Controlling
execution
This
section covers the execution control statements in C++. You must be familiar
with these statements before you can read and write C or C++ code.
C++
uses all C’s execution control statements. These include
if-else,
while,
do-while,
for,
and a selection statement called
switch.
C++ also allows the infamous
goto,
which will be avoided in this book.
True
and false
All
conditional statements use the truth or falsehood of a conditional expression
to determine the execution path. An example of a conditional expression is
A
== B
.
This uses the conditional operator
==
to see if the variable
A
is equivalent to the variable B
.
The expression produces a boolean
true
or
false
(these are keywords only in C++; in C an expression is “true” if it
evaluates to a nonzero value). Other conditional operators are
>,
<,
>=,
etc. Conditional statements are covered more fully later in this chapter.
if-else
The
if-else
statement can exist in two forms: with or without the
else.
The two forms are:
if(expression)
statement
else
statement
The
“expression” evaluates to
true
or
false.
The “statement” means either a simple statement terminated by a
semicolon or compound statement, which is a group of simple statements enclosed
in braces. Any time the word “statement” is used, it always implies
that the statement is simple or compound. Note this statement can also be
another
if,
so they can be strung together.
//: C03:Ifthen.cpp
// Demonstration of if and if-else conditionals
#include <iostream>
using namespace std;
int main() {
int i;
cout << "type a number and 'Enter'" << endl;
cin >> i;
if(i > 5)
cout << "It's greater than 5" << endl;
else
if(i < 5)
cout << "It's less than 5 " << endl;
else
cout << "It's equal to 5 " << endl;
cout << "type a number and 'Enter'" << endl;
cin >> i;
if(i < 10)
if(i > 5) // "if" is just another statement
cout << "5 < i < 10" << endl;
else
cout << "i <= 5" << endl;
else // Matches "if(i < 10)"
cout << "i >= 10" << endl;
} ///:~ It
is conventional to indent the body of a control flow statement so the reader
may easily determine where it begins and ends
[20].
while
while,
do-while
and
for
control looping. A statement repeats until the controlling expression evaluates
to
false.
The form of a
while
loop is
while(expression)
statement
The
expression is evaluated once at the beginning of the loop, and again before
each further iteration of the statement.
This
example stays in the body of the
while
loop until you type the secret number or press control-C.
//: C03:Guess.cpp
// Guess a number (demonstrates "while")
#include <iostream>
using namespace std;
int main() {
int secret = 15;
int guess = 0;
// "!=" is the "not-equal" conditional:
while(guess != secret) { // Compound statement
cout << "guess the number: ";
cin >> guess;
}
cout << "You guessed it!" << endl;
} ///:~ The
while’s
conditional
expression is not restricted to a simple test as in the above example; it can
be as complicated as you like as long as it produces a
true
or
false
result. You will even see code where the loop has no body, just a bare semicolon:
while(/* Do a lot here */)
;
In
these cases the programmer has written the conditional expression not only to
perform the test but also to do the work.
do-while
do
statement
while(expression);
The
do-while
is different from the while because the statement always executes at least
once, even if the expression evaluates to false the first time. In a regular
while,
if the conditional is false the first time the statement never executes.
If
a
do-while
is used in
Guess.cpp,
the variable
guess
does not need an initial dummy value, since it is initialized by the
cin
statement before it is tested:
//: C03:Guess2.cpp
// The guess program using do-while
#include <iostream>
using namespace std;
int main() {
int secret = 15;
int guess; // No initialization needed here
do {
cout << "guess the number: ";
cin >> guess; // Initialization happens
} while(guess != secret);
cout << "You got it!" << endl;
} ///:~ For
some reason, most programmers tend to avoid
do-while
and just work with
while.
for
A
for
loop performs initialization before the first iteration. Then it performs
conditional testing and, at the end of each iteration, some form of
“stepping.” The form of the
for
loop is:
for(initialization;
conditional;
step)
statement
Any
of the expressions
initialization,
conditional
or
step
may be empty. The
initialization
code executes once at the very beginning. The
conditional
is tested before each iteration (if it evaluates to false at the beginning, the
statement never executes). At the end of each loop, the
step
executes.
for
loops are usually used for “counting” tasks:
//: C03:Charlist.cpp
// Display all the ASCII characters
// Demonstrates "for"
#include <iostream>
using namespace std;
int main() {
for(int i = 0; i < 128; i = i + 1)
if (i != 26) // ANSI Terminal Clear screen
cout << " value: " << i <<
" character: " <<
char(i) << endl; // Type conversion
} ///:~ You
may notice that the variable
i
is defined at the point where it is used, instead of at the beginning of the
block denoted by the open curly brace ‘
{’.
