This chapter is from the book
This chapter is from the book
This chapter is from the book
Decisions
The decisions in a loop always relate to the same question: Should we do this (the loop body) again? As humans we would find it boring to be so limited in our decision-making processes. We need to decide not only whether to go to work again today (continuing the loop), but also whether to buy a red shirt or a green one (or no shirt at all), whether to take a vacation, and if so, in the mountains or by the sea.
Programs also need to make these one-time decisions. In a program a decision causes a one-time jump to a different part of the program, depending on the value of an expression. Decisions can be made in C++ in several ways. The most important is with the if...else statement, which chooses between two alternatives. This statement can be used without the else, as a simple if statement. Another decision statement, switch, creates branches for multiple alternative sections of code, depending on the value of a single variable. Finally, the conditional operator is used in specialized situations. We'll examine each of these constructions.
The if Statement
The if statement is the simplest of the decision statements. Our next program, ifdemo, provides an example.
// ifdemo.cpp
// demonstrates IF statement
#include <iostream>
using namespace std;
int main()
{
int x;
cout << "Enter a number: ";
cin >> x;
if( x > 100 )
cout << "That number is greater than 100\n";
return 0;
}
The if keyword is followed by a test expression in parentheses. The syntax of the if statement is shown in Figure 3.7. As you can see, the syntax of if is very much like that of while. The difference is that the statements following the if are executed only once if the test expression is true; the statements following while are executed repeatedly until the test expression becomes false. Figure 3.8 shows the operation of the if statement.
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Here's an example of the ifdemo program's output when the number entered by the user is greater than 100:
Enter a number: 2000 That number is greater than 100
If the number entered is not greater than 100, the program will terminate without printing the second line.
Figure 3.7 Syntax of the if statement.
Figure 3.8 Operation of the if statement.
Multiple Statements in the if Body
As in loops, the code in an if body can consist of a single statement—as shown in the ifdemo example—or a block of statements delimited by braces. This variation on ifdemo, called if2, shows how that looks.
// if2.cpp
// demonstrates IF with multiline body
#include <iostream>
using namespace std;
int main()
{
int x;
cout << "Enter a number: ";
cin >> x;
if( x > 100 )
{
cout << "The number " << x;
cout << " is greater than 100\n";
}
return 0;
}
Here's some output from if2:
Enter a number: 12345 The number 12345 is greater than 100
Nesting ifs Inside Loops
The loop and decision structures we've seen so far can be nested inside one another. You can nest ifs inside loops, loops inside ifs, ifs inside ifs, and so on. Here's an example, prime, that nests an if within a for loop. This example tells you whether a number you enter is a prime number. (Prime numbers are integers divisible only by themselves and 1. The first few primes are 2, 3, 5, 7, 11, 13, 17.)
// prime.cpp
// demonstrates IF statement with prime numbers
#include <iostream>
using namespace std;
#include <process.h> //for exit()
int main()
{
unsigned long n, j;
cout << "Enter a number: ";
cin >> n; //get number to test
for(j=2; j <= n/2; j++) //divide by every integer from
if(n%j == 0) //2 on up; if remainder is 0,
{ //it's divisible by j
cout << "It's not prime; divisible by " << j << endl;
exit(0); //exit from the program
}
cout << "It's prime\n";
return 0;
}
In this example the user enters a number that is assigned to n. The program then uses a for loop to divide n by all the numbers from 2 up to n/2. The divisor is j, the loop variable. If any value of j divides evenly into n, then n is not prime. When a number divides evenly into another, the remainder is 0; we use the remainder operator % in the if statement to test for this condition with each value of j. If the number is not prime, we tell the user and we exit from the program.
Here's output from three separate invocations of the program:
Enter a number: 13 It's prime Enter a number: 22229 It's prime Enter a number: 22231 It's not prime; divisible by 11
Notice that there are no braces around the loop body. This is because the if statement, and the statements in its body, are considered to be a single statement. If you like you can insert braces for readability, even though the compiler doesn't need them.
Library Function exit()
When prime discovers that a number is not prime, it exits immediately, since there's no use proving more than once that a number isn't prime. This is accomplished with the library function exit(). This function causes the program to terminate, no matter where it is in the listing. It has no return value. Its single argument, 0 in our example, is returned to the operating system when the program exits. (This value is useful in batch files, where you can use the errorlevel value to query the return value provided by exit(). The value 0 is normally used for a successful termination; other numbers indicate errors.)
