Presentation on theme: "Computing Tools for Engineering Design (ENME 337 H(3-2))"— Presentation transcript:
1Computing Tools for Engineering Design (ENME 337 H(3-2)) “Spreadsheet Basics”Fig.: 4-line to 1-line multiplexerInstructor:Dr. Daniel Y. KwokOffice: ME321Phone:
2What is a Spreadsheet ?A spreadsheet is a rectangular table (or grid) of information. (It is, therefore, a kind of matrix.)The word came from "spread" in its sense of a newspaper or magazine item (text and/or graphics) that covers two facing pages, extending across the center fold and treating the two pages as one large one.The compound word "spread-sheet" came to mean the format used to present bookkeeping ledgers -- with columns for categories of expenditures across the top, invoices listed down the left margin, and the amount of each payment in the cell where its row and column intersect, for example -- which were traditionally a "spread" across facing pages of a bound ledger (= book for keeping accounting records) or on oversized sheets of paper ruled into rows and columns in that format and approximately twice as wide as ordinary paper.
3What is a spreadsheet?A spreadsheet is the computer equivalent of a paper ledger sheet. It consists of a grid made from columns and rows. It is an environment that can make number manipulation easy and somewhat painless.The math that goes on behind the scenes on the paper ledger can be overwhelming. If you change the loan amount, you will have to start the math all over again (from scratch). But let's take a closer look at the computer version.
4So why do it on a computer? Looking at our previous example it seems pretty evenly matched. Right? WRONG! The nice thing about using a computer and spreadsheet is that you can experiment with numbers without having to RE-DO all the calculations.Lets change the interest rate and then the number of months. Let the COMPUTER do the calculations! Once we have the formulas set up, we can change the variables that are called from the formula and watch the changes.Example: Change the Interest RateDo that on paper and you better get your calculator back out and get an Eraser and hope you punched all the right keys and in the right order.Spreadsheets are instantly updated if one of the entries is changed. NO erasers! NO new formulas! NO calculators!Spreadsheets can be very valuable tools in business. They are often used to play out a series of what-if scenarios! (much like our car purchase here.)
5Origins of the Spreadsheet The concept of an electronic spreadsheet was outlined in the 1961 paper "Budgeting Models and System Simulation" by Richard Mattessich. Some credit for the computerized spreadsheet perhaps belongs to Pardo and Landau, who filed a patent (U.S. Patent no. 4,398,249) on some of the related algorithms in While the patent was originally rejected by the patent office as being a purely mathematical invention, Pardo and Landau won a court case establishing that "something does not cease to become patentable merely because the point of novelty is in an algorithm." This case helped establish the viability of Software patents.The generally recognized inventor of the spreadsheet is Dan Bricklin. Bricklin has spoken of watching his university professor create a table of calculation results on a blackboard. When the professor found an error, he had to tediously erase and rewrite a number of sequential entries in the table, triggering Bricklin to think that he could replicate the process on a computer, using a blackboard/spreadsheet paradigm to view results of underlying formulas.His idea became VisiCalc, the first spreadsheet, and the "killer application" that turned the personal computer from a hobby for computer enthusiasts into a business tool..
6Microsoft ExcelExcelMicrosoft Excel clearly dominates the spreadsheet market. Not too long ago, Lotus was considered the "standard" spreadsheet. Excel now holds that distinction, with an estimated 90% market share.The table below lists the various versions of Excel for Windows that you may encounter.
