Logical Expressions and Conditional Statements

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Logical Expressions

Logical expression is a way of expressing a logical condition, upon which certain parts of the program execution flow can be altered. For example, a furniture store wants the following delivery charges in its invoice program: if the total amount of an order is less than $200.00, delivery is charged as $25.00; if the amount is greater than or equal to $200.00, delivery is free. Here "amount is less than $200.00" is a logical condition that needs to be entered in C# program as a logical express.

The most simplest logical conditions involves relational operators:

• ">": Greater than
• "<": Less than
• ">=": Greater than or equal
• "<=": Less than or equal
• "==": Equal
• "!=": Not equal

A relational operation can be entered into C# programs as a logical expression with the following syntax:

logical_expression:
   arithmetic_expression relational_operator arithmetic_express

Examples of logical expressions are:

• "1 < 2": This is a simple logical expression. The resulting value is true.
• "1 > 2": This is also a simple logical expression. The resulting value is false.
• "quantity * price < 200.00": The arithmetic expression "quantity * price" will be evaluated first. Its result will be then compared with 200.00. The resulting value is true or false, depending on the resulting value of arithmetic expression.
• "exam_score == 100": If the value in exam_score is equal to 100, the resulting value will be true. Otherwise, it will be false.

"if" Statements

The most commonly used conditional statement is the "if" statement, which has the following syntax:

if statement:
   if (logical_expression) {
      embeded_statements
   }

When an "if" statement is encountered in the excution flow, the logical expression will be evaluated first. If the resulting value is true, the embeded statements that are enclosed in braces immediately after the logical expression will be executed. If the resulting value is false, those embeded statements will be skipped.

An "if" statement can also have an "else" clause, which will contain a block of embeded statements to be executed only when the logical expression is evaluated to false. Here is the syntax of "if else" statement:

if statement:
   if (logical_expression) {
      embeded_statements
   } else {
      embeded_statements
   }

In the "else" clause, we could have another logical condition to separate the execution in two more ways:

if statement:
   if (logical_expression) {
      embeded_statements
   } else if (another_logical_expression) {
      embeded_statements
   } else {
      embeded_statements
   }

(Continued on next part...)

Part:   1   2