Lecture 25

Logical Operators

Why are logical operators important?:

logical operators allow your boolean expressions to be more direct
using logical operators can simplify nested if/else statements which are hard to read

What are logical operators?:

&&: and
||: or

How to use logical operators?:

logical operators are placed between 2 boolean expressions, e.g. (1 < 2) && (2 < 3)

expressions containing logical operators simplify like this:

AND Operator `&&`
Expression 1	Expression 2	Result
true	true	true
true	false	false
false	true	false
false	false	false

OR Operator `||`
Expression 1	Expression 2	Result
true	true	true
true	false	true
false	true	true
false	false	false

&& asks "is expression1 AND expression2 true?"
|| asks "is expression1 OR expression2 true?"

Logical Operator Examples

How does C interpret logical operators?

This program shows how logical operators are interpretted in C

logical_operators.c:

Program output:

T && T: 1
T && F: 0
F && T: 0
F && F: 0

T || T: 1
T || F: 1
F || T: 1
F || F: 0

Comparing logical operators with nested if/else:

This program compares using logical operators with nested if/else

logical_operators_vs_nested_if.c:

both blocks of code do the same thing
the block of code using logical operators is more direct and the reader can easily understand the intent
the nested if/else statements allows you to handle "less than 0" differently than "greater than 100"
the logical operator will save you from copying and pasting code if you want to handle "less than 0" and "greater than 100" the same

else if

Why use else if:

using else if conveys more intent than nested if/else statement

How to use else if:

else_if.syntax:

else if Example

Comparing else if with nested if/else:

This program compares using else if with nested if/else

else_if_vs_nested_if.c:

using else if makes the code easier to read and understand
using else if allows the reader to quickly understand the intent of the writer

switch

Why use switch:

using switch conveys more intent than nested if/else statement

How to use switch:

switch.syntax:

switch Example

Comparing switch with nested if/else:

This program compares using switch with nested if/else

switch_vs_nested_if.c:

comparing 0(an int) is different than comparing '0'(a char)
do not forget the break; statement at the end of each case
- without the break; statement, the switch will continue executing code until it reaches a break; statement or exits the switch block of code

else if vs switch

Comparing else if with switch:

This program compares using else if with switch

else_if_vs_switch.c:

else if can match ranges
else if can use variables in the condition, e.g. variable1 <= variable2
switch can only test for equality with constants (no variables)

switch with Characters

Write a program that translates alphabets to their military phonetic alphabet

Alphabet Translation
alphabet	military phonetic alphabet
a	alpha
b	bravo
c	charlie
(anything else)	ran out of funding

Assignment

Homework Problem 25.1, at the end of this problem you should submit to Laulima:

a file named UHusername_25_1.c
- zhaol_25_1.c
Write a program that asks the user for an alphabet letter. If the letter entered is upper case, the program will check for the winning characters A, E, I, O, and U. If the letter entered is lower case, the program will check for the winning characters q and r. The program will tell the user if they've entered in a winning character or not. If the user enters anything besides an upper case or lower case alphabet, then the program will tell the user that the character entered is invalid. The program exits after getting one character from the user.
- avoid nested if/else statements
- use switch, else if, and logical operators whenever possible
- since the switch, else if, and logical operators perform similar functions, be sure to think about which one is a more "natural" fit for the operation at hand (this can be subjective, but for this simple assignment, it should be pretty clear)

Test Cases:

Test Case 1:

Please enter in a letter
[user enters A]
This is a winning character

Test Case 2:

Please enter in a letter
[user enters Z]
This uppercase character is not a winning character

Test Case 3:

Please enter in a letter
[user enters q]
This is a winning character

Test Case 4:

Please enter in a letter
[user enters s]
This lowercase character is not a winning character

Test Case 5:

Please enter in a letter
[user enters +]
This is an invalid character