7Version Released Comments 11985Version 1, for the Macintosh was released.21987The first Windows version was labeled "2" to correspond to the Mac version. This included a run-time version of Windows.31990The first "popular" version. Included lots of usability features.4199251993A major upgrade. Included multi-sheet workbooks and support for VBA.7*1995Known as Excel 95. The first major 32-bit version of Excel**. Feature-wise, it's very similar to Excel 5.81997Known as Excel 97. A new interface for VBA developers, UserForms, data validation, and lots more.91999Known as Excel Can use HTML as a native file format, "self-repair" capability, enhanced clipboard, pivot charts, modeless user forms.102001Known as Excel 2002, this is part of Office XP. It has a long list of new features, but most of them will probably be of little value to the majority of users. Perhaps the most significant feature is the ability to recover your work when Excel crashes. This version features product activation technology (i.e., copy protection). Please consider the ramifications of this before deciding whether to upgrade. 112003It's called Microsoft Office Excel 2003.The new features in this version are: (a) improved support for XML, (b) a new "list range" feature, (c) Smart Tag enhancements, and (d) corrected statistical functions. Most users will not find the upgrade worthwhile. Read John Walkenbach's Excel 2003 Review.* There is no Excel 6. Beginning with Excel 7, the version numbering was changed so all of the Microsoft Office applications would have the same version number.** There was also 32-bit version of Excel 5, but it was not widely distributed.
8Basics of a Spreadsheet So let's get started digging into what makes a spreadsheet work.Spreadsheets are made up ofcolumnsrowsand their intersections are called cellsIn each cell there may be the following types of datatext (labels)number data (constants)formulas (mathematical equations that do all the work)Take a look at the explanations of each of these.
9What is a Column?In a spreadsheet the COLUMN is defined as the vertical space that is going up and down the window. Letters are used to designate each COLUMN'S location.In the above diagram the COLUMN labeled C is highlighted.
10What is a Row?In a spreadsheet the ROW is defined as the horizontal space that is going across the window. Numbers are used to designate each ROW'S location.In the above diagram the ROW labeled 4 is highlighted.
11What is a CELL ?In a spreadsheet the CELL is defined as the space where a specified row and column intersect. Each CELL is assigned a name according to its COLUMN letter and ROW number.In the above diagram the CELL labeled B6 is highlighted. When referencing a cell, you should put the column first and the row second.
12Types of Data data types examples descriptions LABEL In a spreadsheet there are three basic types of data that can be entered.labels - (text with no numerical value)constants - (just a number -- constant value)formulas* - (a mathematical equation used to calculate)data typesexamplesdescriptionsLABELName or Wage or Daysanything that is just textCONSTANT5 or orany numberFORMULA= or = 8*5+3math equation*ALL formulas MUST begin with an equal sign (=)
13Labels in ExcelLabels are text entries. They do not have a value associated with them. We typically use labels to identify what we are talking about.In our first example: the labels are:computer ledgercar loaninterest# of paymentsMonthly Pmt.Again, we use labels to help identify what we are talking about. The labels are NOT for the computer but rather for us so that we can clarify what we are doing.
14Constants in ExcelConstants are entries that have a specific fixed value. If someone asks you how old you are, you would answer with a specific answer. Sure, other people will have different answers, but it is a fixed value for each person.In our first example: the constants were :$12,0009.6%60As you can see from these examples there may be different types of numbers. Sometimes constants are referring to dollars, sometimes referring to percentages, and other times referring to a number of items (in this case 60 months). These are typed into the computer with just the numbers and are changed to display their type of number by formatting (we will talk about this later). Again, we use constants to enter FIXED number data.
15Formulas in ExcelFormulas are entries that have an equation that calculates the value to display. We DO NOT type in the numbers we are looking for; we type in the equation. This equation will be updated upon the change or entry of any data that is referenced in the equation.In our first example, the solution was $ This was NOT typed into the keyboard.The formula that was typed into the spreadsheet was: =PMT(C4/12,C5,-C3)C4 (annual interest rate) was divided by 12 because there are 12 months in a year. Dividing by 12 will give us the interest rate for the payment period – in this case a payment period of one month.
16It is also important to type in the reference to the constants instead of the constants. Had I entered =PMT(.096,60,-12000) my formula would only work for that particular set of data. I could change the months above and the payment would not change. Remember to enter the cell where the data is stored and NOT the data itself.Formulas are mathematical equations. There is a list of the functions available within Excel under the menu INSERT down to Function.Formulas OR Functions MUST BEGIN with an equal sign (=).Again, we use formulas to CALCULATE a value to be displayed.
