How Computers Think and Store Information

Origin of modern computing architectures

• computer = the one who computes

Jacquard Loom

Charles Babbage’s Analytical Machine

Hollerith Census Machine (eventually becomes IBM)

ENIAC: First computer

TIPComputers do not think

• Everyday speech: “the computer thinks”, “Excel calculated that”, “my phone knows”
• More accurate: a computer follows instructions on data
• Those instructions were written by people (or by other programs written by people)
• Excel is one such program: you write many of the instructions as formulas

The loop every computer runs

flowchart LR
  A[Input] --> B[Instructions]
  B --> C[Memory]
  C --> D[Output]
  D --> C

• Input: keyboard, file, sensor, a cell you typed into
• Instructions: a program, a formula, a Python script, a Copilot prompt that becomes code
• Memory: short-term (RAM / unsaved workbook) and long-term (disk / OneDrive / .xlsx file)
• Output: screen, chart, printed report, another file

TIPWhy Excel?

… and not Words or Adobe Acrobat or other applications?

• This course is called Programming and Data Science.
  • What is Charles Babbage’s Analytical Machine used for?
  • What is Hollerith Census Machine used for?
  • What is ENIAC used for?
• Mathematical calculations provide the fundamental drive for automatic computation.
  • Modern computers: still computations, only much bigger and much faster.
• Excel is the perfect application:
  • Driven by business motivation
  • Easy access for the general population

Introduction to Excel

Core concepts of an Excel application (online or offline)

• Workbook: the .xlsx file (the container)
• Worksheet: one grid inside the workbook (Sheet1, Data, Notes)
• Cell: intersection of a column letter and a row number (B2)
• Range: a rectangle of cells (B2:D10)
• Formula bar: shows the stored instruction or value
• Ribbon: Home / Insert / Data / Formulas — where most lab steps live
• Name Box: left of the formula bar; jump to a cell or name a range

Where the wild things are?

• Chip/CPU/Processor: executes an application’s instructions, one after another
• Memory/RAM: contains instructions and data of an application when that application is running. Goes way when the computer is shutdown or the application is closed.
• Storage/SSD/HDD: persistent storage of an application and its data.

Online versus offline

• A .xlsx workbook is a file: a packaged collection of worksheets, charts, and metadata
  • It’s all 0s and 1s!
• Immediate edits are stored in memory
  • Closing Excel without saving is throwing these edits
  • Need autosave or explicit Save/Save As to write to storage (persistence)

• Edits make to online access are saved as a slower rate.

What the raw data looks like?

• Computers store everything as bits (0 and 1)
• A byte is 8 bits; enough for a small integer or one character in older encodings
• You will not count bits in this course
• You will care that the machine must be told whether a pile of bits is a number, text, or a date

Good first habits

• Rename sheets (Data, Notes, Checks) instead of leaving Sheet1
• Freeze the header row when the list grows (View → Freeze Panes)
• Save to OneDrive so AutoSave is persistence
• One idea per sheet when you can: raw data stays raw

Types of information

Why types exist

• 100 the number and "100" the text are not the same
• You can add numbers; you cannot (sensibly) add two ZIP codes stored as text
• Dates are not “how they look”: they are values that can be sorted and subtracted
• True/false (Boolean) values drive IF decisions later in the semester

Types you will use constantly in Excel

What you mean	Typical Excel storage	Danger if you get it wrong
Quantity, money, score	Number	Text that looks like a number will not SUM
Name, ID, ZIP, phone	Text	Leading zeros vanish if stored as Number (08052 → 8052)
When something happened	Date/time (a serial number)	Sorting “September” as text puts it after “August” incorrectly
Yes / no, pass / fail	Boolean or TRUE/FALSE	"TRUE" as text is not the same as TRUE
A rule, not a value	Formula	Copy-paste as values and the rule is gone

What Excel is really storing

Formula bar versus Cell

• The cell display the final interpreted value
• The formula bar is closer to what is stored in memory
• Example: cell shows 50%; formula bar may show 0.5
• Example: cell shows 9/1/2026; Excel may be storing 45901 (a date serial)

Number formats: how data should be viewed

• General, Number, Currency, Percentage, Date, Text change how a value looks
• Changing the format does not change the underlying value (usually)
• Exception: if you type into a cell already formatted as Text, Excel may store digits as text
• Green triangles and ISTEXT / ISNUMBER are early debugging tools

Formulas: how data should be calculated/created

• =B2+C2 means: when asked, fetch B2 and C2, add, put the result here
• Recalculation is the computer running that instruction again
• If B2 is text "10", the instruction may fail or silently do the wrong thing
• This is SLO4 and SLO5 in miniature: right types, then check correctness

Try this in Excel

Lab 1: Build a tiny memory map

1. Create a new workbook and save it to OneDrive as week01-memory.xlsx
2. In A1:A4 enter labels: Quantity, ZIP, Date, Share
3. In B1 enter 12 (Number)
4. In B2 enter 08052 and format the cell as Text before or immediately after typing; confirm the leading zero stays
5. In B3 enter today’s date; change the format between Short Date and Number; watch the serial
6. In B4 enter 0.25 and format as Percentage
7. In C1 enter =B1*B4 and explain in an adjacent cell what instruction you wrote

Lab 1b: Persistence

• Close the file, reopen it: RAM was discarded; the file restored the cells
• Turn AutoSave off temporarily (if you can), type a value, force-quit Excel: that value was only in RAM
• (Do this on a throwaway copy.)

Lab 1b: Persistence

• Close the file, reopen it: RAM was discarded and the file restored the cells
• Turn AutoSave off temporarily (if you can), type a value, force-quit Excel: that value was only in RAM
  • (Do this on a throwaway copy.)