--- title: "If and else" subtitle: "Python Foundations with Turtle · Choices and lists · Lesson 7" --- # If and else **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Traffic lights, game scores, login screens and thermostats all do the same thing: check a condition and pick what happens next. :::teacher-note **In one sentence:** Students make a program choose between two paths with if and else, reading a condition that is true or false. **Students should be able to say:** I can write an if / else that chooses between two actions. I can work out whether a condition is true for a given value. I can say that only one of the two paths runs on each pass. **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 `i` is the counter from the spiral. The new line is `if i < 3:`. Read the code. What will the turtle draw? ```python from turtle import * pensize(6) for i in range(6): if i < 3: pencolor("red") else: pencolor("blue") forward(60) right(60) ``` :::keep-together **What will the turtle draw?** ☐ A hexagon with the sides taking turns, red then blue ☐ A hexagon: three red sides, then three blue ☐ A hexagon with every side red, then every side blue (twelve lines) ☐ A blue hexagon, because else runs last and covers the red My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Three red sides, then three blue. On each pass the program asks one question, i < 3, and runs only one of the two paths. i is 0, 1, 2 for the first three passes (red) and 3, 4, 5 for the last three (blue). ::: ## Run Run the program in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Investigate ```python from turtle import * pensize(6) for i in range(6): if i < 3: pencolor("red") else: pencolor("blue") forward(60) right(60) ``` :::keep-together ### Trace the passes **Fill in the gaps for each pass: is i < 3, and which colour does the program pick?** {{rows 1}} | i | i < 3 | colour | |---|---|---| | 0 | True | red | | 2 | | | | 3 | | | | 5 | | | ::: :::teacher-note **Answer:** True, red, False, blue, False, blue The condition is true for 0, 1 and 2, so the if path runs and the pen is red. From 3 on it is false, so the else path runs and the pen is blue. ::: :::keep-together ### Spot the bug **This should colour only the fourth side red, but the program will not even start. Click the wrong line.** ``` 1 from turtle import * 2 3 pensize(6) 4 for i in range(6): 5 if i = 3: 6 pencolor("red") 7 else: 8 pencolor("blue") 9 forward(60) 10 right(60) ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 5 A single = stores a value. To ask whether two things are equal, the condition needs two: i == 3. ::: :::keep-together ### True or false **A condition works out to a value, like a sum does. What does 5 < 3 work out to?** ☐ 5 ☐ 3 ☐ False ☐ True ::: :::teacher-note **Answer:** False 5 < 3 is a question, and the answer is False. Every condition works out to True or False, and if runs its lines only when it is True. ::: :::typical ## Modify Change the condition so **four** sides are red and two are blue. Run it to check. 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 * pensize(6) for i in range(6): if i < 4: pencolor("red") else: pencolor("blue") forward(60) right(60) ``` ::: ::: ## Mix it up This mixes two lessons: a value that grows, and a decision that reads it. Work it out on paper first. ```python from turtle import * size = 10 pensize(5) for i in range(12): # Choose the colour here: red while size is small, blue once it is big. forward(size) right(90) size = size + 10 ``` :::keep-together **size starts at 10 and grows by 20 every pass. A line is red while size < 50, otherwise blue. How many of the first six lines are red?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 2 size is 10, 30, 50, 70, 90, 110. Only 10 and 30 are less than 50, so two lines are red. ::: ## Make The spiral grows by 10 every pass. Make it **red while the lines are short and blue once they are long**. Then try a different cut-off, or a third colour of your own once you have seen `elif` next lesson. 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 * size = 10 pensize(5) for i in range(12): if size < 60: pencolor("red") else: pencolor("blue") forward(size) right(90) size = size + 10 ``` ::: ## Exit ticket :::keep-together **A loop colours pass i red if i < 2, else blue. For range(5), how many passes are blue?** {{lines-3}} ::: :::teacher-note **Answer:** Three. The red passes are i = 0 and 1, so 2, 3 and 4 are blue. :::