17Basic Formulas in Excel When we are entering formulas into a spreadsheet we want to make as many references as possible to existing data. If we can reference that information we don't have to type it in again. AND more importantly if that OTHER information changes, we DO-NOT have to change the equations.For example, if you work for 23 hours and make $5.36 an hour, how much do you make? We can set up this situation usingthree labelstwo constantsone equationLet's look at this equation in B4:= B1 * B2= 23 * 5.36Both of these equations will produce the same answers, but one is much more useful than the other. DO YOU KNOW which is BEST and WHY?It is BEST if we can Reference as much data as possible as opposed to typing data into equations.
18Change in FormulasIn our last example, things were pretty straightforward. We had number of hours worked multiplied by wage per hour and we got our total pay. Once you have a working spreadsheet you can save your work and use it at a later time. If we referenced the actual cells (instead of typing the data into the equation) we could update the entire spreadsheet by just typing in the NEW Hours worked. And -- you're done!Let's look at the new spreadsheet:hours have been changed to 34wage is the sametotal pay would now be = 34 * 5.36but would still be = B1 * B2If we had typed in ( = 23 * 5.36 ) the first time and just changed the hours worked, our equation in B4 would still be ( = 23 * 5.36 )INSTEAD we typed in references to the data that we wanted to use in the equation. We typed in ( = B1 * B2 ). These are the locations of the data that we want to use in our equation.It is BEST if we can Reference as much data as possible as opposed to typing data into equations.
19Basic Math FunctionsSpreadsheets have many Math functions built into them. Of the most basic operations are the standard multiply, divide, add and subtract. These operations follow the order of operations (just like algebra). Let's look at some examples.For these following examples lets consider the following data:A1 (column A, row 1) = 5A2 (column A, row 2) = 7A3 (column A, row 3) = 8B1 (column B, row 1) = 3B2 (column B, row 2) = 4B3 (column B, row 3) = 6AB15327486OperationSymbolConstant DataReferenced DataAnswerMultiplication*= 5 * 6= A1 * B330Division/= 8 / 4= A3 / B22Addition+= 4 + 7= B2 + A211Subtraction-= 8 - 3= A3 - B15
20Methods of Selecting Cells Selecting cells in an equation is a very important concept of a spreadsheet. We need to know how to reference the data in other parts of the spreadsheet. When entering your selection, you may use the keyboard or the mouse.We can select several cells together if we can specify a starting cell and a stopping cell. This will select ALL the cells within this specified BLOCK of cells.If the cells that we want to work with are not together (non-contiguous cells) we can use the comma to separate the cells or by holding down the control-key (command key on a MAC) and selecting cells or blocks of cells the comma will be inserted automatically to separate these chunks of data.
