Getting Controls

Hour 10 - Getting Controls

It is harder to command than to obey.

—F. Nietzsche

In Hour 7, "Doing the Same Thing Over and Over," you learned to use the for, while, and do-while statements to do the same things over and over. These three statements can be grouped into the looping category that is a part of the control flow statements in C.

In this lesson you'll learn about the statements that belong to another group of control flow statements—conditional branching (or jumping), such as

• The if statement
• The if-else statement
• The switch statement
• The break statement
• The continue statement
• The goto statement

Always Saying "if…"

If life were a straight line, it would be very boring. The same thing is true for programming. It would be too dull if the statements in your program could only be executed in the order in which they appear.

In fact, an important task of a program is to instruct the computer to branch (that is, jump) to different portions of the code and work on different jobs whenever the specified conditions are met.

However, in most cases, you don't know in advance what will come next. What you do know is that something is bound to happen if certain conditions are met. Therefore, you can just write down tasks and conditions in the program. The decisions of when to perform the tasks are made by the conditional branching statements.

In C, the if statement is the most popular conditional branching statement; it can be used to evaluate the conditions as well as to make the decision whether the block of code controlled by the statement is going to be executed.

The general form of the if statement is

if (expression) {

   statement1;

   statement2;

   .

   .

   .

}

Here expression is the conditional criterion. If expression is logical TRUE (that is, nonzero), the statements inside the braces ({ and }), such as statement1 and statement2, are executed. If expression is logical FALSE (zero), then the statements are skipped.

Note that the braces ({ and }) form a block of statements that is under the control of the if statement. If there is only one statement inside the block, the braces can be ignored. The parentheses (( and )), however, must always be used to enclose the conditional expression.

For instance, the following expression

if (x > 0)

  printf("The square root of x is: %f\n", sqrt(x));

tells the computer that if the value of x is greater than zero (that is, positive), it should calculate the square root of x by calling the sqrt() function, and then print out the result. Here the conditional criterion is the relational expression x > 0, which returns 1 for true and 0 for false.

Listing 10.1 gives you another example of using the if statement.

TYPE
Listing 10.1. Using the if statement in decision making.

1:  /* 10L01.c  Using the if statement */

2:  #include <stdio.h>

3:

4:  main()

5:  {

6:     int i;

7:

8:     printf("Integers that can be divided by both 2 and 3\n");

9:     printf("(within the range of 0 to 100):\n");

10:    for (i=0; i<=100; i++){

11:       if ((i%2 == 0) && (i%3 == 0))

12:         printf("   %d\n", i);

13:   }

14:   return 0;

15: }

OUTPUT
After 10L01.exe, the executable of the program in Listing 10.1, is created and run from a DOS prompt, the following output is printed on the screen:

C:\app> 10L01

Integers that can be divided by both 2 and 3

(within the range of 0 to 100):

   0

   6

   12

   18

   24

   30

   36

   42

   48

   54

   60

   66

   72

   78

   84

   90

   96

C:\app>

ANALYSIS
As you see in Listing 10.1, line 6 declares an integer variable, i. Lines 8 and 9 print out two headlines. Starting in line 10, the for statement keeps looping 101 times.

Within the for loop, the if statement in lines 11 and 12 evaluates the logical expression (i%2 == 0) && (i%3 == 0). If the expression returns 1 (that is, the value of i can be divided by both 2 and 3 completely), the value of i is displayed on the screen by calling the printf() function in line 12. Otherwise, the statement in line 12 is skipped.

Note that the braces ({ and }) are discarded in the if statement because there is only one statement under the control of the statement.

The result shown on the screen gives all integers within the range of 0 to 100 that can be divided by both 2 and 3.

The if-else Statement

In the if statement, when the conditional expression is logical TRUE, the computer will jump to the statements controlled by the if statement and execute them right away. If the expression is false, the computer will ignore those statements controlled by the if statement.

From time to time, you will want the computer to execute some other specified statements when the conditional expression of the if statement is logical FALSE. By doing so, you can use another conditional branching statement in C—the if-else statement.

As an expansion of the if statement, the if-else statement has the following form:

if (expression) {

   statement1;

   statement2;

   .

