Simple Selections

Tom Kelliher, CS17

Mar. 11, 1996

We now begin to add decision making capabilities to our programs.

Consider the tax computation program:

• Assume that incomes are between $50,000 and $80,000.
• Federal tax is $5,100 plus 28% of gross income over $34,000.

How about:

federalTax = FEDERAL_BASE 
             + (grossIncome - FEDERAL_EXEMPTION) * FEDERAL_RATE;

Or:

if (grossIncome <= FEDERAL_EXEMPTION)
   federalTax = FEDERAL_BASE;
else
   federalTax = FEDERAL_BASE 
                + (grossIncome - FEDERAL_EXEMPTION) * FEDERAL_RATE;

C++'s selection statements:

• if --- two-way, one-way, multi-way
• switch --- multi-way

Two-Way Selection

Form:

if (<Expression>)
   <ExpressionTrueStatement>;
else
   <ExpressionFalseStatement>;

1. <Expression>:
   1. Named: condition, predicate.
   2. Any expression with a numerical value.
   3. Non-zero value interpreted as ``true.''
   4. Zero value interpreted as ``false.''
   5. First thing evaluated in performing if statement.
2. <ExpressionTrueStatement>
   1. Evaluated if <Expression> interpreted as true.
   2. Single statement or a compound block { ... }.
3. <ExpressionFalseStatement>
   1. Evaluated if <Expression> interpreted as false.
   2. Single statement or a compound block { ... }.
4. <Expression> always evaluated, exactly one of the other two is evaluated.

Equality Operators

• == --- has value 1 if operands are exactly equal, otherwise has value 0.

  Easily confused with =:

  int i = 3;
  
  if (i == 3)
     cout << "i is 3.\n";
  else
     cout << "i isn't 3.\n";
  
  i = 6;
  
  if (i = 3)
     cout << "i is 3.\n";
  else
     cout << "i isn't 3.\n";
  

• != --- has value 1 if operands are not equal, otherwise has value 0.

  int i = 3;
  
  if (i != 3)
     cout << "i isn't 3.\n";
  else
     cout << "i is 3.\n";
  

  Same effect as == if?

Relational Operators

1. <
2. <=
3. >
4. >=

Examples:

int i = 4, j = 4, k = 5, l;

l = i == j;
l = i > k;
cout << (l < j) << endl;   // parentheses required --- see next section
i = 4;
j = 10;
k = 6;
l = i < j < k;   // left associative

Simplifying the Predicate

Consider deciding if a number is divisible by 10:

if (n % 10 == 0)
   cout << "Divisible by 10.\n";
else
   cout << "Not divisible by 10.\n";

if (n % 10)
   cout << "Not divisible by 10.\n";
else
   cout << "Divisible by 10.\n";

Simpler?

Easier to read?

New Precedence Table

1. () (left)
2. Unary +, -, ++, -- (right)
3. *, /, % (left)
4. Binary +, - (left)
5. <<, >> (left)
6. <, <=, >, >= (left)
7. ==, != (left)
8. Assignment =, +=, -=, *=, /=, %= (right)

Multi-Way Selection

Filing Status for a Tax Program

if (filingStatus == 1)
   cout << "Single";
else
   if (filingStatus == 2)
      cout << "Married filing jointly";
   else
      if (filingStatus == 3)
         cout << "Married filing separately";
      else
         if (filingStatus == 4)
            cout << "Head of household";
         else
            cout << "Error in filing status;";

Easier to read:

if (filingStatus == 1)
   cout << "Single";
else if (filingStatus == 2)
   cout << "Married filing jointly";
else if (filingStatus == 3)
   cout << "Married filing separately";
else if (filingStatus == 4)
   cout << "Head of household";
else
   cout << "Error in filing status;";

Selection based on two variables

• Customer has good credit standing (value above 6):
  • payment is balance if balance under $20.01.
  • payment if $10 if balance between $20.01 and $100.00
  • else payment is 10% of balance.
• Customer has poor credit standing:
  • payment is balance.

if (creditStanding > 6)
{
   if (balance <= 20.0)
      minPayment = balance;
   else if (balance <= 100)
      minPayment = 10.0
   else
      minPayment = 0.1 * balance;
}
else
   minPayment = balance;

Exercises

Chapter 5 Study Guide Programming Exercises 1, 2, 3, 5, 9.