--- marp: true theme: default html: true --- # Text as numbers Data representation ยท Data as numbers --- ## Today **We are learning to** text is stored as a list of numbers, one for each character. Every character has its own code: A is 65, a is 97, a space is 32 and even the digit 7 has a code (55). Unicode gives codes to over a hundred thousand characters, including emoji. - I can say how a word is stored as numbers. - I can find the code of a letter, a space or a digit. - I can explain why some characters need more than one byte. --- ## Why it matters > Every message you send is stored as a list of numbers. What list is your name? --- ๐Ÿ’ฌ Discussion ## Predict **In text, **A** is stored as 65 and **B** as 66. How is the word **CAB** stored?** - One number for the whole word - As the letters C, A, B themselves - 3, 1, 2 - 67, 65, 66 --- โœ๏ธ Activity ## Try it {{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fsystems%2Fdata%2Ftext-as-numbers%2Fstep-2&code=widget%3A+decode%0Anumbers%3A+72%2C+105%0Aas%3A+text%0Aswitch%3A+off%0Atype%3A+on%0Atable%3A+on%0A&fresh=1}} [Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/systems/data/text-as-numbers/?full=step-2#step-2) --- โœ… Check ## What happened? * 67, 65, 66. Each character has its own code, and the capital letters count on from A = 65, so C is 67. The computer stores only the numbers. --- ๐Ÿ’ฌ Discussion ## Encode Hi 7! Now find lower case c. $$\text{H} \to 72$$ 1) $\text{i} \to 105$ โ€” Lower case has its own codes, starting at a = 97. Each is 32 more than its capital (I is 73). 2) $\text{space} \to 32$ โ€” A space is a character too, so it has a code. 3) $\text{7} \to 55$ โ€” The digit 7 is a character. Digits start at 0 = 48, so 7 is 55. 4) $\text{!} \to 33$ โ€” Symbols have codes as well. 5) $\text{Hi 7!} \to 72, 105, 32, 55, 33$ โ€” Text is the list of codes, in order. --- โœ… Check ## Encode Hi 7! **Lower case letters start at a = 97. What number stores lower case **c**?** * Answer: 99 --- โœ… Check ## Find the mistake **Sam stores the text "Year 7" as numbers. Click the line where the working first goes wrong.** ``` 1 Store the text "Year 7" as numbers. 2 Y is 89, e is 101, a is 97, r is 114. 3 The space between the words is 32. 4 The 7 is a number already, so it is stored as 7. 5 So "Year 7" is 89, 101, 97, 114, 32, 7. ``` * Answer: line 4 --- โœ… Check ## Capital or lower case? **Why do **A** and **a** have different codes?** - They are different characters, so the computer must be able to tell them apart - So that lower case takes up more space - Lower case letters are smaller, so they need a bigger number - They don't: A and a are both 65 * Answer: They are different characters, so the computer must be able to tell them apart --- โœ๏ธ Activity ## Vary: change one character - **Lower case:** - **One capital:** - **A digit:** - **A space:** - **Beyond A to Z:** [Open this step](https://studio.learningwaypoints.com/tutorials/systems/data/text-as-numbers/?full=step-4#step-4) --- โœ… Check ## Last letter **Capital letters run from A = 65. What is the code for **Z**?** * Answer: 90 --- โœ… Check ## A list of codes **Which list stores the text **Go 2**?** - 7, 15, 2 - 71, 111, 50 - 71, 111, 32, 50 - 71, 111, 2 * Answer: 71, 111, 32, 50 --- โœ… Check ## Passwords **Your password is **cat** but you type **Cat**. Why does it fail?** - Passwords ignore capitals - C is stored as 67 but c is 99, so the codes don't match - The computer can't read capital letters - Cat is longer than cat * Answer: C is stored as 67 but c is 99, so the codes don't match --- โœ… Check ## Code to character **What character has the code 97 + 25 = 122?** - y - The number 122 - z - Z * Answer: z --- โœ… Check ## Review: same numbers **Review: a text file holds 72, 105. A paint program opens it. What does it show?** - Two shades of grey - The word Hi - Nothing - The numbers 72 and 105 as text * Answer: Two shades of grey --- โœ๏ธ Activity ## Make: change case with numbers Stretch: the Caesar cipher in Secret messages adds the same number to every code. What would adding 1 to every code in HI make? [Open this step](https://studio.learningwaypoints.com/tutorials/systems/data/text-as-numbers/?full=step-6#step-6) --- โœ… Check ## Exit ticket **Write the codes for the text Hi 5.** * 72, 105, 32, 53 (the digit 5 is 48 + 5).