The if...else Statement
The if statement lets you do something if a condition is true. If it isn't true, nothing happens. But suppose we want to do one thing if a condition is true, and do something else if it's false. That's where the if...else statement comes in. It consists of an if statement, followed by a statement or block of statements, followed by the keyword else, followed by another statement or block of statements. The syntax is shown in Figure 3.9.
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Figure 3.9 Syntax of the if...else statement.
Here's a variation of our if example, with an else added to the if:
// ifelse.cpp
// demonstrates IF...ELSE statememt
#include <iostream>
using namespace std;
int main()
{
int x;
cout << "\nEnter a number: ";
cin >> x;
if( x > 100 )
cout << "That number is greater than 100\n";
else
cout << "That number is not greater than 100\n";
return 0;
}
If the test expression in the if statement is true, the program prints one message; if it isn't, it prints the other.
Here's output from two different invocations of the program:
Enter a number: 300 That number is greater than 100 Enter a number: 3 That number is not greater than 100
The operation of the if...else statement is shown in Figure 3.10.
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Figure 3.10 Operation of the if...else statement.
The getche() Library Function
Our next example shows an if...else statement embedded in a while loop. It also introduces a new library function: getche(). This program, chcount, counts the number of words and the number of characters in a phrase typed in by the user.
// chcount.cpp
// counts characters and words typed in
#include <iostream>
using namespace std;
#include <conio.h> //for getche()
int main()
{
int chcount=0; //counts non-space characters
int wdcount=1; //counts spaces between words
char ch = 'a'; //ensure it isn't '\r'
cout << "Enter a phrase: ";
while( ch != '\r' ) //loop until Enter typed
{
ch = getche(); //read one character
if( ch==' ' ) //if it's a space
wdcount++; //count a word
else //otherwise,
chcount++; //count a character
} //display results
cout << "\nWords=" << wdcount << endl
<< "Letters=" << (chcount-1) << endl;
return 0;
}
So far we've used only cin and >> for input. That approach requires that the user always press the Enter key to inform the program that the input is complete. This is true even for single characters: The user must type the character, then press Enter. However, as in the present example, a program often needs to process each character typed by the user without waiting for an Enter. The getche() library function performs this service. It returns each character as soon as it's typed. It takes no arguments, and requires the conio.h header file. In chcount the value of the character returned from getche() is assigned to ch. (The getche() function echoes the character to the screen. That's why there's an e at the end of getche. Another function, getch(), is similar to getche() but doesn't echo the character to the screen.)
The if...else statement causes the word count wdcount to be incremented if the character is a space, and the character count chcount to be incremented if the character is anything but a space. Thus anything that isn't a space is assumed to count as a character. (Note that this program is fairly naïve; it will be fooled by multiple spaces between words.)
Here's some sample interaction with chcount:
For while and do Words=4 Letters=13
The test expression in the while statement checks to see if ch is the '\r' character, which is the character received from the keyboard when the Enter key is pressed. If so, the loop and the program terminate.
Assignment Expressions
The chcount program can be rewritten to save a line of code and demonstrate some important points about assignment expressions and precedence. The result is a construction that looks rather peculiar but is commonly used in C++ (and in C).
Here's the rewritten version, called chcnt2:
// chcnt2.cpp
// counts characters and words typed in
#include <iostream>
using namespace std;
#include <conio.h> // for getche()
int main()
{
int chcount=0;
int wdcount=1; // space between two words
char ch;
while( (ch=getche()) != '\r' ) // loop until Enter typed
{
if( ch==' ' ) // if it's a space
wdcount++; // count a word
else // otherwise,
chcount++; // count a character
} // display results
cout << "\nWords=" << wdcount << endl
<< "Letters=" << chcount << endl;
return 0;
}
The value returned by getche() is assigned to ch as before, but this entire assignment expression has been moved inside the test expression for while. The assignment expression is compared with '\r' to see whether the loop should terminate. This works because the entire assignment expression takes on the value used in the assignment. That is, if getche() returns 'a', then not only does ch take on the value 'a', but the expression
(ch=getche())
also takes on the value 'a'. This is then compared with '\r'.
The fact that assignment expressions have a value is also used in statements such as
x = y = z = 0;
This is perfectly legal in C++. First, z takes on the value 0, then z = 0 takes on the value 0, which is assigned to y. Then the expression y = z = 0 likewise takes on the value 0, which is assigned to x.
