selection structures in c (ii) h&k chapter 4 instructor – gokcen cilingir cpt s 121 (june 30,...
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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;
4C. Hundhausen, A.
O’Fallon
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.
5C. Hundhausen, A.
O’Fallon
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;
}
6C. Hundhausen, A.
O’Fallon
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.
7C. Hundhausen, A.
O’Fallon
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;
}
8C. Hundhausen, A.
O’Fallon
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.
9C. Hundhausen, A.
O’Fallon
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’ problemTo 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”);
11C. Hundhausen, A.
O’Fallon
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?
12C. Hundhausen, A.
O’Fallon
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!
13C. Hundhausen, A.
O’Fallon
Nested if statements (7) Example: Nested if with return
statementsint 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 */}
14C. Hundhausen, A.
O’Fallon
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
15C. Hundhausen, A.
O’Fallon
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.
16C. Hundhausen, A.
O’Fallon
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
17C. Hundhausen, A.
O’Fallon
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
18C. Hundhausen, A.
O’Fallon
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.
19C. Hundhausen, A.
O’Fallon
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;
}
20C. Hundhausen, A.
O’Fallon
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
ReferencesJ.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.