Making Reference Diagrams

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En provenance du cours de University of Michigan

Python Basics

1051 notes

University of Michigan

Python Basics

1051 notes

Cours 1 sur 5 dans Specialization Python 3 Programming

This course introduces the basics of Python 3, including conditional execution and iteration as control structures, and strings and lists as data structures. You'll program an on-screen Turtle to draw pretty pictures. You'll also learn to draw reference diagrams as a way to reason about program executions, which will help to build up your debugging skills. The course has no prerequisites. It will cover Chapters 1-9 of the textbook "Fundamentals of Python Programming," which is the accompanying text (optional and free) for this course. The course is for you if you're a newcomer to Python programming, if you need a refresher on Python basics, or if you may have had some exposure to Python programming but want a more in-depth exposition and vocabulary for describing and reasoning about programs. This is the first of five courses in the Python 3 Programming Specialization.

À partir de la leçon

Sequence Mutation and Accumulation Patterns

In week four we will present deeper knowledge on using lists, strings, and python objects in general. We will also cover how to use the accumulation pattern with lists and with strings. The final assignment will test your knowledge and skills through application, much like previous assessments and assignments did, though with a more difficult set of tasks now that you have learned the basics.

Introduction - Way of the Programmer0:47

Making Reference Diagrams3:40

Accumulator Pattern Strategies8:44

Conclusion - Way of the Programmer0:54

Rencontrer les enseignants

Paul Resnick
Michael D. Cohen Collegiate Professor
School of Information
Steve Oney
Assistant Professor
School of Information
Jaclyn Cohen
Lecturer
School of Information

When in doubt, make a reference diagram.

Even if you only might be in doubt,

make a reference diagram.

It's going to seem slow,

painful, even to watch me do it.

When you're doing it for the first few times it's going to

be even slower and more painful,

but it's worth it because it gives you a way of understanding

things that can be pretty confusing otherwise.

So, here's a little code example.

Four lines and then we're printing out what's the result.

But it's not so easy to figure out exactly what's going to print out.

So, let's make a diagram to follow it.

We're going to basically simulate like what CodeLens would do.

We're going to do it ourselves.

So, x on line one gets bound to a list of three items: one, two, three.

On line two, y becomes an alias for x.

On line three we have this x plus equals.

Now, I gave you the suggestion that when you're dealing with lists,

just don't use plus equals.

But sometimes you have to read

other people's code and they don't follow that good advice.

So, suppose you had this x plus equals,

that's the version where we're going to take the new list: four and five,

and append them on to the old list,

so that we now have a list of five items,

and line three changes the object that x is pointing to.

Since y is pointing to that same object,

y now has five items as well.

Next, on line four,

we're going to use the better version that is a little easier to understand

where we do the normal thing of

figuring out the right hand side which takes the list: one,

two, three, four, five,

and appends another list to it,

making a new list.

So, then we have one, two,

three, four, five, that's a list.

We concatenate on this other list which contains six and then we reassign it.

That's what this equal is doing so that y no longer points to what it used to point to,

it now points to this new value.

X, however, still points to the old value.

So, when we print out x on line five,

we're going to get five items,

and we print out y on line six,

we're going to get all six items.

Let's check and see if that analysis is correct.

Sure enough, line five prints out five items,

line six prints out all six.

Trying to do this in your head,

especially if you ever end up with

more than four lines of code that are doing things like this,

very difficult to do.

So remember, if there's anything even remotely confusing,

make a reference diagram.

Practice now even when it might not be so confusing.

You'll thank me later. On to the next.

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