The parentheses around the assignment expression in
(ch=getche())
are necessary because the assignment operator = has a lower precedence than the relational operator !=. Without the parentheses the expression would be evaluated as
while( ch = (getche() != '\r') ) // not what we want
which would assign a true or false value to ch (not what we want).
The while statement in chcnt2 provides a lot of power in a small space. It is not only a test expression (checking ch to see whether it's '\r'); it also gets a character from the keyboard and assigns it to ch. It's also not easy to unravel the first time you see it.
Nested if...else Statements
You're probably too young to remember adventure games on early character-mode MS-DOS systems, but let's resurrect the concept here. You moved your "character" around an imaginary landscape and discovered castles, sorcerers, treasure, and so on, using text—not pictures—for input and output. The next program, adifelse, models a small part of such an adventure game.
// adifelse.cpp
// demonstrates IF...ELSE with adventure program
#include <iostream>
using namespace std;
#include <conio.h> //for getche()
int main()
{
char dir='a';
int x=10, y=10;
cout << "Type Enter to quit\n";
while( dir != '\r' ) //until Enter is typed
{
cout << "\nYour location is " << x << ", " << y;
cout << "\nPress direction key (n, s, e, w): ";
dir = getche(); //get character
if( dir=='n') //go north
y--;
else
if( dir=='s' ) //go south
y++;
else
if( dir=='e' ) //go east
x++;
else
if( dir=='w' ) //go west
x--;
} //end while
return 0;
} //end main
When the game starts, you find yourself on a barren moor. You can go one "unit" north, south, east, or west, while the program keeps track of where you are and reports your position, which starts at coordinates 10,10. Unfortunately, nothing exciting happens to your character, no matter where you go; the moor stretches almost limitlessly in all directions, as shown in Figure 3.11. We'll try to provide a little more excitement to this game later on.
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Here's some sample interaction with adifelse:
Your location is 10, 10 Press direction key (n, s, e, w): n Your location is 10, 9 Press direction key (n, s, e, w): e Your location is 11, 9 Press direction key (n, s, e, w):
You can press the Enter key to exit the program.
This program may not cause a sensation in the video arcades, but it does demonstrate one way to handle multiple branches. It uses an if statement nested inside an if...else statement, which is nested inside another if...else statement, which is nested inside yet another if...else statement. If the first test condition is false, the second one is examined, and so on until all four have been checked. If any one proves true, the appropriate action is taken—changing the x or y coordinate—and the program exits from all the nested decisions. Such a nested group of if...else statements is called a decision tree.
Figure 3.11 The barren moor.
Matching the else
There's a potential problem in nested if...else statements: You can inadvertently match an else with the wrong if. badelse provides an example:
// badelse.cpp
// demonstrates ELSE matched with wrong IF
#include <iostream>
using namespace std;
int main()
{
int a, b, c;
cout << "Enter three numbers, a, b, and c:\n";
cin >> a >> b >> c;
if( a==b )
if( b==c )
cout << "a, b, and c are the same\n";
else
cout << "a and b are different\n";
return 0;
}
We've used multiple values with a single cin. Press Enter following each value you type in; the three values will be assigned to a, b, and c.
What happens if you enter 2, then 3, and then 3? Variable a is 2, and b is 3. They're different, so the first test expression is false, and you would expect the else to be invoked, printing a and b are different. But in fact nothing is printed. Why not? Because the else is matched with the wrong if. The indentation would lead you to believe that the else is matched with the first if, but in fact it goes with the second if. Here's the rule: An else is matched with the last if that doesn't have its own else.
Here's a corrected version:
if(a==b) if(b==c) cout << "a, b, and c are the same\n"; else cout << "b and c are different\n";
We changed the indentation and also the phrase printed by the else body. Now if you enter 2, 3, 3, nothing will be printed. But entering 2, 2, 3 will cause the output
b and c are different
If you really want to pair an else with an earlier if, you can use braces around the inner if:
if(a==b)
{
if(b==c)
cout << "a, b, and c are the same";
}
else
cout << "a and b are different";
Here the else is paired with the first if, as the indentation indicates. The braces make the if within them invisible to the following else.
The else...if Construction
The nested if...else statements in the adifelse program look clumsy and can be hard—for humans—to interpret, especially if they are nested more deeply than shown. However, there's another approach to writing the same statements. We need only reformat the program, obtaining the next example, adelseif.
