Lab 2: Stormy Weather

The purpose of this lab is to give you practice:

• using Boolean expressions,
• reading and writing if expressions,
• writing functions that use if expressions, and
• writing examples/tests for the functions you write.

This description is adapted from Wellesley CS111 and The National Center for Women and Information Technology. In pair programming, two programmers share one computer. One student is the “driver”, who controls the keyboard and mouse. The other is the “navigator”, who observes, asks questions, suggests solutions, and thinks about slightly longer-term strategies. The two programmers switch roles about every 20 minutes.

If you and your partner have different levels of experience and comfort with programming, you may worry that the less experienced partner will hold the other back or that there’s no benefit to participating actively, but pair programming studies show that paired work is consistently better than work the stronger partner does alone. It is each partner’s job to understand the whole task; that means asking questions when necessary and answering them when possible.

Working in pairs should make you better at programming than would working alone. The resulting work of pair programming nearly always outshines that of the solitary programmer, with pairs producing better code in less time.

You’ve been asked to review a program written by a coworker. The problem they were asked to solve was to return the storm category for a given wind speed measurement, using the Saffir–Simpson scale:

Your coworker shows you this solution:

fun saffir-simpson(wind-speed :: Number) -> String:
  doc: "Saffir-Simpson category for a given wind speed in miles per hour"
  if wind-speed > 0:
    "Tropical depression"
  else if wind-speed > 38:
    "Tropical storm"
  else if wind-speed > 73:
    "Category 1"
  else if wind-speed > 95:
    "Category 2"
  else if wind-speed > 110:
    "Category 3"
  else if wind-speed > 129:
    "Category 4"
  else if wind-speed > 156:
    "Category 5"
  end
end

This looks good… but it doesn’t work.

#|
  This is my answer...
  continuing...
  as long as I need it to!
|#

Now you know why this solution is incorrect, but can you fix it?

Feel free to start by copy-and-pasting the broken function.

Consider making a weather app that will send alerts when there’s a storm on campus. An important first step is asking whether a report of a storm is for an area on campus or not.

fun at-vassar(latitude :: Number, longitude :: Number) -> Boolean:
  ...
end

Be sure to test your function! You may want to use these coordinates:

Surprisingly, even though the at-vassar function is asking a true–false question, it doesn’t need an if expression!

Whenever you find yourself writing an if expression like

if 42 > 0:
  true
else:
  false
end

you can – and should – replace it with just the question:

42 > 0

This does the same thing – it returns true when the question is true and false when the question is false!

A Category 5 storm is extremely dangerous, so we want to send everyone an alert if there’s a Category 5 storm over Vassar!

fun extreme-alert(wind-speed :: Number, latitude :: Number,
    longitude :: Number) -> Boolean:
  ...
end

This function should be very short. As in Part 2, you don’t need to use an if. You also don’t need to deal with the wind speed or the latitude and longitude directly; instead, consider how you can use the saffir-simpson and at-vassar functions you’ve already written.

Some people would like alerts even if a storm isn’t that severe, so we’ll make a function that checks if the wind speed corresponds to a Tropical Storm or a Category 1–5 hurricane and the classification is for Vassar.

fun severe-alert(wind-speed :: Number, latitude :: Number,
    longitude :: Number) -> Boolean:
  ...
end

Here are some tests you can try (or make your own!):

# Hurricane at Main Building
severe-alert(190, 41.686804, -73.895664) is true
# Gentler weather at Main Building
severe-alert(10, 41.686804, -73.895664) is false
# Hurricane at the Griffith Observatory, Los Angeles
severe-alert(190, 34.119108, -118.300388) is false

As in Part 3, you should be able to do this using the saffir-simpson and at-vassar functions you already wrote. Consider how you can keep the body of this function short using Boolean operators and functions.

• When you’ve completed the exercises, show your code to your instructor or one of the coaches.
• Then upload your lab02.arr file to the Lab 2 assignment on Gradescope. You should submit a single copy of the lab with both your names – see these instructions.