This is in contrast to traditional procedural languages (including C), which
require that all variables be defined at the beginning of the block. This will
be discussed later in this chapter.
The
break
and continue Keywords
Inside
the body of any of the looping constructs
while,
do-while
or
for,
you
can control the flow of the loop using
break
and
continue.
break
quits the loop without executing the rest of the statements in the loop.
continue
stops the execution of the current iteration and goes back to the beginning of
the loop to begin a new iteration.
As
an example of the use of
break
and
continue,
this program is a very simple menu system:
//: C03:Menu.cpp
// Simple menu program demonstrating
// the use of "break" and "continue"
#include <iostream>
using namespace std;
int main() {
char c; // To hold response
while(true) {
cout << "MAIN MENU:" << endl;
cout << "l: left, r: right, q: quit -> ";
cin >> c;
if(c == 'q')
break; // Out of "while(1)"
if(c == 'l') {
cout << "LEFT MENU:" << endl;
cout << "select a or b: ";
cin >> c;
if(c == 'a') {
cout << "you chose 'a'" << endl;
continue; // Back to main menu
}
if(c == 'b') {
cout << "you chose 'b'" << endl;
continue; // Back to main menu
}
else {
cout << "you didn't choose a or b!"
<< endl;
continue; // Back to main menu
}
}
if(c == 'r') {
cout << "RIGHT MENU:" << endl;
cout << "select c or d: ";
cin >> c;
if(c == 'c') {
cout << "you chose 'c'" << endl;
continue; // Back to main menu
}
if(c == 'd') {
cout << "you chose 'd'" << endl;
continue; // Back to main menu
}
else {
cout << "you didn't choose c or d!"
<< endl;
continue; // Back to main menu
}
}
cout << "you must type l or r or q!" << endl;
}
cout << "quitting menu..." << endl;
} ///:~ If
the user selects ‘q’ in the main menu, the
break
keyword is used to quit, otherwise the program just continues to execute
indefinitely. After each of the sub-menu selections, the
continue
keyword is used to pop back up to the beginning of the while loop.
The
while(true)
statement is the equivalent of saying “do this loop forever.” The
break
statement allows you to break out of this infinite while loop when the user
types a ‘q.’
switch
A
switch
statement selects from among pieces of code based on the value of an integral
expression. Its form is:
switch(selector)
{
case
integral-value1
:
statement;
break;
case
integral-value2
:
statement;
break;
case
integral-value3
:
statement;
break;
case
integral-value4
:
statement;
break;
case
integral-value5
:
statement;
break;
(...)
default:
statement;
}
Selector
is an expression that produces an integral value. The
switch
compares the result of
selector
to each
integral-value.
If it finds a match, the corresponding statement (simple or compound) executes.
If no match occurs, the
default
statement
executes.
You
will notice in the above definition that each
case
ends with a
break,
which causes execution to jump to the end of the
switch
body (the closing brace that completes the
switch).
This is the conventional way to build a
switch
statement, but the
break
is optional. If it is missing, your
case
“drops
through” to the one after it. That is, the code for the following
case
statements execute until a
break
is encountered. Although you don’t usually want this kind of behavior, it
can be useful to an experienced programmer.
The
switch
statement is a very clean way to implement multi-way selection (i.e., selecting
from among a number of different execution paths), but it requires a selector
that evaluates to an integral value at compile-time. If you want to use, for
example, a
string
object as a selector, it won’t work in a
switch
statement. For a
string
selector, you must instead use a series of
if
statements and compare the
string
inside the conditional.
The
menu example shown previously provides a particularly nice example of a
switch:
//: C03:Menu2.cpp
// A menu using a switch statment
#include <iostream>
using namespace std;
int main() {
bool quit = false; // Flag for quitting
while(quit == false) {
cout << "Select a, b, c or q to quit: ";
char response;
cin >> response;
switch(response) {
case 'a' : cout << "you chose 'a'" << endl;
break;
case 'b' : cout << "you chose 'b'" << endl;
break;
case 'c' : cout << "you chose 'c'" << endl;
break;
case 'q' : cout << "quitting menu" << endl;
quit = true;
break;
default : cout << "Please use a,b,c or q!"
<< endl;
}
}
} ///:~ The
quit
flag
is a
bool,
short for “Boolean,” which is a type you’ll only find in C++.
It can only have the keyword values
true
or
false.
Selecting
‘q’ sets the
quit
flag to
true.
The next time the selector is evaluated,
quit
== false
returns
false
so the body of the
while
does not execute.
[20]
Note that all conventions seem to end after the agreement that some sort of
indentation take place. The feud between styles of code formatting is unending.
See appendix A for the description of this book’s coding style.
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