--- title: "Functions that give answers" subtitle: "Python functions and events · Functions · Lesson 4" --- # Functions that give answers **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Calculators, maps and games are full of functions that take something in and give an answer back. :::teacher-note **In one sentence:** Students write functions that give an answer back with return, see why print is not the same, and combine small functions to build a street. **Students should be able to say:** I can write a function that gives a value back with return. I can explain the difference between return (hand the value back) and print (only show it). I can build a program from several small functions working together. **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 `metres(5)` gives back a number. What do you think `forward(metres(5))` does? How far will the turtle move, and how far does the second `forward` take it? ```python from turtle import * scale = 20 def metres(m): return m * scale forward(metres(5)) right(90) forward(metres(3)) ``` :::keep-together My prediction, and why: {{lines-3}} ::: ## Run Run the program in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Investigate `print(metres(5))` shows the answer under the drawing. Change `scale` and watch both the printed value and the drawing change. ```python from turtle import * scale = 20 def metres(m): return m * scale print(metres(5)) forward(metres(5)) right(90) forward(metres(3)) ``` :::keep-together ### What comes back? **scale is 20. What number does metres(5) give back?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 100 metres(5) returns 5 * 20 = 100, so forward(metres(5)) moves 100. ::: :::keep-together ### Spot the bug **This version prints 100, then stops with a TypeError on the last line. Click the line that causes it.** ``` 1 from turtle import * 2 3 scale = 20 4 5 6 def metres(m): 7 print(m * scale) 8 9 10 forward(metres(5)) ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 7 print(m * scale) shows the value but does not give it back. It should be return m * scale. ::: :::keep-together ### Explain return **What does the word return do in metres?** ☐ It draws the shape again ☐ It prints the answer ☐ It gives the answer back to the code that called the function ☐ It stops the program ::: :::teacher-note **Answer:** It gives the answer back to the code that called the function return hands the value back, so forward(metres(5)) can use it. print only shows a value; it does not give it back. ::: :::typical ## Modify `room` calls `metres` to turn sizes into pixels. Functions are working together here. **Write area.** Write `def area(width, depth)` that **returns** the area. **Print the area.** Print the area of the room that the program draws. 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. **Write area:** ```python from turtle import * scale = 20 def metres(m): return m * scale def room(width, depth): for i in range(2): forward(metres(width)) right(90) forward(metres(depth)) right(90) def area(width, depth): return width * depth room(5, 4) ``` **Print the area:** ```python from turtle import * scale = 20 def metres(m): return m * scale def room(width, depth): for i in range(2): forward(metres(width)) right(90) forward(metres(depth)) right(90) def area(width, depth): return width * depth room(5, 4) print(area(5, 4)) ``` ::: ::: ## Mix it up Work this out on paper first. :::keep-together **What does label(50) give back?** ☐ 50 ☐ big ☐ nothing ☐ small ::: :::teacher-note **Answer:** small 50 > 50 is False, because 50 is not more than 50. So the else path runs and the function returns "small". ::: ## Make a street This time there is no starter. Build the street in two stages. **Draw one house.** Write a function that draws one house. **Draw five in a row.** Use a loop to repeat it for a **street of five houses**. 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. **Draw one house:** ```python from turtle import * def house(size): for i in range(4): forward(size) right(90) left(60) forward(size) right(120) forward(size) right(120) forward(size) right(180) house(60) ``` **Draw five in a row:** ```python from turtle import * def house(size): for i in range(4): forward(size) right(90) left(60) forward(size) right(120) forward(size) right(120) forward(size) right(180) penup() goto(-200, 0) pendown() for i in range(5): house(50) penup() forward(80) pendown() ``` ::: :::wider ## Vary a street of houses This is a finished street. Make the houses all the same size, then make them shrink along the street. Add a sixth house, or change the gap between them. 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 * def house(size): for i in range(4): forward(size) right(90) left(60) forward(size) right(120) forward(size) right(120) forward(size) right(180) def move(distance): penup() forward(distance) pendown() penup() goto(-220, 0) pendown() for i in range(6): house(70 - i * 10) move(75) ``` ::: ::: ## Exit ticket :::keep-together **def double(n): return n * 2. What does forward(double(30)) do?** {{lines-3}} ::: :::teacher-note **Answer:** double(30) gives back 60, so the turtle moves forward 60 steps. :::