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# Selection structures in C (II) H&K Chapter 4 - PowerPoint PPT Presentation

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!.

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Selection structures in C (II) H&K Chapter 4

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## Selection structures in C (II)H&K Chapter 4

Instructor – GokcenCilingir

Cpt S 121 (June 30, 2011)

Washington State University

### You try it!

• What will be the output of the code below?

intx = 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;

### 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.

### 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;

}

### 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.

### 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;

}

### 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.

### 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 conditionalif ((humidity < 20) && (temp >= 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”);

### 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?

### 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!

### 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 */

}

### 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

### 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.

### 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 the begin and end curly braces

Don't forget to end each case with break

If at_bat does not match any other case labels, the default case is executed

### 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

### 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.

### 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;

}

### 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 defaultcase

• 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.)

### References

• J.R. Hanly & E.B. Koffman, Problem Solving and Program Design in C (6th Ed.), Addison-Wesley, 2010

• P.J. Deitel & H.M. Deitel, C How to Program (5th Ed.), Pearson Education , Inc., 2007.