--- title: "Name a value" subtitle: "Python Foundations with Turtle · Commands, loops and values · Lesson 3" --- # Name a value **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Games keep score, maps keep scale and apps keep your settings. All of them store a value under a name and calculate with it. :::teacher-note **In one sentence:** Students give numbers names (variables) and let the program calculate the turn, so one change at the top updates the whole shape. **Students should be able to say:** I can store a value in a variable and use its name instead of the number. I can work out a value with a calculation, such as angle = 360 / sides. I can change a whole drawing by changing one variable. **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 `angle = 360 / sides` stores the result of a calculation. What will the turtle draw? How many sides, and why does `right(angle)` turn by that amount? ```python from turtle import * sides = 5 size = 80 angle = 360 / sides for i in range(sides): forward(size) right(angle) ``` :::keep-together **How many sides will the shape have?** My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Five sides. angle = 360 / 5 = 72, so the turtle turns 72 degrees at each corner and the shape closes. ::: ## Run Run the program in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Investigate `print(angle)` shows a value under the drawing. Copy it whenever you want to see what a name holds; the console lessons explain how it works. Change `sides` and watch what `angle` becomes, then work through the checks. ```python from turtle import * sides = 5 size = 80 angle = 360 / sides print(angle) for i in range(sides): forward(size) right(angle) ``` :::keep-together ### Calculate the angle **If sides = 6, what is angle?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 60 360 / 6 = 60, so the turtle turns 60 degrees at each corner. ::: :::keep-together ### Spot the bug **This program should draw a 5-sided shape, but it does not close. Click the wrong line.** ``` 1 from turtle import * 2 3 sides = 5 4 size = 80 5 angle = 360 * sides 6 7 for i in range(sides): 8 forward(size) 9 right(angle) ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 5 angle = 360 * sides is too big. To split one full turn between the corners it should be 360 / sides. ::: :::keep-together ### A name keeps its value A name holds one value at a time. Lines that ran earlier keep what they used. ```python size = 50 forward(size) size = 100 forward(size) ``` **The program stores size = 50 and draws a line. Later it stores size = 100 and draws another line. How long was the first line?** ☐ 100 ☐ 50 ☐ 150 ☐ Storing size twice is an error ::: :::teacher-note **Answer:** 50 A line uses the value size holds at that moment. Storing a new value changes the lines drawn after it, not the ones already drawn. ::: :::keep-together ### Spot the bug in the loop **This should draw a hexagon, but the turtle draws one long line and turns once at the end. Click the wrong line.** ``` 1 from turtle import * 2 3 sides = 6 4 size = 80 5 angle = 360 / sides 6 7 for i in range(sides): 8 forward(size) 9 right(angle) ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 9 right(angle) is not indented, so it is not part of the loop. Indent it under the for line so both lines repeat. ::: :::typical ## Modify Change one number at the top to draw a hexagon, then a 12-sided shape. Run it each time. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} :::teacher-note **One possible solution.** Other correct programs are fine; check that the program runs and does what the task asks. ```python from turtle import * sides = 12 size = 40 angle = 360 / sides for i in range(sides): forward(size) right(angle) ``` ::: ::: ## Make Make your own shape generator. Put three names at the top (`sides`, `size` and `colour`) and use all three below. Then change each one and watch the shape change. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} :::teacher-note **One possible solution.** Other correct programs are fine; check that the program runs and does what the task asks. ```python from turtle import * sides = 6 size = 80 angle = 360 / sides colour = "purple" pencolor(colour) for i in range(sides): forward(size) right(angle) ``` ::: :::wider ## Resize a room plan Plans and maps store a **scale**: how many pixels are one metre. This program draws a 6 m by 4 m room. Change the room to 8 m by 5 m, then change `scale` and notice the whole plan resizes. Then add a second room beside it. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} :::teacher-note **One possible solution.** Other correct programs are fine; check that the program runs and does what the task asks. ```python from turtle import * scale = 20 # pixels per metre width = 8 # metres depth = 5 for i in range(2): forward(width * scale) right(90) forward(depth * scale) right(90) # A second room beside the first: 4 m by 5 m. forward(width * scale) for i in range(2): forward(4 * scale) right(90) forward(depth * scale) right(90) ``` ::: ::: ## Exit ticket :::keep-together **A program has sides = 8 and angle = 360 / sides. How far does the turtle turn at each corner?** {{lines-3}} ::: :::teacher-note **Answer:** 45 degrees (360 divided by 8). :::