Choosing between different courses of action makes a program much more useful and flexible. Decision making constructs allow for many different logical branches of code to be incorporated into a single program.
One of the fundamental properties of a programming language is the ability to repetitively execute a sequence of statements. These looping capabilities enable programmers to develop concise programs containing repetitive processes that could otherwise require an excessive number of statements.
The if-else statement allows conditional execution of a group of statements. The general form of an if-else statement is
if(expression)
{
statement-block-1;
}
else
{
statement-block-2;
}
where expression can be a combination of logical and relational operators and their operands, a function call, or a mathematical expression. If the expression is true, then statement-block-1 is executed otherwise statement-block-2 is executed. Only expressions evaluating to zero(0) are considered to be false in C and C++.
Statements subordinate to if and else clauses can be compound statements; meaning several statements grouped together and bounded by opening and closing curly braces. It is recommended that subordinate statements be indented to indicate the subordination. Most programmers indent from three to five spaces, some use as many as eight spaces. It is also recommended that code not be placed after column seventy-two (72), this makes all the code on a line visible on the display and usually makes for a readable printout.
A:
if(x == y)
a = b + c;
B:
if(c >= 10)
{
cout << "\nC is over 10";
++a;
}
else
{
cout << "\nNot yet!";
c++;
}
Notice that the first example, A, does not use an else clause. The else clause is optional and is only used if needed. Also, note that curly braces are not used on example A because there is only a single statement subordinate to the if.
if-else statements can be nested. There is no reasonable limit imposed by the compiler to the number of nested if-else statements, although there should be a practical limit. It is recommended that if nested if-else statements are used, that the nest not go over four deep. This limit is more a matter of readability and not anything imposed by the compiler.
For example, the following shows nested if-else statements. Notice how quickly the code progresses toward the right edge of the display, even with short variable names. Also, this example has a major error in logic. See if you can find it.
if( a > 10 )
if( b < 5 )
b = a * 10;
else
if( a < 5 )
if ( b > 10 )
b = a * b;
else
if( c > 5 && b == 10 )
if( c < 10 )
c = b * a;
else
c = a * c;
In nested if-else statements, the else is associated with the nearest if, unless bracketed within curly braces which form a compound statement. The error in the above sample is at the very beginning where
if( a > 10 )
if( b < 5 )
b = a * 10;
else
The else is associated by the compiler with the if( b < 5 ) statement not with if( a > 10 ). The intention of the programmer was to associate the else with if( a > 10 ). In order to accomplish the correct association, the code block should appear as
if( a > 10 )
{
if( b < 5 )
b = a * 10;
}
else
Notice the use of the curly braces to limit the scope of the if( b < 5 ) statement. Many errors in logic occur because of incorrect else statement association. Use the curly braces if there is any doubt about how the compiler will associate if with else statements.
The switch allows the conditional execution of one of a number of groups of statements based on the value of an expression. The general form of a switch statement is:
switch( expression )
{
case constant-value-1:
statement-block-1;
break;
case constant-value-2:
statement-block-2;
break;
.
.
.
default:
statement-block-n;
break;
}
Where expression must produce an integer or character result and each of the constant values must be an integer or character value that could be a possible value of the expression.
The expression in the switch statement is evaluated and compared to each of the case constant values in order. If a match is found, execution is started at that case statement. If no match is found, control is transferred to the default statement, if one has been coded.
#include <iostream.h>
ing main()
{
int day;
cout << "\nEnter a number for"
<< "the day of the week:";
cin >> day;
switch( day )
{
case 0:
cout << "Sunday";
break;
case 1:
cout << "Monday");
break;
case 2:
cout << "Tuesday";
break;
case 3:
cout << "Wednesday";
break;
case 4:
cout << "Thursday";
break;
case 5:
cout << "Friday";
break;
case 6:
cout << "Saturday";
break;
default:
cout << "No Such Day";
break;
}
return 0;
}
The break statement causes an immediate exit from the switch. Without the break, the execution will fall thru to the next case and will continue to fall thru until a break statement is reached. Sometimes the desired logic is to have multiple case statements cause one code block to be executed. For example, assume that the keyboard is being read and that when a newline character or a carriage return character is read a null byte is to placed at the end of an array of characters.