21=sum(Type In) or =sum(Click On) For the following examples lets consider the table below:A1 (column A, row 1) = 5A2 (column A, row 2) = 7A3 (column A, row 3) = 8B1 (column B, row 1) = 3B2 (column B, row 2) = 4B3 (column B, row 3) = 6AB15327486This is just a discussion of selection methods. If we wanted to add the cells in the (To Select) you would type in=sum(Type In) or =sum(Click On)
22To Select Type In Click On A1 click on A1 A1, A2, A3 A1:A3 with button downdrag to A3A1, B1A1:B1drag to B1A1, B3type in comma (or hold down the control key on a PC) (or hold down the command key on a MAC)click on B3A1, A2, B1, B2A1:B2drag to B2
24Sum FunctionProbably the most popular function in any spreadsheet is the SUM function. The Sum function takes all of the values in each of the specified cells and totals their values. The syntax is:=SUM(first value, second value, etc)In the first and second spots you can enter any of the following (constant, cell, range of cells).Blank cells will return a value of zero to be added to the total.Text cells can not be added to a number and will produce an error.Lets use the table here for the discussion that follows: We will look at several different specific examples that show how the typical function can be used! Notice that in A4 there is a TEXT entry. This has NO numeric value and can not be included in a total.A1252503754test5
25Examples: Example Cells to ADD Answer A 1 25 2 50 3 75 4 test 5 =sum (A1:A3)A1, A2, A3150=sum (A1:A3, 100)A1, A2, A3 and 100250=sum (A1+A4)A1, A4#VALUE!=sum (A1:A2, A5)A1, A2, A575
26Average FunctionThere are many functions built into many spreadsheets. One of the first ones that we are going to discuss is the Average function. The average function finds the average of the specified data. (Simplifies adding all of the indicated cells together and dividing by the total number of cells.) The syntax is as follows.=Average (first value, second value, etc.)Text fields and blank entries are not included in the calculations of the Average Function.Lets use the table here for the discussion that follows: We will look at several different specific examples that show how the average function can be used!A12525037541005
27Examples: Example Cells to Average Answer A 1 25 2 50 3 75 4 100 5 =average (A1:A4)A1, A2, A3, A462.5= average(A1:A4, 300)A1, A2, A3, A4 and 300110= average(A1:A5)A1, A2, A3, A4, A5= average(A1:A2, A4)A1, A2, A458.33
28MAX FunctionThe next function we will discuss is Max (which stand for Maximum). This will return the largest (max) value in the selected range of cells.Blank entries are not included in the calculations of the Max Function.Text entries are not included in the calculations of the Max Function.Lets use the table here for the discussion that follows.We will look at several different specific examples that show how the Max functions can be used! that show how the average function can be used!A1102203304test5
29Examples: Example of Max Cells to look at Ans. Max A 1 10 2 20 3 30 4 test5Examples:Example of MaxCells to look atAns. Max=max (A1:A4)A1, A2, A3, A430=max (A1:A4, 100)A1, A2, A3, A4 and 100100=max (A1, A3)A1, A3=max (A1, A5)A1, A510
30MIN FunctionThe next function we will discuss is Min (which stands for minimum). This will return the smallest (Min) value in the selected range of cells.Blank entries are not included in the calculations of the Min Function.Text entries are not included in the calculations of the Min Function.Lets use the table here for the discussion that follows. We will look at several different specific examples that show how the min functions can be used!A1102203304test5
31Examples: Example of Min Cells to look at Ans. Min A 1 10 2 20 3 30 4 test5Examples:Example of MinCells to look atAns. Min= min (A1:A4)A1, A2, A3, A410= min (A2:A3, 100)A2, A3 and 10020= min (A1, A3)A1, A3= min (A1, A5)A1, A5 (displays the smallest number)
32COUNT FunctionThe next function we will discuss is Count. This will return the number of entries (actually counts each cell that contains number data) in the selected range of cells.Blank entries are not counted.Text entries are NOT counted.Lets use the table here for the discussion that follows. We will look at several different specific examples that show how the Count functions can be used!A1102203304test5
33Examples: Example of Count Cells to look at Answer A 1 10 2 20 3 30 4 test5Examples:Example of CountCells to look atAnswer= Count (A1:A3)A1, A2, A33= Count (A1:A3, 100)A1, A2, A3 and 1004= Count (A1, A3)A1, A32= Count (A1, A4)A1, A41= Count (A1, A5)A1, A5
34CountA FunctionThe next function we will discuss is CountA. This will return the number of entries (actually counts each cell that contains number data OR text data) in the selected range of cells.Blank entries are not Counted.Text entries ARE Counted.Lets use the table here for the discussion that follows. We will look at several different specific examples that show how the CountA functions can be used!A1102203304test5
