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In
C++, zero is considered false, and all other values
are considered true, although true is usually represented
by 1. Thus, if an expression is false, it is equal
to zero, and if an expression is equal to zero,
it is false. If a statement is true, all you know
is that it is nonzero, and any nonzero statement
is true.
Relational
Operators
The
relational operators are used to determine whether
two numbers are equal, or if one is greater or less
than the other. Every relational statement evaluates
to either 1 (TRUE) or 0 (FALSE). The relational
operators are presented later, in Table 4.1.
If
the integer variable myAge has the value 39, and
the integer variable yourAge has the value 40, you
can determine whether they are equal by using the
relational "equals" operator:
myAge
== yourAge; // is the value in myAge the same
as in yourAge?
This
expression evaluates to 0, or false, because the
variables are not equal. The expression
myAge
> yourAge; // is myAge greater than yourAge?
evaluates
to 0 or false.
WARNING: Many novice C++ programmers
confuse the assignment operator (=) with
the equals operator (==). This can create
a nasty bug in your program.
There
are six relational operators: equals (==),
less than (<), greater than (>),
less than or equal to (<=), greater than
or equal to (>=), and not equals (!=).
Table 4.1 shows each relational operator, its use,
and a sample code use.
Table 4.1. The
Relational Operators.
|
Name
Operator
|
Sample
Evaluates
|
|
Equals
==
|
100
== 50; false
50 == 50; true
|
|
Not
Equals !=
|
100
!= 50; true
50 != 50; false
|
|
Greater
Than >
|
100
> 50; true
50 > 50; false
|
|
Greater
Than >=
|
100
>= 50; true
or Equals 50 >= 50; true
|
|
Less
Than < 1
|
00
< 50; false
50 < 50; false
|
|
Less
Than <=
|
100
<= 50; false
or Equals 50 <= 50; true
|
DO
remember that relational operators return the value
1 (true) or 0 (false). DON’T confuse the
assignment operator (=) with the equals relational
operator (==). This is one of the most common C++
programming mistakes—be on guard for it.
The
if Statement
Normally,
your program flows along line by line in the order
in which it appears in your source code. The if
statement enables you to test for a condition (such
as whether two variables are equal) and branch to
different parts of your code, depending on the result.
The
simplest form of an if statement is this:
|
if
(expression)
statement;
|
The
expression in the parentheses can be any expression
at all, but it usually contains one of the relational
expressions. If the expression has the value 0,
it is considered false, and the statement is skipped.
If it has any nonzero value, it is considered true,
and the statement is executed. Consider the following
example:
if
(bigNumber > smallNumber)
bigNumber
= smallNumber;
This
code compares bigNumber and smallNumber. If bigNumber
is larger, the second line sets its value to the
value of smallNumber.
Because
a block of statements surrounded by braces is exactly
equivalent to a single statement, the following
type of branch can be quite large and powerful:
|
if
(expression)
{
statement1;
statement2;
statement3;
}
|
Here’s
a simple example of this usage:
if
(bigNumber > smallNumber)
{
bigNumber
= smallNumber;
cout
<< "bigNumber: " << bigNumber <<
"\n";
cout
<< "smallNumber: " << smallNumber
<< "\n";
}
This
time, if bigNumber is larger than smallNumber, not
only is it set to the value of smallNumber, but
an informational message is printed. Listing 4.4
shows a more detailed example of branching based
on relational operators.
Listing
4.4. A demonstration of branching based on relational
operators.
1:
// Listing 4.4 - demonstrates if statement
2:
// used with relational operators
3:
#include <iostream.h>
4:
int main()
5:
{
6:
int RedSoxScore, YankeesScore;
7:
cout << "Enter the score for the Red Sox:
";
8:
cin >> RedSoxScore;
9:
10:
cout << "\nEnter the score for the Yankees:
";
11:
cin >> YankeesScore;
12:
13:
cout << "\n";
14:
15:
if (RedSoxScore > YankeesScore)
16:
cout << "Go Sox!\n";
17:
18:
if (RedSoxScore < YankeesScore)
19:
{
20:
cout << "Go Yankees!\n";
21:
cout << "Happy days in New York!\n";
22:
}
23:
24:
if (RedSoxScore == YankeesScore)
25:
{
26:
cout << "A tie? Naah, can’t be.\n";
27:
cout << "Give me the real score for the Yanks:
";
28:
cin >> YankeesScore;
29:
30:
if (RedSoxScore > YankeesScore)
31:
cout << "Knew it! Go Sox!";
32:
33:
if (YankeesScore > RedSoxScore)
34:
cout << "Knew it! Go Yanks!";
35:
36:
if (YankeesScore == RedSoxScore)
37:
cout << "Wow, it really was a tie!";
38:
}
39:
40:
cout << "\nThanks for telling me.\n";
41:
return 0;
42:
}
Output:
Enter the score for the Red Sox: 10
Enter the score for the Yankees: 10
A tie? Naah, can’t be
Give me the real score for the Yanks: 8
Knew it! Go Sox!
Thanks for telling me.
