--- title: "Text as numbers" subtitle: "Data representation · Data as numbers · Lesson 2" --- # Text as numbers **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Every message you send is stored as a list of numbers. What list is your name? :::teacher-note **In one sentence:** Students type words and read their character codes to see that text is a list of codes, one per character, including spaces, digits and lower case, and that Unicode gives codes to every character (AC9TDI8K03 node 2). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **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 My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 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. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: Hi 7! :::keep-together ### Encode Hi 7! 1. $\text{H} \to 72$ — Capital letters start at A = 65. H is the 8th letter: 65 + 7. 2. $\text{i} \to 105$ — Lower case has its own codes, starting at a = 97. Each is 32 more than its capital (I is 73). 3. $\text{space} \to 32$ — A space is a character too, so it has a code. 4. $\text{7} \to 55$ — The digit 7 is a character. Digits start at 0 = 48, so 7 is 55. 5. $\text{!} \to 33$ — Symbols have codes as well. 6. $\text{Hi 7!} \to 72, 105, 32, 55, 33$ — Text is the list of codes, in order. Now find lower case c. **Lower case letters start at a = 97. What number stores lower case **c**?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 99 a is 97, b is 98, c is 99. Each lower case letter is 32 more than its capital (C is 67). ::: :::keep-together ### 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. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 4 Line 4: in text, the 7 is a character like any other, and its code is 55 (the digits start at 0 = 48). 7 is the code for a control character, not the digit. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** They are different characters, so the computer must be able to tell them apart A and a look different and mean different things in passwords and names, so each has its own code. That is why passwords are case sensitive. ::: :::typical ## Vary: change one character **Lower case.** **One capital.** **A digit.** **A space.** **Beyond A to Z.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Codes :::keep-together ### Last letter **Capital letters run from A = 65. What is the code for **Z**?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 90 65 + 25 = 90. ::: :::keep-together ### A list of codes **Which list stores the text **Go 2**?** ☐ 7, 15, 2 ☐ 71, 111, 50 ☐ 71, 111, 32, 50 ☐ 71, 111, 2 ::: :::teacher-note **Answer:** 71, 111, 32, 50 G 71, o 111, space 32, digit 2 is 50. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** C is stored as 67 but c is 99, so the codes don't match The computer compares codes, and 67 is not 99. ::: :::keep-together ### Code to character **What character has the code 97 + 25 = 122?** ☐ y ☐ The number 122 ☐ z ☐ Z ::: :::teacher-note **Answer:** z 97 is a, so 97 + 25 is the 26th lower case letter, z. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** Two shades of grey The same numbers, read with a different rule: two dark greys. ::: ## 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? Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **Write the codes for the text Hi 5.** {{lines-3}} ::: :::teacher-note **Answer:** 72, 105, 32, 53 (the digit 5 is 48 + 5). :::