35Examples: Example of CountA Cells to look at Answer A 1 10 2 20 3 30 4 test5Examples:Example of CountACells to look atAnswer= CountA (A1:A3)A1, A2, A33= CountA (A1:A3, 100)A1, A2, A3 and 1004= CountA (A1, A3)A1, A32= CountA (A1, A4)A1, A4= CountA (A1, A5)A1, A51
36IF FunctionThe next function we will discuss is IF. The IF function will check the logical condition of a statement and return one value if true and a different value if false. The syntax is=IF (condition, value-if-true, value-if-false)value returned may be either a number or textif value returned is text, it must be in quotesLets use the table here for the discussion that follows. We will look at several different specific examples that show how the IF functions can be used!AB1PriceOver a dollar?2$.95No3$1.37Yes4comparing #returning #5140000.08684530.05
37Examples: Example of IF typed into column B Compares Answer 1PriceOver a dollar?2$.95No3$1.37Yes4comparing #returning #5140000.08684530.05Examples:Example of IF typed into column BComparesAnswer=IF (A2>1,"Yes","No")is ( .95 > 1)No=IF (A3>1, "Yes", "No")is (1.37 > 1)Yes=IF (A5>10000, .08, .05)is (14000 > 10000)0.08=IF (A6>10000, .08, .05)is (8453 > 10000)0.05
38PMT (loan stuff) Function The PMT function returns the periodic (in this case monthly) payment for an annuity (in this case a loan). This is the PMT function that was used for the car purchase in the first example. There are a few things that we must know in order for this function to work. To calculate the loan we must know a combination of the following(rate) interest rate per period(NPER) number of payments until repaid(PV) present value of the loan (amount we are borrowing)(FV) future value of the money (for saving or investing)(type) enter 0 or 1 to indicate when payments are due.=PMT(rate, NPER, PV, FV, type)
39Examples: Equation goes into c7 =PMT(C4/12,C5,-C3) C4 is the yearly interest and since it's compounded monthly we divide by 12C5 is the number of months (# of payments)-C3 is the amount of money we have (borrow - negative)Note that the rate is per period. If we have an annual interest rate of 9.6% and we are calculating monthly payments, we must divide the annual interest rate by 12 to calculate the monthly interest rate.
40SIN COS TAN etc. Functions Excel has most of the math and trig functions built into it. If you need to use the SIN, COS, TAN functions, they can be typed into any cell. If you wanted to find:format for degrees formula = sin (angle * pi()/180) the argument angle is in degreesformat for radians formula = sin (angle) the argument angle is in radiansanglesincostanREF=sin(REF)=cos(REF)=tan(REF)0.001.00300.500.870.58450.7190180-1.00To calculate trig functions in degrees you must convert them - otherwise excel will calculate them in radians.You can type in either an actual number for the REF or you can also type in a reference from the excel spreadsheet (like A2).
41Function WizardIn Excel there is a help tool for functions called the Function Wizard.There are two ways to get the function wizard. If you look at the Standard Toolbar, the function wizard icon looks like the icon on the right.The other way to get to the function wizard is to go to the Menu INSERT -- down to FUNCTION.Either way you get there, at this point Excel will list all of the functions available. Upon choosing the function, Excel will prompt you for the information it needs to complete the function. Mini descriptions are available for each of the cells. It is often necessary for you to understand the functions in order to be able to figure out these descriptions.Yeah, I know it would have been nice to know this earlier, but it is important for you to understand how the functions work before you start using the Function Wizard. It is faster to type the basic function in from the keyboard as opposed to going through the steps of this tool.Well, that is all of the functions we are going to cover.On to the next phase of the course.
42Copying FormulasSometimes when we enter a formula, we need to repeat the same formula for many different cells. In the spreadsheet we can use the copy and paste command. The cell locations in the formula are pasted relative to the position we Copy them from.Cells information is copied from its relative position. In other words in the original cell (C1) the equation was (A1+B1). When we paste the function it will look to the two cells to the left. So the equation pasted into (C2) would be (A2+B2). And the equation pasted into (C3) would be (A3+B3).ABC153=A1+B128=A2+B246=A3+B3=? + ?If you have a lot of duplicate formulas you can also perform what is referred to as a FILL DOWN. (discussed next).