#include <iostream.h>
int main()
{
char buffer[80];
int ch, idx = 0, lgth = sizeof( buffer) -1;
cout << "\nEnter a string of characters: ";
while ( idx < lgth && ch != '\n' && ch != '\r' )
{
cin >> ch;
switch( ch )
{
case '\n':
case '\r':
buffer[idx] = '\0';
break;
case '\b':
--idx;
if( idx < 0 )
idx = 0;
putchar( '\b' );
putchar( ' ' );
putchar( '\b' );
break;
default:
buffer[idx] = ch;
++idx;
break;
}
}
return 0;
}
The while loop allows for the repeated execution of a group of statements as long as a condition is true. The condition is checked each time before the code is executed. The general form of the loop construct is:
while (expression)
{
statement-block;
}
If statement-block comprises a group of statements, then the group must be bracketed by opening and closing curly braces. The statement or group of statements inside the while loop will be executed until the expression is false
#include <iostream.h>
int main()
{
int ch = 0;
cout << "\nCharacter Set";
while ( ch < 256 )
{
cout << "\nChar: " << char(ch)
<< " Hex: " << hex << ch
<< " Oct: " << oct << ch
<< " ASCII: " << ch ;
++ch;
}
return 0;
}
The for loop is a specialized form of the while loop, which includes an initialization statement, a conditional expression and a third expression, referred to as the increment expression, which is executed at the end of the loop before control is transferred back to the conditional expression test at the loop beginning. The general form of the for loop is:
for(init-expression;
conditional-expression; increment-expression)
{
statement-block;
}
All three expressions composing the for loop are optional. The expressions can be any legal expression, function call, or mathematical statement. The statements are executed as follows:
init-expression;
loop:
if conditional-expression is true
begin
statement-block;
increment-expression
end
else
exit the loop;
end of loop:
The following example prints the character set being used on the current machine. The program prints character, the ASCII value, the hexadecimal value and the octal value of the character.
#include <iostream.h>
#define MAX_LINES 23
int main()
{
char ans;
int ch, lines, stop = 0, maxchars;
//
// how many characters in current character set
//
cout << "\nHow many characters in the character set: ";
cin >> maxchars;
//
// print the ASCII and extended ASCII characters
//
for(ch = 0, lines = 0; ch < maxchars && !stop; ++ch)
{
cout << "\nCHAR: " << char( ch )
<< " ASCII: " << ch
<< " HEX : " << hex << ch
<< " OCT : " << oct << ch
;
if( ++lines > MAX_LINES )
{
cout << "\nContinue(Y/N)?";
cin >> ans;
if(ans == 'N' || ans == 'n')
{
stop = 1;
break;
}
lines = 0;
}
}
return 0;
}
This loop is like the while loop, except that the condition is checked after the loop is executed. The general form is as follows:
do
{
statement-block;
} while(expression);
The statement or statement-block are executed at least once, even if the expression is false on the first test.
#include <iostream.h>
#define ESC 27
int main()
{
int ch;
do
{
cout << "\nPress any key:";
cin >> ch;
cout << "\nCHAR: " << ch << HEX: " << ch;
} while(ch != ESC);
return 0;
}
The break terminates execution of a loop or switch-case and transfers execution to the first statement following the loop or the switch-case. If the break statement is within a loop that is nested within another loop, the break statement only exits the loop the holds the break statement, not the outer loop.
The continue causes execution to pass to the end of the current loop. For example:
for(i=1; i < 100; i++)
{
if( i == 10 )
continue;
.
.
.
}
when i is equal to 10 execution will go to the increment- expression, i++ and bypass any code between the if and the increment-expression.
C++ allows declarations within blocks after code statements. This allows a programmer to declare an entity closer to its first point of application. For example, an index can be declared within a loop, as follows:
for( int i = 0; i < 12; i++ )
The new operator :: is used to resolve name conflicts. For example, if the automatic variable vector_sum is declared within a function and there exists a global variable vector_sum, the specifier ::vector_sum allows the global variable to be accessed within the scope of the automatic variable vector_sum.
The reverse is not true. It is not possible for the automatic variable vector_sum to be accessed from outside its scope. The scope qualifier :: is also used in connection with classes.
// **::** scope resolution operator example
int i = 1; // external or global i
#include <iostream.h>
int main()
{
int i = 2; // redeclares i locally to main
{ // an inner block within a function
cout << "Enter inner block" << endl;
int n = i; // the global i is still visible
int i = 3; // hides the global i which can only be
// referenced by using the :: operator
// print the local i and the global i
cout << i << " i << ::i " << ::i << endl;
cout << "n = " << n << endl;
} // end of inner block
cout << "Enter outer block" << endl;
// print the current local i and the global i
cout << i << " i ** ::i " << ::i << endl;
return 0;
}
**
The output of this code is:
Each identifier or name has a scope that indicates the region of the program where that identifier can be used. C++ supports the notion of the following types of scope:
#. Local: Names appearing inside a block statement (enclosed in a pair of curly braces {...}) have local scope. You can use these only within that block and only after the actual declaration. Note that you cannot define a function in a local scope. (This means you cannot define one function inside the body of another.) #. File: Names appearing outside all blocks and class declarations have file scope. These can be used within that file anywhere after the point where they are declared. #. Class: Names of data and function members of a class (or
struct) have class scope.