Analysis: This program asks for user
input of scores for two baseball teams, which are
stored in integer variables. The variables are compared
in the if statement on lines 15, 18, and 24.
If one score is higher than the other, an informational
message is printed. If the scores are equal, the
block of code that begins on line 24 and ends on
line 38 is entered. The second score is requested
again, and then the scores are compared again.
Note
that if the initial Yankees score was higher than
the Red Sox score, the if statement on line 15 would
evaluate as FALSE, and line 16 would not be invoked.
The test on line 18 would evaluate as true, and
the statements on lines 20 and 21 would be invoked.
Then the if statement on line 24 would be tested,
and this would be false (if line 18 was true). Thus,
the program would skip the entire block, falling
through to line 39.
In
this example, getting a true result in one if statement
does not stop other if statements from being tested.
Indentation
Styles
Listing
4.3 shows one style of indenting if statements.
Nothing is more likely to create a religious war,
however, than to ask a group of programmers what
is the best style for brace alignment. Although
there are dozens of variations, these appear to
be the favorite three:
·
Putting the initial brace after the condition
and aligning the closing brace under the if
to close the statement block.
if
(expression){
statements
}
·
Aligning the braces under the if and indenting
the statements.
if
(expression)
{
statements
}
·
Indenting the braces and statements.
if
(expression)
{
statements
}
This
tutorial uses the middle alternative, because I
find it easier to understand where blocks of statements
begin and end if the braces line up with each other
and with the condition being tested. Again, it doesn’t
matter much which style you choose, as long as you
are consistent with it.
else
Often
your program will want to take one branch if your
condition is true, another if it is false. In Listing
4.3, you wanted to print one message (Go Sox!) if
the first test (RedSoxScore > Yankees) evaluated
TRUE, and another message (Go Yanks!) if it evaluated
FALSE.
The
method shown so far, testing first one condition
and then the other, works fine but is a bit cumbersome.
The keyword else can make for far more readable
code:
|
if
(expression)
statement;
else
statement;
|
Listing
4.5. Demonstrating the else keyword.
1:
// Listing 4.5 - demonstrates if statement
2:
// with else clause
3:
#include <iostream.h>
4:
int main()
5:
{
6:
int firstNumber, secondNumber;
7:
cout << "Please enter a big number: ";
8:
cin >> firstNumber;
9:
cout << "\nPlease enter a smaller number:
";
10:
cin >> secondNumber;
11:
if (firstNumber > secondNumber)
12:
cout << "\nThanks!\n";
13:
else
14:
cout << "\nOops. The second is bigger!";
15:
16:
return 0;
17:
}
Output:
Please enter a big number: 10
Please enter a smaller number: 12
Oops. The second is bigger!
Analysis: The if statement on line
11 is evaluated. If the condition is true, the statement
on line 12 is run; if it is false, the statement
on line 14 is run. If the else clause on line 13
were removed, the statement on line 14 would run
whether or not the if statement was true. Remember,
the if statement ends after line 12. If the else
was not there, line 14 would just be the next line
in the program.
Remember
that either or both of these statements could be
replaced with a block of code in braces.
The
if Statement
The
syntax for the if statement is as follows: Form
1
|
if
(expression)
statement;
next
statement;
|
If
the expression is evaluated as TRUE, the statement
is executed and the program continues with the next
statement. If the expression is not true, the statement
is ignored and the program jumps to the next statement.
Remember that the statement can be a single statement
ending with a semicolon or a block enclosed in braces.
Form 2
|
if
(expression)
statement1;
else
statement2;
next
statement;
|
If
the expression evaluates TRUE, statement1 is executed;
otherwise, statement2 is executed. Afterwards, the
program continues with the next statement. Example
1
Example
if
(SomeValue < 10)
cout
<< "SomeValue is less than 10");
else
cout
<< "SomeValue is not less than 10!");
cout
<< "Done." << endl;
Advanced
if Statements
It
is worth noting that any statement can be used in
an if or else clause, even another if or else statement.
Thus, you might see complex if statements in the
following form:
|
if
(expression1)
{
if
(expression2)
statement1;
else
{
if
(expression3)
statement2;
else
statement3;
}
}
else
statement4;
|
This
cumbersome if statement says, "If expression1 is
true and expression2 is true, execute statement1.
If expression1 is true but expression2 is not true,
then if expression3 is true execute statement2.
If expression1 is true but expression2 and expression3
are false, execute statement3. Finally, if expression1
is not true, execute statement4." As you can see,
complex if statements can be confusing!
Listing
4.6 gives an example of such a complex if statement.
Listing
4.6. A complex, nested if statement.
1:
// Listing 4.5 - a complex nested
2:
// if statement
3:
#include <iostream.h>
4:
int main()
5:
{
6:
// Ask for two numbers
7:
// Assign the numbers to bigNumber and littleNumber
8:
// If bigNumber is bigger than littleNumber,
9:
// see if they are evenly divisible
10:
// If they are, see if they are the same number
11:
12:
int firstNumber, secondNumber;
13:
cout << "Enter two numbers.\nFirst: ";
14:
cin >> firstNumber;
15:
cout << "\nSecond: ";
16:
cin >> secondNumber;
17:
cout << "\n\n";
18:
19:
if (firstNumber >= secondNumber)
20:
{
21:
if ( (firstNumber % secondNumber) == 0) // evenly
divisible?