43Fill DownOften we have several cells that need the same formula (in relationship) to the location it is to be typed into. There is a short cut that is called Fill Down. There are a number of ways to perform this operation. One of the ways is toselect the cell that has the original formulahold the shift key down and click on the last cell (in the series that needs the formula)under the edit menu go down to fill and over to downCells information is copied from its relative position. In other words in the original cell (C1) the equation was (A1+B1). When we paste the function it will look to the two cells to the left. So the equation pasted into (C2) would be (A2+B2). And the equation pasted into (C3) would be (A3+B3). And the equation pasted into (C4) would be(A4+B4).ABC153=A1+B128fill down46
44Absolute PositioningSometimes it is necessary to keep a certain position that is not relative to the new cell location. This is possible by inserting a $ before the Column letter or a $ before the Row number (or both). This is called Absolute Positioning.If we were to fill down with this formula we would have the exact same formula in all of the cells C1, C2, C3, and C4. The dollar signs Lock the cell location to a FIXED position. When it is copied and pasted it remains EXACTLY the same (no relative).ABC153=$A$1+$B$12846
45Fill RightWe can also fill right. We must select the original cell (and the cells to the right) and select from the Edit menu -- Fill and Right.ABC1=A2+$B$3=B2+$B$3=C2+$B$32653710498If we were to fill right from A1 to C1 we would get the formulas displayed to the left. Notice that the second part of the equation is FIXED or (ABSOLUTE REFERENCE so always references B3 which is 10).
47Formatting TextSpreadsheets can be pretty dry, so we need some tools to dress them up a little. We can use most of the tricks in our word processor to do the formatting of text. We can use : bold face, italics, underline, change the color, align (left, right, center), font size, font, etc.We need to select the cell (or group of cells) that we wish to change the formatting and then go from the FORMAT menu -- down to CELLS -- click on FONT. Here is a picture of what you will see there. Notice that you can choose to change the alignment as well as several other options.
48Formatting NumbersWe often need to format the numbers to display the appropriate number of decimals, dollar signs, percentage, red (for negative dollars), etc. It is best to keep numbers describing similar items as uniform as possible.Example:If we have the number , we would probably have to make the column wider and at the least bore most people. We need to set the number of decimal places to what is important. If this was a dollar figure that had calculated tax it should be $3.53.Here is a screen displaying what you would see if you select a cell (or group of cells) and from the FORMAT menu -- go down to format -- click on number.
49Column WidthA question that everyone (who has ever worked on a spreadsheet) has asked at one time or another is, "Where did all my numbers go?" or same question, "Where did all of those ####### come from and why are they in my spreadsheet?" The problem is the number trying to be displayed in a particular cell does not have enough width to display properly. To clear up the problem we just need to make the column wider. You can do this many ways.Here are two ways to change the column width:Select the column (or columns) with the problem by clicking on their labels (letters). Then you choose the MENU FORMAT. Go down to COLUMN and over to WIDTH and type in a new number for the column width.Move the arrow to the right side of the column label and click and drag the mouse to the right (to make wider) or left (to make smaller). Let up on the mouse button when the column is wide enough.Notice the cursor changes to a vertical line with arrows pointing left and right.In many spreadsheets you can also change the vertical height of a row by moving the lower edge of the row title (number).
50Inserting A ColumnSometimes we (all) make mistakes or things change. If you have a spreadsheet designed and you forgot to include some important information, you can insert a column into an existing spreadsheet. What you must do is click on the column label (letter) and choose in Columns from the Insert menu. This will insert a column immediately left of the selected column.As you can see from this example there was a blank column inserted into the spreadsheet. You might wonder if this will affect your referenced formulas. Yes, the Referenced cells are changed to their new locations. For example: Cell C4 was =C3+B4 and now is =D3+B4
51Inserting A RowLikewise, we can also insert rows. With the row label (number) selected you must choose the Row from the Insert menu. Again this will insert a row before the row you have selected.The formulas will be updated to their corresponding locations. C3 was = C2+B3 NOW C4=C2+B4
52Charts or GraphingNumbers can usually be represented quicker and to a larger audience in a picture format. Excel has a chart program built into its main program.The Chart Wizard will step you through questions that will (basically) draw the chart from the data that you have selected. There are many types of charts. The two most widely used are the bar chart and the pie chart.The BAR Chart is usually used to display a change (growth or decline) over a time period. You can quickly compare the numbers of two different bar charts to each other.The PIE Chart is usually used to look at what makes up a whole Something. If you had a pie chart of where you spent your money you could look at the percentages of dollars spent on food (or any other category).You can add legends, titles, and change many of the display variables.
