Lecture 06

Boolean Expressions

What are boolean expressions?

Expressions that return true or false (on/off, 1/0)
Expressions that use the logical operators, read more here http://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B (Section Comparision Operators)
Main logical operators
- >, greater than
- <, less than
- >=, greater than or equal to
- <=, less than or equal to
- ==, equal to
- !=, not equal to

Why are boolean expressions important?:

It allow the program to ask true/false questions

How are boolean expressions used?:

Program outputs:

1 means true
0 means false

2 <  3: 1
2 >  3: 0

2 <= 3: 1
3 <= 3: 1
2 >= 3: 0
3 >= 3: 1

3 == 3: 1
2 == 3: 0

3 != 3: 0
2 != 3: 1

What is the if/else control statement?

The if/else control statement is a group of code that tells the program to execute some code (and not others) based on a condition

Why is the if/else control statement important?

that means: it allows your program to handle different scenarios
a program without branching is like a train that:
- starts at Honolulu
- ends at Waipahu
a program with branching is like a train that:
- starts at Honolulu
- pauses at Pearl City and thinks about where to go
- it could end up in Mililani
- it could end up in Waipahu

How to use the if/else control statement?

A group of code that includes:

the false block of code is optional
when there's no false block of code, then the control statement is called the if control statement

Syntax:

the keywords are if and else
the if keyword starts the if/else control statement, if (condition)
the condition follows the if keyword
the condition is wrapped in parenthesis, (condition)
the true and false blocks of code are wrapped in curly braces, { /*true block of code*/ }
the else keyword is between the ending curly braces of the true block and the starting curly braces of the false block, { /* true block */ } else { /* false block */ }
read more at http://www.tutorialspoint.com/cprogramming/if_else_statement_in_c.htm

Example if/else, if.c:

Things to note:

a condition is a boolean expression (or function) that returns true or false

there's a true block of code
- these lines of code are executed when the condition is true
there's a false block of code
- these lines of code are executed when the condition is false
if.c is utilizing the if/else control statement
- read more at http://www.tutorialspoint.com/ansi_c/c_control_statements.htm (Section Branching, Sub-Section If Statement)
- read more at http://www.tutorialspoint.com/cprogramming/if_else_statement_in_c.htm

More if/else examples, more_if.c:

Rename if.c (e.g. zhaol_06_1.c) and then modify it so magical numbers are from 100 to 199:

test case1:

Enter a magical number:
[user enters 5]
Your number needs more magic :(

test case2:

Enter a magical number:
[user enters 105]
Your number is very magical!

test case3:

Enter a magical number:
205
Your number is too magical :(