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  • You may not remove a slip of paper from a compartment without replacing it by another slip of paper on which you have written a value.
  • You may not put a slip of paper in a compartment without removing the one already there.

Checkout counter example

In understanding how you might behave as a human computer program, consider yourself to have a job working at the checkout counter of a small grocery store in the 1930s.

You have two tools to work with:

  • A mechanical adding machine
  • The rack of compartments described above


Each morning, the owner of the grocery store tells you to insert a name in the slot above each compartment and to place a little slip of paper with a number written on it inside each compartment. (In programming jargon, we would refer to this as initialization.)

Each of the names on the compartments represents a type of grocery such as

  • Beans
  • Apples
  • Pears

No two compartments can have the same name.

No compartment is allowed to have more than one slip of paper inside it.

The price of a can of beans

When you place a new slip of paper in a compartment, you must be careful to remove and destroy the one that was already there.

Each slip of paper that you insert into a compartment contains the price for the type of grocery identified by the label on the compartment.

For example, the slip of paper in the compartment labeled Beans may contain the value 15, meaning that each can of beans costs 15 cents.

The checkout procedure

As each customer comes to your checkout counter during the remainder of the day, you execute the following procedure:

  • Examine each grocery item to determine its type.
  • Read the price stored in the compartment corresponding to that type of grocery.
  • Add that price to that customer's bill using your mechanical adding machine.

In programming jargon, we would say that as you process each grocery item for the same customer, you are looping . We would also say that you are executing a procedure or an algorithm .

When you have processed all of the grocery items for a particular customer, you would

  • Press the TOTAL key on the adding machine,
  • Turn the crank, and
  • Present the customer with the bill.

A schematic representation of the procedure

We might represent the procedure in schematic form as shown in Figure 1 .

Figure 1 . A checkout counter algorithm.
For each customer, do the following: For each item, do the following:a. Identify the type of grocery item b. Get the price from the compartmentc. Add the price to accumulated total End loop on grocery itemsPresent customer with a bill End loop on a specific customer

Common programming activities

This procedure illustrates the three activities commonly believed to be the fundamental activities of any computer program:

  • sequence
  • selection
  • loop


A sequence of operations is illustrated by the three items labeled a, b, and c in Figure 1 because they are executed in sequential order.


The process of identifying the type of grocery item is often referred to as selection . A selection operation is the process of selecting among two or more choices.

Questions & Answers

how do they get the third part x = (32)5/4
kinnecy Reply
can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
okay, so you have 6 raised to the power of 2. what is that part of your answer
I don't understand what the A with approx sign and the boxed x mean
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
I'm not sure why it wrote it the other way
I got X =-6
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
oops. ignore that.
so you not have an equal sign anywhere in the original equation?
Commplementary angles
Idrissa Reply
im all ears I need to learn
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what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
Kevin Reply
a perfect square v²+2v+_
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Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Embra Reply
if |A| not equal to 0 and order of A is n prove that adj (adj A = |A|
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rolling four fair dice and getting an even number an all four dice
ramon Reply
Kristine 2*2*2=8
Bridget Reply
Differences Between Laspeyres and Paasche Indices
Emedobi Reply
No. 7x -4y is simplified from 4x + (3y + 3x) -7y
Mary Reply
is it 3×y ?
Joan Reply
J, combine like terms 7x-4y
Bridget Reply
how do you translate this in Algebraic Expressions
linda Reply
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Object-oriented programming (oop) with java. OpenStax CNX. Jun 29, 2016 Download for free at https://legacy.cnx.org/content/col11441/1.201
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