53Next topic: Microsoft Excel and Visual basic (VBA) How to create programs in Excel using Macros and VB to solve diverse engineering problems in Heat Transfer, Numerical Methods, Fluid Mechanics, etc.
54Excel VBA Programming For Dummies DetailsAuthor: John WalkenbachISBN:Publisher: Wiley, 2004, 390 pp., List price $24.99 (US)Target Reader: This book is intended for experienced Excel users who want to learn the basics of VBA programming. It's good for Excel 2000, 2002, and 2003.Comments: This is a light-hearted guide to learning VBA programming. Despite the title, you'll find lots of useful information, tips, and techniques. The book doesn't have a CD, but the sample files are available to download from the Web.
55Excel VBA Programming For Dummies Table of ContentsPart I: Introducing VBAPart II: How VBA Works With ExcelPart III: Programming ConceptsPart IV: Developing Custom Dialog BoxesPart V: Creating Custom Menus and ToolbarsPart VI: Putting It All TogetherPart VII: The Part of Tens
56Spreadsheet Programming Just as the early programming languages were designed to generate spreadsheet printouts, programming techniques themselves have evolved to process tables (= spreadsheets = matrices) of data more efficiently in the computer itself. A spreadsheet program is designed to perform general computation tasks using spatial relationships rather than time as the primary organizing principle. Many programs designed to perform general computation use timing, the ordering of computational steps, as their primary way to organize a program. A well defined entry point is used to determine the first instructions, and all other instructions must be reachable from that point.In a spreadsheet, however, a set of cells is defined, with a spatial relation to one another. In the earliest spreadsheets, these arrangements were a simple two-dimensional grid. Over time, the model has been expanded to include a third dimension, and in some cases a series of named grids. The most advanced examples allow inversion and rotation operations which can slice and project the data set in various ways.The cells are functionally equivalent to variables in a sequential programming model. Cells often have a formula, a set of instructions which can be used to compute the value of a cell. Formulas can use the contents of other cells or external variables such as the current date and time. It is often convenient to think of a spreadsheet as a mathematical graph, where the nodes are spreadsheet cells, and the edges are references to other cells specified in formulas. This is often called the dependency graph of the spreadsheet. References between cells can take advantage of spatial concepts such as relative position and absolute position, as well as named locations, to make the spreadsheet formulas easier to understand and manage.Spreadsheets usually attempt to automatically update cells when the cells on which they depend have been changed. The earliest spreadsheets used simple tactics like evaluating cells in a particular order, but modern spreadsheets compute a minimal recomputation order from the dependency graph. Later spreadsheets also include a limited ability to propagate values in reverse, altering source values so that a particular answer is reached in a certain cell. Since spreadsheet cells formulas are not generally invertible, though, this technique is of somewhat limited value.Many of the concepts common to sequential programming models have analogues in the spreadsheet world. For example, the sequential model of the indexed loop is usually represented as a table of cells, with similar formulas. Cyclic dependency graphs produce the traditional construct known as the infinite loop. Most spreadsheets allow iterative recalculation in the presence of these cyclic dependencies, which can be either directly controlled by a user or which stop when threshold conditions are reached.The power of spreadsheets derives largely from the fact that human beings have a well developed intuition about spaces and a well developed notion of dependency between items. Thus, many people find it easier to perform complex calculations in a spreadsheet than writing the equivalent sequential program.