Selection structures in C (II) H&K Chapter 4 Instructor – Gokcen Cilingir Cpt S 121 (June 30, 2011) Washington State University

You try it! What will be the output of the code below? int x = 0; if (x = 3) printf(“x is %d\n”, x); else printf(“x is %d\n”, x);

You try it! Explain the difference between the statements on the left and the statements on the right. What will x be in each situation given that its initial value was 1? if (x >= 0) x = x + 1; else if (x >= 1) x = x + 2; if (x >= 0) x = x + 1; if (x >= 1) x = x + 2;

C. Hundhausen, A. O’Fallon4 Nested if statements (1) Consider the following scenario: A high school baseball team awards merit points to players based on their offensive performance. A single (encoded 's') is worth 1 point, a double (encoded 'd') is worth 2 points, a triple (encoded 't') is worth 3 points, and a home run (encoded 'h') is worth 4 points. Any at-bat that leads to an out (encoded 'o') worth 0 points. Write a C statement that, given an at- bat character, properly awards points.

C. Hundhausen, A. O’Fallon5 Nested if statements (2) We can write a nested if statement to handle this situation: char at_bat; int points; printf("Enter the at-bat code (s,d,t,h,o): "); scanf(" %c",&at_bat); if (at_bat == 's') /* single */ { points = 1; } else if (at_bat == 'd') /* double */ { points = 2; } else if (at_bat == 't') /* triple */ { points = 3; } else if (at_bat == 'h') /* home run */ { points = 4; } else /* out */ { points = 0; }

C. Hundhausen, A. O’Fallon6 Nested if statements (3) Consider the following updated scenario: A high school baseball team awards merit points to players based on their offensive performance and the class standing ('f' = freshman, 'o' = sophomore, 'j' = junior, and 's' = senior). In particular, freshmen and sophomores earn an extra point for home runs, whereas juniors and seniors do not earn any points for singles. Write a C if-statement that, given an at- bat character and a class standing character, properly awards points.

C. Hundhausen, A. O’Fallon7 Nested if statements (4) We can write an even more nested if statements to handle this situation: char at_bat, class_standing; int points;... if (at_bat == 's') /* single */ { if (class_standing == 'f') || (class_standing == 'o') points = 1; else points = 0; } else if (at_bat == 'd') /* double */ { points = 2; } else if (at_bat == 't') /* triple */ { points = 3; } else if (at_bat == 'h') /* home run */ { if (class_standing == 'j') || (class_standing == 's') points = 4; else points = 5; } else /* out */ { points = 0; }

C. Hundhausen, A. O’Fallon8 Nested if statements (3) Nested if statements vs. compound conditionals ◦ Consider the following scenario: The National Weather Service would like to identify hourly weather reports in which the relative humidity is low (below 20%, the temperature is pleasant (between 75 and 85), and the winds are calm (0 to 10 m.p.h.). Assuming that the variables humidity, temp, and wind_speed hold those values, write an if statement that prints out a message when the conditions are met.

C. Hundhausen, A. O’Fallon9 Nested if statements (4) Nested if statements vs. compound conditionals (cont.) ◦ Alternative 1: Nested if if (humidity < 20) if (temp >= 75) if (temp <= 85) if (wind_speed <= 10) printf("Perfect conditions!\n"); ◦ Alternative 2: Compound if conditional if ((humidity = 75) && (temp <= 85) && (wind_speed <= 10)) printf("Perfect conditions!\n");

‘Dangling else’ problem To which if statement does the else belong to? if(y != 0 ) if (x != 0) result = x/y; else printf(“Error: y is equal to 0\n”);

C. Hundhausen, A. O’Fallon11 Nested if statements (5) Important to note that the C compiler always matches an else with the most recent incomplete if ◦ Example: if (humidity < 20) if (temp <= 32) printf("It's a cool, dry day.") if (wind < 10) printf("Luckily, the winds are calm."); else printf("The humidity is low, it's above freezing."); else if (humidity < 60) if (temp <= 32) printf("It's cold, with moderate humidity."); else printf("It's above freezing, with moderate humidity."); ◦ Do you see a problem here? How would you fix it?

C. Hundhausen, A. O’Fallon12 Nested if statements (6) Guidelines for using nested if statements ◦ Use braces to enclose all if branches, even if they contain only one statement  This will help you avoid the problem of mismatching if and else branches ◦ If possible, structure conditions so each alternative falls on false branch of previous condition (else if…) ◦ If each if branch contains a return statement, there's no need for an else clause, and you can avoid deep nesting ◦ In conditionals, don't mistake = for ==  The C compiler won't be able to catch this error, and you're condition will always evaluate to true!

