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# Lesson 4.02: For Loops Using Range
## Learning Objectives
Students will be able to...
* Define and identify: **range**.
* Use the `range` and `len` function to to update lists via for loops.
## Materials/Preparation
* [4.02 Slide Deck](https://github.com/TEALSK12/2nd-semester-introduction-to-computer-science/raw/master/units/4_unit/slidedecks/Intro%20Python%204.02%20TEALS.pptx)
* [Do Now][]
* [Lab - Getting Loopy][] ([docx][]) ([pdf][])
* [Associated Reading](https://tealsk12.github.io/2nd-semester-introduction-to-computer-science/readings.md.html#associatedreadings/4.2)
* Read through the Do Now, lesson, and lab so that you are familiar with the requirements and can assist students.
## Pacing Guide
| **Duration** | **Description** |
| ---------- | ----------- |
| 5 Minutes | Do Now |
| 10 Minutes | Lesson |
| 35 Minutes | Lab |
| 5 Minutes | Debrief |
## Instructor's Notes
### 1. Do Now
* Display the Do Now on the board.
* Students experiment with and are introduced to the `range` function.
### 2. Lesson
#### Part 1 of the Do Now
* Ask students what the `range` function did.
* Remind them that there are reference docs online.
* Show the docs for the range function. Note that it can take in a third value that is optional.
* Work together with the students to write a `for` loop using the `range` function.
#### Part 2 of the Do Now
* Ask the students what happened
* Ask the students why these values might be helpful
* They are a list of the indices!
#### Part 3 of the Do Now (many students likely didn't finish)
* Ask students to write the first line of the loop on the board
* Work together as a class to come to a solution that is demonstrated for all to see.
#### Debugging Loops
In the sample "Function Contains Two Errors" code, we expect the return value `True`, but we get an IndexError instead.
Suggest printing the values of the indices immediately before the line where the error appears (right after while statement).
First error: the index of the last character is `3`, so the initial value for `j` should be different:
`j = len(word2)-1`.
Second error: we need to include the character at index 0, so we need to change the `while` condition:
`while j >= 0:`
### 3. Lab
* Students re-write the fruit pluralize program from yesterday's do now, but without creating a new list.
* Students write a function that reverses the letters in a string.
### 4. Debrief
* Talk about any issues the students had with the lab today.
* Discuss how lists are mutable, so you don't have to return a new value. Instead, the list is just updated as the loop runs.
#### Video explanation on Python Lists being Mutable
[](https://youtu.be/_y3PqL4lIzw?t=181)
## Accommodation/Differentiation
* If students are having a hard time with the fruit pluralize, consider altering the fruit program to not allow fruit that ends in y.
* Some students may have issues with grabbing the last item of a string, consider providing tips or scaffolding for students that are struggling with this.
* Go over the bonus question if any students got to it. Discuss having a function inside of the loop and how that operated.
[Do Now]: do_now.md.html
[Lab - Getting Loopy]: lab.md.html
[pdf]: https://github.com/TEALSK12/2nd-semester-introduction-to-computer-science/raw/master/units/4_unit/02_lesson/lab.pdf
[docx]: https://github.com/TEALSK12/2nd-semester-introduction-to-computer-science/raw/master/units/4_unit/02_lesson/lab.docx