// adelseif.cpp
// demonstrates ELSE...IF with adventure program
#include <iostream>
using namespace std;
#include <conio.h> //for getche()
int main()
{
char dir='a';
int x=10, y=10;
cout << "Type Enter to quit\n";
while( dir != '\r' ) //until Enter is typed
{
cout << "\nYour location is " << x << ", " << y;
cout << "\nPress direction key (n, s, e, w): ";
dir = getche(); //get character
if( dir=='n') //go north
y--;
else if( dir=='s' ) //go south
y++;
else if( dir=='e' ) //go east
x++;
else if( dir=='w' ) //go west
x--;
} //end while
return 0;
} //end main
The compiler sees this as identical to adifelse, but we've rearranged the ifs so they directly follow the elses. The result looks almost like a new keyword: else if. The program goes down the ladder of else ifs until one of the test expressions is true. It then executes the following statement and exits from the ladder. This format is clearer and easier to follow than the if...else approach.
The switch Statement
If you have a large decision tree, and all the decisions depend on the value of the same variable, you will probably want to consider a switch statement instead of a ladder of if...else or else if constructions. Here's a simple example called platters that will appeal to nostalgia buffs:
// platters.cpp
// demonstrates SWITCH statement
#include <iostream>
using namespace std;
int main()
{
int speed; //turntable speed
cout << "\nEnter 33, 45, or 78: ";
cin >> speed; //user enters speed
switch(speed) //selection based on speed
{
case 33: //user entered 33
cout << "LP album\n";
break;
case 45: //user entered 45
cout << "Single selection\n";
break;
case 78: //user entered 78
cout << "Obsolete format\n";
break;
}
return 0;
}
This program prints one of three possible messages, depending on whether the user inputs the number 33, 45, or 78. As old-timers may recall, long-playing records (LPs) contained many songs and turned at 33 rpm, the smaller 45's held only a single song, and 78s were the format that preceded LPs and 45s.
The keyword switch is followed by a switch variable in parentheses.
switch(speed)
Braces then delimit a number of case statements. Each case keyword is followed by a constant, which is not in parentheses but is followed by a colon.
case 33:
The data type of the case constants should match that of the switch variable. Figure 3.12 shows the syntax of the switch statement.
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Figure 3.12 Syntax of the switch statement.
Before entering the switch, the program should assign a value to the switch variable. This value will usually match a constant in one of the case statements. When this is the case (pun intended!), the statements immediately following the keyword case will be executed, until a break is reached.
Here's an example of platter's output:
Enter 33, 45, or 78: 45 Single selection
The break Statement
platters has a break statement at the end of each case section. The break keyword causes the entire switch statement to exit. Control goes to the first statement following the end of the switch construction, which in platters is the end of the program. Don't forget the break; without it, control passes down (or "falls through") to the statements for the next case, which is usually not what you want (although sometimes it's useful).
If the value of the switch variable doesn't match any of the case constants, control passes to the end of the switch without doing anything. The operation of the switch statement is shown in Figure 3.13. The break keyword is also used to escape from loops; we'll discuss this soon.
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Figure 3.13 Operation of the switch statement.
switch Statement with Character Variables
The platters example shows a switch statement based on a variable of type int. You can also use type char. Here's our adelseif program rewritten as adswitch:
// adswitch.cpp
// demonstrates SWITCH with adventure program
#include <iostream>
using namespace std;
#include <conio.h> //for getche()
int main()
{
char dir='a';
int x=10, y=10;
while( dir != '\r' )
{
cout << "\nYour location is " << x << ", " << y;
cout << "\nEnter direction (n, s, e, w): ";
dir = getche(); //get character
switch(dir) //switch on it
{
case 'n': y--; break; //go north
case 's': y++; break; //go south
case 'e': x++; break; //go east
case 'w': x--; break; //go west
case '\r': cout << "Exiting\n"; break; //Enter key
default: cout << "Try again\n"; //unknown char
} //end switch
} //end while
return 0;
} //end main
A character variable dir is used as the switch variable, and character constants 'n', 's', and so on are used as the case constants. (Note that you can use integers and characters as switch variables, as shown in the last two examples, but you can't use floating-point numbers.)
Since they are so short, the statements following each case keyword have been written on one line, which makes for a more compact listing. We've also added a case to print an exit message when Enter is pressed.
The default Keyword
In the adswitch program, where you expect to see the last case at the bottom of the switch construction, you instead see the keyword default. This keyword gives the switch construction a way to take an action if the value of the loop variable doesn't match any of the case constants. Here we use it to print Try again if the user types an unknown character. No break is necessary after default, since we're at the end of the switch anyway.