22:
{
23:
if (firstNumber == secondNumber)
24:
cout << "They are the same!\n";
25:
else
26:
cout << "They are evenly divisible!\n";
27:
}
28:
else
29:
cout << "They are not evenly divisible!\n";
30:
}
31:
else
32:
cout << "Hey! The second one is larger!\n";
33:
return 0;
34:
}
Output:
Enter two numbers.
First: 10
Second: 2
They are evenly divisible!
Analysis: Two numbers are prompted
for one at a time, and then compared. The first
if statement, on line 19, checks to ensure that
the first number is greater than or equal to the
second. If not, the else clause on line 31 is executed.
If the first if is true, the block of code beginning
on line 20 is executed, and the second if statement
is tested, on line 21. This checks to see whether
the first number modulo the second number yields
no remainder. If so, the numbers are either evenly
divisible or equal. The if statement on line 23
checks for equality and displays the appropriate
message either way.
If
the if statement on line 21 fails, the else statement
on line 28 is executed.
Using
Braces in Nested if Statements
Although
it is legal to leave out the braces on if statements
that are only a single statement, and it is legal
to nest if statements, such as
if
(x > y) // if x is bigger than y
if
(x < z) // and if x is smaller than z
x
= y; // then set x to the value in z
when
writing large nested statements, this can cause
enormous confusion. Remember, whitespace and indentation
are a convenience for the programmer; they make
no difference to the compiler. It is easy to confuse
the logic and inadvertently assign an else statement
to the wrong if statement. Listing 4.7 illustrates
this problem.
Listing
4.7. A demonstration of why braces help clarify
which else statement goes with which if statement.
////1:
// Listing 4.7 - demonstrates why braces
2:
// are important in nested if statements
3:
#include <iostream.h>
4:
int main()
5:
{
6:
int x;
7:
cout << "Enter a number less than 10 or greater
than 100: ";
8:
cin >> x;
9:
cout << "\n";
10:
11:
if (x > 10)
12:
if (x > 100)
13:
cout << "More than 100, Thanks!\n";
14:
else // not the else intended!
15:
cout << "Less than 10, Thanks!\n";
16:
17:
return 0;
18:
}
Output:
Enter a number less than 10 or greater than 100:
20
Less than 10, Thanks!
Analysis: The programmer intended to
ask for a number between 10 and 100, check for the
correct value, and then print a thank-you note.
If the if statement on line 11 evaluates TRUE, the
following statement (line 12) is executed. In this
case, line 12 executes when the number entered is
greater than 10. Line 12 contains an if statement
also. This if statement evaluates TRUE if the number
entered is greater than 100. If the number is not
greater than 100, the statement on line 13 is executed.
If
the number entered is less than or equal to 10,
the if statement on line 10 evaluates to
FALSE. Program control goes to the next line
following the if statement, in this case
line 16. If you enter a number less than 10, the
output is as follows:
Enter a number less than 10 or greater than
100: 9
The
else clause on line 14 was clearly intended
to be attached to the if statement on line
11, and thus is indented accordingly. Unfortunately,
the else statement is really attached to
the if statement on line 12, and thus this
program has a subtle bug.
It
is a subtle bug because the compiler will not complain.
This is a legal C++ program, but it just doesn’t
do what was intended. Further, most of the times
the programmer tests this program, it will appear
to work. As long as a number that is greater than
100 is entered, the program will seem to work just
fine.
Listing 4.8 fixes the problem by putting in the
necessary braces.
Listing
4.8. A demonstration of the proper use of braces
with an if statement
1:
// Listing 4.8 - demonstrates proper use of braces
2:
// in nested if statements
3:
#include <iostream.h>
4:
int main()
5:
{
6:
int x;
7:
cout << "Enter a number less than 10 or greater
than 100: ";
8:
cin >> x;
9:
cout << "\n";
10:
11:
if (x > 10)
12:
{
13:
if (x > 100)
14:
cout << "More than 100, Thanks!\n";
15:
}
16:
else // not the else intended!
17:
cout << "Less than 10, Thanks!\n";
18:
return 0;
19:
}
Output:
Enter a number less than 10 or greater than 100:
20
Analysis: The braces on lines 12 and
15 make everything between them into one statement,
and now the else on line 16 applies to the if on
line 11 as intended. The user typed 20, so the if
statement on line 11 is true; however, the if statement
on line 13 is false, so nothing is printed. It would
be better if the programmer put another else clause
after line 14 so that errors would be caught and
a message printed.
NOTE: The programs shown in this book
are written to demonstrate the particular issues
being discussed. They are kept intentionally simple;
there is no attempt to "bulletproof" the code to
protect against user error. In professional-quality
code, every possible user error is anticipated and
handled gracefully.
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