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Conditional
Execution
Objectives
Having read this section you should be able to:
- Program control with if ,
if-else and switch
structures - have a better idea of what C understands as true and
false.
Program Control
It is time to turn our attention to a different problem -
conditional execution. We often need to be able to choose
which set of instructions are obeyed according to a condition.
For example, if you're keeping a total and you need to display
the message 'OK' if the value is greater than
zero you would need to write something like:
if (total>O) printf("OK");
This is perfectly reasonable English, if somewhat terse, but
it is also perfectly good C. The if
statement allows you to evaluate a > condition and only carry out the statement, or
compound statement, that follows if the
condition is true. In other words the
printf will only be obeyed if the
condition total > O is
true.
If the condition is false then the program continues with the
next instruction. In general the if statement is
of the following form:
if (condition) statement;
and of course the statement can be a
compound statement.
Here's an example program using two if
statements:
#include <stdio.h>
main()
{
int a , b;
do {
printf("\nEnter first number: ");
scanf("%d" , &a);
printf("\nEnter second number: ");
scanf("%d" , &b);
if (a<b) printf("\n\nFirst number is less than second\n\n");
if (b<a) printf("Second number is less than first\n\n");
} while (a < 999);
}
[program]
Here's another program using an if keyword and
a compound statement or a block:
#include <stdio.h>
main()
{
int a , b;
do {
printf("\nEnter first number: ");
scanf("%d" , &a);
printf("\nEnter second number: ");
scanf("%d" , &b);
if (a<b) {
printf("\n\nFirst number is less than second\n");
printf("Their difference is : %d\n" , b-a);
printf("\n");
}
printf("\n");
} while (a < 999);
}
[program]
The if statement lets you execute or skip an
instruction depending on the value of the
condition. Another possibility is that you
might want to select one of two possible statements - one to be
obeyed when the condition is true and one to be obeyed
when the condition is false. You
can do this using the
if (condition) statement1;
else statement2;
form of the if statement.
In this case statement1 is carried out if the
condition is true and
statement2 if the
condition is false.
Notice that it is certain that one of the two statements will
be obeyed because the condition has to be
either true or false! You may be puzzled by the
semicolon at the end of the if part of the
statement. The if (condition) statement1
part is one statement and the else statement2
part behaves like a second separate statement, so there has to be
semi-colon terminating the first statement.
Logical Expressions
So far we have assumed that the way to write the
conditions used in loops and
if statements is so obvious that we don't need to
look more closely. In fact there are a number of deviations from
what you might expect. To compare two values you can use the
standard symbols:
| > | (greater than) |
| < | (less than) |
| >= | (for greater than or equal to ) |
| <= | (for less than or equal to) |
| == | (to test for equality) |
The reason for using two equal signs for equality is that the
single equals sign always means store a value in a variable -
i.e. it is the assignment operator. This causes beginners lots of
problems because they tend to write:
if (a = 10) instead of if (a ==
10)
The situation is made worse by the fact that the statement
if (a = 10) is legal and causes no compiler error
messages! It may even appear to work at first because, due to a
logical quirk of C, the assignment actually evaluates to
the value being assigned and a non-zero value is treated as
true (see below). Confused? I agree it is confusing, but
it gets easier. . .
Just as the equals condition is written differently from what
you might expect so the non-equals sign looks a little odd. You
write not equals as !=. For example:
if (a != 0)
is 'if a is not equal to zero'.
An example program showing the if else
construction now follows:
#include <stdio.h>
main ()
{
int num1, num2;
printf("\nEnter first number ");
scanf("%d",&num1);
printf("\nEnter second number ");
scanf("%d",&num2);
if (num2 ==0) printf("\n\nCannot devide by zero\n\n");
else printf("\n\nAnswer is %d\n\n",num1/num2);
}
[program]
This program uses an if and
else statement to prevent division by 0 from
occurring.
True and False in C
Now we come to an advanced trick which you do need to know about,
but if it only confuses you, come back to this bit later. Most
experienced C programmers would wince at the expression
if(a!=0).
The reason is that in the C programming language
dosen't have a concept of a Boolean variable, i.e. a type class
that can be either true or false. Why bother when
we can use numerical values. In C true is
represented by any numeric value not equal to 0 and false
is represented by 0. This fact is usually well hidden and can be
ignored, but it does allow you to write
if(a != 0) just as if(a)
because if a isn't zero then this also acts as
the value true. It is debatable if this sort of shortcut
is worth the three characters it saves. Reading something
like
if(!done)
as 'if not done' is clear, but
if(!total) is more dubious.
Using break and continue Within Loops
The break statement allows you to exit a loop
from any point within its body, bypassing its normal termination
expression. When the break statement is
encountered inside a loop, the loop is immediately terminated, and
program control resumes at the next statement following the loop.
The break statement can be used with all three of C's loops. You can have as many statements within a loop
as you desire. It is generally best to use the
break for special purposes, not as your normal
loop exit. break is also used in conjunction with
functions and case statements which will be
covered in later sections.
The continue statement is somewhat the
opposite of the break statement. It forces the
next iteration of the loop to take place, skipping any code in
between itself and the test condition of the loop. In
while and do-while loops, a
continue statement will cause control to
go directly to the test condition and then continue the looping
process. In the case of the for loop, the
increment part of the loop continues. One good use of
continue is to restart a statement sequence when
an error occurs.
#include <stdio.h>
main()
{
int x ;
for ( x=0 ; x<=100 ; x++) {
if (x%2) continue;
printf("%d\n" , x);
}
}
[program]
Here we have used C's modulus operator:
%. A expression:
a % b
produces the remainder when a is divided by
b; and zero when there is no remainder.
Here's an example of a use for the break
statement:
#include <stdio.h>
main()
{
int t ;
for ( ; ; ) {
scanf("%d" , &t) ;
if ( t==10 ) break ;
}
printf("End of an infinite loop...\n");
}
[program]
Select Paths with switch
While if is good for choosing between two
alternatives, it quickly becomes cumbersome when several
alternatives are needed. C's solution to this problem is
the switch statement. The switch
statement is C's multiple selection statement. It is used
to select one of several alternative paths in program execution
and works like this: A variable is successively tested against a
list of integer or character constants. When a match is found,
the statement sequence associated with the match is executed. The
general form of the switch statement is:
switch(expression)
{
case constant1: statement sequence; break;
case constant2: statement sequence; break;
case constant3: statement sequence; break;
.
.
.
default: statement sequence; break;
}
Each case is labelled by one, or more, constant expressions
(or integer-valued constants). The default
statement sequence is performed if no matches are found. The
default is optional. If all matches fail and
default is absent, no action takes place.
When a match is found, the statement
sequence associated with that case
are executed until break is encountered.
An example program follows:
#include <stdio.h>
main()
{
int i;
printf("Enter a number between 1 and 4");
scanf("%d",&i);
switch (i)
{
case 1:
printf("one");
break;
case 2:
printf("two");
break;
case 3:
printf("three");
break;
case 4:
printf("four");
break;
default:
printf("unrecognized number");
} /* end of switch */
}
[program]
This simple program recognizes the numbers 1 to 4 and prints
the name of the one you enter. The switch
statement differs from if, in that
switch can only test for equality, whereas the
if conditional expression can be of any type.
Also switch will work with only
int and char types. You cannot
for example, use floating-point numbers. If the statement
sequence includes more than one statement they will have to be
enclosed with {} to form a compound statement.
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