Conditional (Branching) Statements

Chapter 4

Control Structures

A computer can proceed:

1. In sequence
2. Selectively (branch): making a choice
3. Repetitively (iteratively): looping
4. By calling a function

Here are subsections of flowcharts showing the first three:

So far, our program instructions have only proceeded in sequence. We will not add selectivity.

Let’s look inside the decisions (“diamonds”) – this is where we need to make a decision about what we are going to do next. For instance, if we are assigning grades, if the grade is between 90 and 100 it is an A, if 80-90 it’s a B, etc. So we decide what to do based on our input.

• Conditional statements: only executed if certain conditions are met
• Condition: represented by a logical (Boolean) expression that evaluates to a logical (Boolean) value of true or false

There are two structures for these conditional statements:

1. if…then…else statements
2. switch statements

If…then…else Statement

if (guard statement)
{
    // If true, then do whatever is in these {}
}
else
{
    // If false, do this instead
}

Examples

1. Just if statement:

   if (grade > 90)
   {
       letter = 'A';
   }

2. If…then…else:

   if (grade > 70)
   {
       letter = 'P'; // P for pass
   }
   else
   {
       letter = 'F'; // F for fail
   }

3. Block of if…then…else statements (nested if/else):

   if (grade >= 90)
   {
       letter = 'A';
   }
   else if (grade >= 80)
   {
       letter = 'B';
   }
   else if (grade >= 70)
   {
       letter = 'C';
   }
   else if (grade >= 60)
   {
       letter = 'D';
   }
   else
   {
       letter = 'F';
   }

Expression of char values with relational operators:

• The result depends on the machine’s collating sequence.
• ASCII character set

Nesting

Nesting: one control statement is located within another.

An else is associated with the most recent if that has not been paired with an else.

if (month == 2) // Determine how many days are in February this year.
{
    if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0)
        days = 29; // leap year
    else
        days = 28; // normal year
}

Logical Expressions

Logical (Boolean) expressions:

• Expressions such as 4 < 6 and 'R' > 'T'
• Returns an integer value of 1 if the logical expression evaluates to true
• Returns an integer value of 0 otherwise

The bool Data Type

• The data type bool has logical (Boolean) values true and false
• bool, true, and false are reserved words
• The identifier true has the integer value 1 (or not 0) and false has the value 0.

Relational (Boolean) Operators

Operator	Description	Syntax
==	equal to	num1 == num2
!=	not equal to	num1 != num2
<	less than	num1 < num2
<=	less than or equal to	num1 <= num2
>	greater than	num1 > num2
>=	greater than or equal to	num1 >= num2

String Comparisons

Relational operators can be applied to strings.

• Strings are compared character by character, starting with the first character.
• The comparison continues until either a mismatch is found or all characters are found equal.
• If two strings of different lengths are compared and the comparison is equal to the last character of the shorter string
  • The shorter string is less than the larger string

Multiple Comparisons

You can combine multiple comparisons into a single expression with logical (Boolean) operators.

Unlike relational operators (which compare values), logical operators take Boolean expressions and produce a Boolean result.

This video covers how to perform multiple comparisons using the logical operators AND (&&) and OR (||). We also cover the NOT (!) operator.

Operator	Description	Syntax
!	not	!val
&&	and	val1 && val2
||	or	val1 || val2

Things to look out for:

• A “dangling” else (You cannot have an else without a matching if directly preceding it.)

• Else statements cannot have conditionals; only if statements.

  else (num > 3) // this is an error; you need: else if (num > 3)

• Using a = when you mean ==

  • The appearance of = in place of == resembles a silent killer (no syntax error, but a semantic error).

• Forgetting to break in a switch statement.

• Everything is a number (even Booleans and characters)

• Any value that is not 0 is evaluated as true when interpreted as a Boolean.

• Don’t forget to use && and || when doing multiple comparisons

  0 < num1 < 10 // Error: is true for any number greater than 0!`  
  (0 < num1) < 10 // Here is the order of operations. See the problem?  
  0 < num1 && num1 < 10 // Correct: true for num1 values between 1 and 10

A dangling else means that you have an else that is going with the wrong if statement.

Self-Check Questions

1. What is a conditional?
2. What is the difference between a = and ==?
3. Be able to create an if…else; if…else…if…else; and switch statements.