CSCI 1110 - C++ Lecture Notes
Chapter 9 - Additional Control Structures

The Switch Statement

break and continue Statements

Do-While Statement

For Statement

Choosing a Looping Statement

Case Study - Monthly Rainfall Averages

The Switch Statement

• Nested if statements can be used to select from one of several paths in a program but the logic is sometimes difficult to follow:
  
  
if (grade = 'A')
gradePt = 4.0;
else if (grade = 'B')
gradePt = 3.0;
else if (grade = 'C')
gradePt = 2.0;
else if (grade = 'D')
gradePt = 1.0;
else if (grade = 'F')
gradePt = 0.0;
else
cout << "Invalid grade";
 


• The C++ switch statement provides a control structure for this type of situation:
  
  
switch (grade)
{
case 'A' :
gradePt = 4.0;
break;
case 'B' :
gradePt = 3.0;
break;
case 'C' :
gradePt = 2.0;
break;
case 'D :
gradePt = 1.0;
break;
case 'F' :
gradePt = 0.0;
break;
default :
cout << "Invalid grade";
}
 


• switch syntax diagram:

• Example:
  
  
// letters.cc
// Letters inputs characters and
// counts the number of vowels
// and consonants.
#include <iostream.h>
#include <ctype.h>

int main()
{

int vowels = 0;
int consonants = 0;
char ch;

cin >> ch;
while (cin)
{

if (!isalpha(ch))
{
cin >> ch;
continue;
}
ch = tolower(ch);
switch (ch)
{
case 'a':
case 'e':
case 'i':
case 'o':
case 'u':
vowels++;
break;
default:
consonants++;
}
cin >> ch;
}
cout << "The input contained "
     << vowels << " vowels and "
     << consonants << "consonants."
} // end main
 


• The switch expression is an integral expression that is used to select one of the cases in the switch statement.
  
  
• If the value of the switch expression does not match any of the case constants, control is transfered to the default case. If there is no default case, control is passed to the statement following the brace at the end of the switch statement.

• The break statement is used to exit from the innermost switch or loop control structure.
  
  
loopCount = 1;
while (TRUE)
{
cin >> num1;
if ( !cin || num1 >= 100)
break;
cin >> num2;
if ( !cin || num2 <= 50)
break;
cout << sqrt(float(num1 + num2))
     << endl;
loopCount++;
if (loopCount > 10)
break;
}
 


• When the break statement is executed, control is transfered to the statement following the switch or loop.
  
  
• If the break statement is missing from a case in a switch statement, the program continues to execute the statements in the following case:

 
switch(nameFormat)
{
case FULL_NAME :
cout << "Amanda ";
case INITIAL_AND_LAST_NAME :
cout << "B. ";
case LAST_NAME :
cout << "Recondwithe";
}
 


• The continue statement can be used with a loop to cause it to suspend execution of the current iteration and continue with the next iteration.
  
  
// Output first 500 non-negative values
dataCount = 0;
while (dataFile && dataCount < 500)
{
dataFile >> inputVal;
if (!dataFile)
break;
if (inputVal < 0)
continue;
cout << inputVal;

dataCount++;

}
 


Note that when a negative value is read, the cout statement and the dataCount++; statement are skipped, but the loop continues by reading another value.
 

Do-While Statement

• The while loop that we have been using tests its condition at the beginning of the loop:

• There are some applications where a loop that tests its condition at the bottom of the loop is better. The do ... while statement provides such a loop:

• The Statement in the do ... while can be a single statement or a block of statements enclosed in braces. The statement will always be executed at least once:

• Examples:

 
 

 

While Solution	do While Solution
dataFile >> inChar;  while (inChar != '.')    dataFile >> inChar;	do    dataFile >> inChar;  while (inChar != '.');
cout << "Enter age: ";  cin >> age;  while (age < 0)  {    cout << "Age is negative";    cout << "Enter age: ";    cin >> age;  }	do  {    cout << "Enter age: ";    cin >> age;    if (age < 0)     cout << "Age is negative";  } while (age < 0);

 

For Statement

• A lot of loops are count controlled loops. The for statement makes it easy to build such loops:

• The for loop tests at the bottom of the loop just as the while loop does:

• Examples:
 

for (count = 1; count <= n; count++)
    cout << count << endl;
 

is equivalent to:
count = 1;
while (count <= n)
{
cout << count << endl;
count++;
}
 
The following for loops execute the same number of times:
 
for (count = 1; count <= 50; count++)
cout << count;
 
for (count = 50; count >= 1; count--)
cout << count;
 
• Loops can be nested to any depth:

for (count = 0; count < 10; count++)
{
for (digit = 0; digit <= count; digit++)
cout << digit;
cout << endl;
}

prints the following output:

0
01
012
0123
01234
012345
0123456
01234567
012345678
0123456789
 



•  Any of the components of the for statement may be null:
 

 for ( ; inVal != 999; )

cin >> inVal;
 
is equivalent to:
 
while (inVal != 999)
cin >> inVal;
 
and
 
 for ( ; ; ; )
{
cin >> inVal;
if (inVal < 0)
break;
}
is equivalent to:
 
while (1)
{
cin >> inVal;
if (inVal < 0)
break;
}
 
 

Choosing a Looping Statement

• Guidelines for deciding when to use the looping statments:
  
  
1. If the loop is a simple count-controlled loop, the for statement is the best choice.
   
   
2. If the loop is an event-controlled loop whose body is always executed at least once, a do while statement is appropriate.
   
   
3. If the loop is and event-controlled loop and nothing is known about the first exectuion, use a while or for statement.
   
   
4. When in doubt, use a while statement.
   
   
5. An infinite loop with a break statement sometimes clarifies the code, it often reflects and undisciplined loop design. Use it only after careful consideration.

Case Study - Monthly Rainfall Averages

• Case study from text pages 486-491.
  
  
• Problem: Meteorologist have recorded monthly rainfall amounts at several sites throught a region of the country. You have been asked to write an interactive program that lets the user enter one year's rainfall amounts at a particular site and prints out the average of the 12 values. After the data for a site is processed, the program asks whether the user would like to repeat the process for another recording site. The program must trap erroneous input data (negative values for rainfall amounts and invalid responses fo the "Do you wish to continue?" prompt).
  
  
• See the text for the pseudocode and structure chart for the case study.
  
  
• Rainfall Averages Program.