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Python Python Collections (2016, retired 2019) Sets Set Math

Kristian Vrgoc
Kristian Vrgoc
3,046 Points
Posted by Kristian Vrgoc
Kristian Vrgoc
3,046 Points

Sets.py, Task 2 of 2: understanding the if statement.

Hey Guys,

I got the solution on the forum. I understand what the if statement is doing, but why in this way? My comments are added in the code.

Kind regards

Kristian

sets.py 
COURSES = {
    "Python Basics": {"Python", "functions", "variables",
                      "booleans", "integers", "floats",
                      "arrays", "strings", "exceptions",
                      "conditions", "input", "loops"},
    "Java Basics": {"Java", "strings", "variables",
                    "input", "exceptions", "integers",
                    "booleans", "loops"},
    "PHP Basics": {"PHP", "variables", "conditions",
                   "integers", "floats", "strings",
                   "booleans", "HTML"},
    "Ruby Basics": {"Ruby", "strings", "floats",
                    "integers", "conditions",
                    "functions", "input"}
}

def covers(arg):
    course_list = []
    for key, value in COURSES.items():
        if value.intersection(arg):
            course_list.append(key)
    return course_list

def covers_all(arg):
    name_list = []
    for keys, values in COURSES.items():
            if len(arg & values) == len(arg):       # The outcome is always == len(arg), right? Like a infinite while loop?
                name_list.append(keys)              # Therefore it is like a trick to get to the goal, am I getting this right?
    return name_list

3 Answers

Steven Parker
Steven Parker
231,236 Points
Steven Parker
Steven Parker
231,236 Points

The outcome of "len(arg & values)" would only be the same as "len(arg)" when every item in "arg" was contained in "values". That's the point, to be sure that it "covers all". If it only covers some items, the lengths would differ.

This could also be done in a similar way to the first task by using "issubset" instead of "intersection".

Dave StSomeWhere
Dave StSomeWhere
19,870 Points
Dave StSomeWhere
Dave StSomeWhere
19,870 Points

Your first comment is incorrect, the length of the intersection of the arg and values will only be equal to the len of arg if all entries in arg are in the values variable. Basically this is verifying that all the topics exist in the course. 

def covers_all(arg):
    name_list = []
    for keys, values in COURSES.items():
            if len(arg & values) == len(arg):       # The outcome is always == len(arg), right? Like a infinite while loop?
                name_list.append(keys)              # Therefore it is like a trick to get to the goal, am I getting this right?
    return name_list

Does that help, Dave :tropical_drink: :palm_tree:

Steven Parker
Steven Parker
231,236 Points
Steven Parker
Steven Parker
231,236 Points

Kristian Vrgoc — Glad to help. You can mark the question solved by choosing a "best answer".
And happy coding!