--- title: "Totals and the biggest" subtitle: "Data detective · Reading data · Lesson 2" --- # Totals and the biggest **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Average speed, top scorer, longest song: most questions about data are a total, an average or a biggest. :::teacher-note **In one sentence:** Students total a column, divide for an average and track the longest song with a biggest-so-far variable, then restrict the average to one genre, find the oldest song and write genre stats that cope with no matches. **Students should be able to say:** I can add up a column with a total that starts at 0. I can work out an average from a total and a count. I can keep the biggest (or smallest) so far as the loop goes. **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 Follow `total` through the loop: what is it after each number? Then what does `/ 3` give? ```python total = 0 for n in [3, 4, 5]: total = total + n print(total / 3) ``` :::keep-together **What does it print?** ☐ 4.0 ☐ 5 ☐ 12 ☐ 12.0 My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 4.0. total goes 3, then 7, then 12, and 12 / 3 is 4.0. Dividing with / always gives a decimal number. ::: ## Run Run the program in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Investigate ```python import csv with open("music.csv") as f: songs = list(csv.DictReader(f)) total = 0 longest = songs[0] for song in songs: minutes = float(song["minutes"]) total = total + minutes if minutes > float(longest["minutes"]): longest = song print("Average length: " + str(round(total / len(songs), 1)) + " minutes") print("Longest: " + longest["title"] + " (" + longest["minutes"] + " minutes)") ``` :::keep-together ### Follow the loop **The first three songs are Jolene (2.7), Bohemian Rhapsody (5.9) and Billie Jean (4.9). Fill in total and the longest minutes after each one.** {{rows 1}} | song | minutes | total | longest minutes | |---|---|---|---| | Jolene | 2.7 | 2.7 | 2.7 | | Bohemian Rhapsody | 5.9 | | | | Billie Jean | 4.9 | | | ::: :::teacher-note **Answer:** 8.6, 5.9, 13.5, 5.9 total grows with every song. longest changes only when a song is longer than the one it holds, so Billie Jean (4.9) does not replace Bohemian Rhapsody (5.9). ::: :::keep-together ### Where longest starts **Why does longest start as songs[0], not 0?** ☐ Python does not allow 0 there ☐ It holds a whole song (a dictionary), so it starts as a real song: the first one ☐ songs[0] is always the longest ☐ 0 would make the program slower ::: :::teacher-note **Answer:** It holds a whole song (a dictionary), so it starts as a real song: the first one longest is a song, so the program can print its title at the end. Starting with the first song means there is always something to compare with. ::: :::keep-together ### Spot the bug **This should add up the minutes of every song, but it prints only 3.3, the length of the last song. Click the line that is wrong.** ``` 1 import csv 2 3 with open("music.csv") as f: 4 songs = list(csv.DictReader(f)) 5 6 total = 0 7 for song in songs: 8 total = float(song["minutes"]) 9 print("Total: " + str(total) + " minutes") ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 8 total = float(...) replaces the total each time. total = total + float(...) adds each song on. ::: :::typical ## Modify **Average rock song.** Print the average length of the **rock** songs only. `len(songs)` counts every song, so you need a count of your own. **The oldest song.** Also print the **oldest** song and its year, in the same loop. 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. **Average rock song:** ```python import csv with open("music.csv") as f: songs = list(csv.DictReader(f)) total = 0 count = 0 longest = songs[0] for song in songs: if song["genre"] == "rock": minutes = float(song["minutes"]) total = total + minutes count = count + 1 if float(song["minutes"]) > float(longest["minutes"]): longest = song print("Average rock song: " + str(round(total / count, 1)) + " minutes") print("Longest: " + longest["title"] + " (" + longest["minutes"] + " minutes)") ``` **The oldest song:** ```python import csv with open("music.csv") as f: songs = list(csv.DictReader(f)) total = 0 longest = songs[0] oldest = songs[0] for song in songs: minutes = float(song["minutes"]) total = total + minutes if minutes > float(longest["minutes"]): longest = song if int(song["year"]) < int(oldest["year"]): oldest = song print("Average length: " + str(round(total / len(songs), 1)) + " minutes") print("Longest: " + longest["title"] + " (" + longest["minutes"] + " minutes)") print("Oldest: " + oldest["title"] + " (" + oldest["year"] + ")") ``` ::: ::: ## Make: genre stats Ask for a genre, then print how many songs it has, their average length and its **shortest** song. If nobody types a genre that is there, say so instead of crashing (dividing by a count of 0 is an error). 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 import csv with open("music.csv") as f: songs = list(csv.DictReader(f)) genre = input("Which genre? ").strip().lower() matches = [] for song in songs: if song["genre"].lower() == genre: matches.append(song) if len(matches) == 0: print("No songs in that genre.") else: total = 0 shortest = matches[0] for song in matches: minutes = float(song["minutes"]) total = total + minutes if minutes < float(shortest["minutes"]): shortest = song print(str(len(matches)) + " songs, " + str(round(total / len(matches), 1)) + " minutes on average") print("Shortest: " + shortest["title"] + " (" + shortest["minutes"] + " minutes)") ``` ::: :::wider ## Where this leads Next: count every genre at once, and draw the counts as a bar chart. ::: ## Exit ticket :::keep-together **Why does an average of the pop songs need its own count?** {{lines-3}} ::: :::teacher-note **Answer:** len(songs) counts every song; the average of some songs divides their total by how many of them there are, counted inside the if. :::