((y % 4 == 0) && (y % 100 != 0)) || (y % 400 == 0)

10. Bad Example if (x < 5) cout << ”A”; if (x > 5) cout << ”B”; else cout << ”C”;

11. If Blocking Rule • For any if statement that requires (1) more than a single line, or (2) an else clause, always use braces to enclose in a separate block the statements under the control of the if statement .

12. Single-line if statement if (condition) statement where: condition is the Boolean value to test statement is a single statement to be executed if condition is true

13. Multiline if statement if (condition) { statements; } where condition is the Boolean value to test statements is a block of statements to be executed if condition is true

14. if-else statements if (condition) { statementsT ; } else { statementsF ; }

15. Cascading if statement if (condition1) { statements1 ; } else if (condition2) { statements2; . . . } else { statementsnone }

16. Iteration and Loops • Iteration - process of repeating an operation • Essential for large amounts of data - repeating statements shortens programs • Loop - any portion of program repeated via control statement • e.g. while loop

17. The while statement while (condition) { statements } where condition tested before first and every iteration statements executed each time condition true

18. while operation • Condition test performed before every cycle, including first. • If condition is initially false, body of loop not executed at all. • Condition test performed only at beginning of each loop cycle. • Even if condition becomes false during cycle, cycle is completed.

19. while example • Sum up digits of a number sum = 0; while (n > 0) { sum = sum + n % 10; n = n / 10; }

20. Infinite loops • A loop cycle that never stops – infinite loop • Condition of while statement never false • e.g. while (n >= 0) { sum += n % 10; n /= 10; }

21. Repeat-N-Times Idiom • for (i = 0; i < N; i++) {statements to be repeated } • May want to add up 10 input numbers for (i = 0; i < 10; i++) { cin >> value; sum = sum + value; }

22. Index Variable in Counting Loop for (i = 0; i < N; i++) • Variable i is called the index variable • Any variable may be used - traditionally i • Must be declared at beginning of function • Initial value of i in loop is 0 - first cycle • Next cycles value is 1, 2, etc. • On last cycle, value is N – 1 • After loop, value of i is N

23. Non-Zero Counting Loop • It is possible to modify for loop to begin at another number for (i = first; i <= last; i++) • Note the <= operator instead of < • Value of i ranges from first to last • Used if value of i required in body of loop

24. for statement for (init; test; step) { statements } where init is an expression evaluated at beginning test is a condition for continuing the loop step is an expression to prepare next cycle

25. for example • Countdown from 10 to 0 for (t=10; t >= 0; t--) { cout << t; } cout << ”Liftoff!\n”;

26. for more • Note that expressions init, test, and step are optional, but the semicolons must appear for (;;) - infinite loop for (;t<10;) same as while (t<10)

27. Nested for loops • Create a 10 x 10 multiplication table for (row=1; row<=10; row++) { for (col=1; col<=10; col++) { cout << (row * col); } cout << endl; }