What can a computer do?

These notes are based on Section 1.4 of the book.
- You should read this briefly, we will concentrate on this material in detail later.
We have seen two things that computers can do well.
- Follow instructions
- Do basic computations
  - gallons = roundup(cones/24)
You may have missed it, but they can also store information
- in our algorithm use used gallons, and cones as places to store information.
- we call these places variables.
- when writing algorithms we will be somewhat sloppy with variables.
  - Write them down as you need them.
  - Don't worry too much about the type.
- We will examine variables much more closely in the future.
Another thing computers can do well is compare simple things.
- We will use the symbols for the following operations
- < less than
  
  > greater than
  
  == equal
  
  <= less than or equal
  
  >= greater than or equal
  
  != not equal

The ability to compare gives two more things computers do well.

Selection (if)

Based on a comparison we can do different things.
We write these as if or if else statements.
And we can use this type of structure in algorithms.

For example, if we give a discount of 10% to children 5 and under at our ice cream store we might write the following algorithm

get age
if age <= 5
   calculate the discount by multiplying the price by 10% 
   discount = itemPrice * .1
else
   the discount is 0
   discount = 0

Note
- I intended the items inside of the if statement.
- Used the comparison operators.
- And used formulas when appropriate
Please use if statements like this.
DO NOT DO

If the age is five or less compute the discount by multiplying the price by 10%, otherwise there is no discount.

Repetition (loops)

Based on a comparison we can repeat something until the comparison changes.
We have two basic ways we loop: while and for

If we want to calculate a total bill for a customer at our ice cream store we might do something like

for each person getting a cone
   get the age
   if age <= 5
      set the discount to be 10% of the cost.
   else 
      set the discount to be 0
   cost = price - discount
   total = total + cost
print the total

This says to repeat the process for each person getting a cone.
We could also have written that:

while there is another person getting a cone
   get the age
   if age <= 5
      set the discount to be 10% of the cost.
   else 
      set the discount to be 0
   cost = price - discount
   total = total + cost
print the total

Pick which ever is easier for you right now.
We will learn more about each later.
Notice the indentation to mark the "body" of the loop.

So computers can do the following well

Follow simple instructions.
Perform simple math, including comparison
Store and retrieve data
Make simple decisions
Repeat instructions.

<	less than
>	greater than
==	equal
<=	less than or equal
>=	greater than or equal
!=	not equal