A switch statement is a common approach to analyzing input entered by the user. Each of the possible characters is represented by a case.
It's a good idea to use a default statement in all switch statements, even if you don't think you need it. A construction such as
default: cout << "Error: incorrect input to switch"; break;
alerts the programmer (or the user) that something has gone wrong in the operation of the program. In the interest of brevity we don't always include such a default statement, but you should, especially in serious programs.
switch Versus if...else
When do you use a series of if...else (or else if) statements, and when do you use a switch statement? In an else if construction you can use a series of expressions that involve unrelated variables and are as complex as you like. For example:
if( SteamPressure*Factor > 56 ) // statements else if( VoltageIn + VoltageOut < 23000) // statements else if( day==Thursday ) // statements else // statements
In a switch statement, however, all the branches are selected by the same variable; the only thing distinguishing one branch from another is the value of this variable. You can't say
case a<3: // do something break;
The case constant must be an integer or character constant, like 3 or 'a', or an expression that evaluates to a constant, like 'a'+32.
When these conditions are met, the switch statement is very clean—easy to write and to understand. It should be used whenever possible, especially when the decision tree has more than a few possibilities.
The Conditional Operator
Here's a strange sort of decision operator. It exists because of a common programming situation: A variable is given one value if something is true and another value if it's false. For example, here's an if...else statement that gives the variable min the value of alpha or the value of beta, depending on which is smaller:
if( alpha < beta ) min = alpha; else min = beta;
This sort of construction is so common that the designers of C++ (actually the designers of C, long ago) invented a compressed way to express it: the conditional operator. This operator consists of two symbols, which operate on three operands. It's the only such operator in C++; other operators operate on one or two operands. Here's the equivalent of the same program fragment, using a conditional operator:
min = (alpha<beta) ? alpha : beta;
The part of this statement to the right of the equal sign is called the conditional expression:
(alpha<beta) ? alpha : beta // conditional expression
The question mark and the colon make up the conditional operator. The expression before the question mark
(alpha<beta)
is the test expression. It and alpha and beta are the three operands.
If the test expression is true, the entire conditional expression takes on the value of the operand following the question mark: alpha in this example. If the test expression is false, the conditional expression takes on the value of the operand following the colon: beta. The parentheses around the test expression aren't needed for the compiler, but they're customary; they make the statement easier to read (and it needs all the help it can get). Figure 3.14 shows the syntax of the conditional statement, and Figure 3.15 shows its operation.
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Figure 3.14 Syntax of the conditional operator.
Figure 3.15 Operation of the conditional operator.
The conditional expression can be assigned to another variable or used anywhere a value can be used. In this example it's assigned to the variable min.
Here's another example: a statement that uses a conditional operator to find the absolute value of a variable n. (The absolute value of a number is the number with any negative sign removed, so it's always positive.)
absvalue = n<0 ? -n : n;
If n is less than 0, the expression becomes -n, a positive number. If n is not less than 0, the expression remains n. The result is the absolute value of n, which is assigned to absvalue.
Here's a program, condi.cpp, that uses the conditional operator to print an x every eight spaces in a line of text. You might use this to see where the tab stops are on your screen.
// condi.cpp
// prints 'x' every 8 columns
// demonstrates conditional operator
#include <iostream>
using namespace std;
int main()
{
for(int j=0; j<80; j++) //for every column,
{ //ch is 'x' if column is
char ch = (j%8) ? ' ' : 'x'; //multiple of 8, and
cout << ch; //' ' (space) otherwise
}
return 0;
}
Some of the right side of the output is lost because of the page width, but you can probably imagine it:
x x x x x x x x x
As j cycles through the numbers from 0 to 79, the remainder operator causes the expression (j % 8) to become false—that is, 0—only when j is a multiple of 8. So the conditional expression
(j%8) ? ' ' : 'x'
has the value ' ' (the space character) when j is not a multiple of 8, and the value 'x' when it is.
You may think this is terse, but we could have combined the two statements in the loop body into one, eliminating the ch variable:
cout << ( (j%8) ? ' ' : 'x' );
Hotshot C++ (and C) programmers love this sort of thing—getting a lot of bang from very little code. But you don't need to strive for concise code if you don't want to. Sometimes it becomes so obscure it's not worth the effort. Even using the conditional operator is optional: An if...else statement and a few extra program lines will accomplish the same thing.