C. Hundhausen, A. O’Fallon13 Nested if statements (7) Example: Nested if with return statements int get_points(char at_bat) { if (at_bat == 's') /* single */ return 1; /* assertion: at_bat != 's' */ if (at_bat == 'd') /* double */ return 2; /* assertion: at_bat != 's' && at_bat != 'd' */ if (at_bat == 't') /* triple */ return 3; /* assertion: at_bat != 's' && at_bat != 'd' && at_bat != 't' */ if (at_bat == 'h') /* home run */ return 4; /* assertion: at_bat != 's' && at_bat != 'd' && at_bat != 't' && at_bat != 'h' */ return 0; /* out */ }

C. Hundhausen, A. O’Fallon14 switch Statements (1) One issue with nested if statements is readability ◦ The deeper the nesting, the more difficult it can be to figure out what's happening Another issue is that the programmer could mistakenly "mis-nest" if statements, as in the previous example In cases in which the nesting is based on the value of a single variable, a switch statement may be a better alternative

C. Hundhausen, A. O’Fallon15 switch Statements (2) Let's revisit the previous baseball scenario: A high school baseball team awards merit points to players based on their offensive performance. A single (encoded 's') is worth 1 point, a double (encoded 'd') is worth 2 points, a triple (encoded 't') is worth 3 points, and a home run (encoded 'h') is worth 4 points. Any at- bat that leads to an out (encoded 'o') worth 0 points. Write a C statement that, given an at-bat character, properly awards points.

C. Hundhausen, A. O’Fallon16 switch Statements (3) The switch statement provides a cleaner way to handle this scenario: char at_bat; int points; points = 0; printf("Enter the at-bat code (s,d,t,h,o): "); scanf(" %c",&at_bat); switch (at_bat) { case 's': /* single */ points = points + 1; break; case 'd': /* double */ points = points + 2; break; case 't': /* triple */ points = points + 3; break; case 'h': /* home run */ points = points + 4; break; case 'o': points = points + 0; break; default: /* Anything but 's','d','t','h', 'o' */ printf("Unrecognized at-bat code."); break; } Don't forget to end each case with break Don't forget the begin and end curly braces If at_bat does not match any other case labels, the default case is executed

C. Hundhausen, A. O’Fallon17 switch Statements (4) What if we also want to allow capital letter codes? switch (at_bat) { case 's': /* single */ case 'S': points = points + 1; break; case 'd': /* double */ case 'D': points = points + 2; break; case 't': /* triple */ case 'T': points = points + 3; break; case 'h': /* home run */ case 'H': points = points + 4; break; case 'o': case 'O': points = points + 0; break; default: /* Anything but 's','d','t','h' */ printf("Unrecognized at-bat code."); break; } A single case can contain an arbitrary number of case labels

C. Hundhausen, A. O’Fallon18 switch Statements (5) Let's revisit the following scenario: A high school baseball team awards merit points to players based on their offensive performance and the class standing ('f' = freshman, 'o' = sophomore, 'j' = junior, and 's' = senior). In particular, freshmen and sophomores earn an extra point for home runs, whereas juniors and seniors do not earn any points for singles. Write a C if-statement that, given an at- bat character and a class standing character, properly awards points.

C. Hundhausen, A. O’Fallon19 switch Statements (6) We can write this more clearly as a switch statement with embedded if statements : switch (at_bat) { case 's': /* single */ if (class_standing == 'f') || (class_standing == 'o') points = 1; break; case 'd': /* double */ points = 2; break; case 't': /* triple */ points = 3; break; case 'h': /* home run */ if ((class_standing == 'f') || (class_standing == 'o')) points = 5; else points = 4; break; case 'o': points = 0; break; default: /* Anything but 's','d','t','h' */ printf("Unrecognized at-bat code."); break; }

C. Hundhausen, A. O’Fallon20 switch Statements (7) Notes on switch statements ◦ You can only switch based on a value of char or int. You cannot switch based on the value of a double. double era = 1.67; switch (era) { … /* can't do this */ An arbitrary number of statements can follow a case label It's a good idea always to define a default case A common mistake is to forget to end a case with break. If a break is forgotten, execution "falls" through to the next case label! (C can be cruel.)

21 References J.R. Hanly & E.B. Koffman, Problem Solving and Program Design in C (6 th Ed.), Addison- Wesley, 2010 P.J. Deitel & H.M. Deitel, C How to Program (5 th Ed.), Pearson Education, Inc., 2007.