   .

   .

}

else {

   statement_A;

   statement_B;

   .

   .

   .

}

Here if expression is logical TRUE, the statements controlled by if, including statement1 and statement2, are executed. The statements, such as statement_A and statement_B, inside the statement block and following the else keyword are executed if expression is not logical TRUE.

The program in Listing 10.2 shows how to use the if-else statement.

TYPE
Listing 10.2. Using the if-else statement.

1:  /* 10L02.c  Using the if-else statement */

2:  #include <stdio.h>

3:

4:  main()

5:  {

6:     int i;

7:

8:     printf("Even Number   Odd Number\n");

9:     for (i=0; i<10; i++)

10:       if (i%2 == 0)

11:          printf("%d", i);

12:       else

13:          printf("%14d\n", i);

14:

15:    return 0;

16: }

OUTPUT
The following result is obtained by running the executable file 10L02.exe:

C:\app>10L02

Even Number   Odd Number

0             1

2             3

4             5

6             7

8             9

C:\app>

ANALYSIS
Line 6 of Listing 10.2 declares an integer variable, i. The printf() function in line 8 displays a headline on the screen.

The integer variable i is initialized in the first expression field of the for statement in line 9. Controlled by the for statement, the if-else statement in lines 10_13 is executed 10 times. According to the if-else statement, the printf() function in line 11 prints out even numbers if the relational expression i%2 == 0 in line 10 returns 1 (that is, TRUE). If the relational expression returns 0 (that is, FALSE), the printf() function controlled by the else keyword in line 12 outputs odd numbers to the standard output.

Because the if-else statement is treated as a single statement, the braces { and } are not needed to form a block of statement in the for statement. Likewise, there are no braces used in the if-else statement because the if and else keywords each control a single statement, respectively, in lines 11 and 13.

Note that the minimum width of 14 is specified in the printf() function in line 13, so the output of the odd numbers is listed to the right side of the even numbers, as you can see in the output section. Because the program in Listing 10.2 checks numbers in a range of 0 to 9, the output shows that 0, 2, 4, 6, and 8 are even numbers, and 1, 3, 5, 7, and 9 are odd ones.

Nested if Statements

As you saw in the previous sections, one if statement enables a program to make one decision. In many cases, a program has to make a series of decisions. To enable it to do so, you can use nested if statements.

Listing 10.3 demonstrates the usage of nested if statements.

TYPE
Listing 10.3. Using nested if statements.

1:  /* 10L03.c  Using nested if statements */

2:  #include <stdio.h>

3:

4:  main()

5:  {

6:     int i;

7:

8:     for (i=-5; i<=5; i++){

9:        if (i > 0)

10:           if (i%2 == 0)

11:               printf("%d is an even number.\n", i);

12:           else

13:               printf("%d is an odd number.\n", i);

14:       else if (i == 0)

15:           printf("The number is zero.\n");

16:       else

17:           printf("Negative number: %d\n", i);

18:    }

19:    return 0;

20: }

OUTPUT
After running the executable file 10L03.exe, I obtain the following output:

C:\app>10L03

Negative number: -5

Negative number: -4

Negative number: -3

Negative number: -2

Negative number: -1

The number is zero.

1 is an odd number.

2 is an even number.

3 is an odd number.

4 is an even number.

5 is an odd number.

C:\app>

ANALYSIS
Listing 10.3 contains a for loop, starting in line 8 and ending in line 18. According to the expressions of the for statement in line 8, any tasks controlled by the for statement are executed up to 11 times.

First, a decision has to be made based on the return value of the relational expression i > 0 in the if statement of line 9. The i > 0 expression is used to test whether the value of i is positive or negative (including zero.) If the return value is 1, the computer jumps to the second (that is, nested) if statement in line 10.

Note that line 10 contains another relational expression, i%2 == 0, which tests whether the integer variable i is even or odd. Therefore, the second decision of displaying even numbers or odd numbers has to be made according to the return value of the second relational expression, i%2 == 0. The printf() function in line 11 prints out an even number if the return value is 1. Otherwise, the statement in line 13 is executed, and an odd number is shown on the screen.