Selective Control Statements

By default a C program runs its statements one after another, from top to bottom. This straight-line flow is called sequential execution. Many problems, however, need the program to choose between alternatives, skip some statements, or repeat a block. The statements that let a program depart from the plain top-to-bottom order are called control statements (also called control structures); they define the order in which the statements of a program are executed.

C's control statements fall into two broad groups:

1. Conditional (selective) control statements — they carry out a logical test whose result is either true or false, and then transfer control to different statements depending on that result. Because the program makes a decision and branches accordingly, this is described as decision making and branching.
2. Iterative (looping) statements — they repeat a block of statements a number of times (covered in the next section).

This section covers the four conditional control statements on the Karnataka 2nd PUC Electronics syllabus: the if, if-else, nested-if and switch statements. In every case the condition is a logical expression that yields true or false, and the statement it controls may be a single (simple) statement or a compound statement — a group of two or more statements enclosed in braces.

[!NOTE]

Several of the program listings printed in this KTBS textbook carry typographical slips (missing or stray semicolons, a mismatched quote) and a few logic slips; many of these are fixed in the board's official corrigendum. The code shown below is written in corrected, compilable form, and wherever a correction changes what the program actually does, it is pointed out.

if-statement

The if-statement executes a statement or a block of statements only when a condition is satisfied. Since control takes just one of two paths — perform the block, or skip it — it is called one-way branching.

if (condition)
{
    statement;
}

The condition is a logical expression that results in true or false. If the condition is true the statement runs; if it is false, control passes straight to the next statement in the program. The statement may be simple (one statement) or compound (several statements in braces).

For example, to print a number only when it is even, the test should ask whether the remainder after dividing by 2 is zero:

if ((numb % 2) == 0)
    printf("%d is an even number\n", numb);

[!NOTE]

The textbook's own version of this example writes the test as (numb % 2) != 0 while still printing "even number". That condition is actually true for odd numbers, so the printed logic is inverted. The board's corrigendum corrects the test to check for a zero remainder (an even number), as shown above.

if-else statement

When one of two alternative actions is to be carried out, the if-else statement is used — two-way branching.

if (condition)
{
    statement1;
}
else
{
    statement2;
}

If the logical test is true, statement1 executes; otherwise statement2 executes. After whichever branch runs, control continues with the statement that follows the if-else. A typical use is to check whether two numbers are equal, or to accept a character and respond differently for one particular key.

[!NOTE]

In the textbook's "check whether two numbers are equal" example the true branch mistakenly repeats the input prompt, printf("Enter the values of M, N\n");, instead of announcing the result — yet its sample run correctly shows "M and N are equal". The listing and its output disagree. The corrigendum corrects the true branch to printf("M and N are equal\n");.

Nested-if statement

When there are more than two alternatives to select from, one if-statement is placed inside another, most often as an else-if ladder. A nested-if is simply an if statement enclosed within another if statement.

if (condition1)
{
    if (condition2)
        statement1;
    else
        statement2;
}
else if (condition3)
    statement3;
else
    statement4;

Reading the ladder: statement1 runs when both condition1 and condition2 are true; statement2 runs when condition1 is true but condition2 is false; if condition1 is false, control drops to condition3, running statement3 when it is true and statement4 otherwise. Nested-if is how a program picks the largest of three integers, or sorts a measured value (such as a person's height) into one of several ranges.

switch statement

The switch statement selects one task from among several, based on the value of an integer or character expression — multiple-way branching. It is usually clearer than a long else-if ladder when the choice depends on a single variable.

switch (expression)
{
    case label1:
        block1;
        break;
    case label2:
        block2;
        break;
    /* ... more cases ... */
    default:
        default_block;
}
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