Trying to figure out how to get a result I want with this increment/dictionary problem

Python Help
1

Hey, everybody. I’m new to programming and Python. In the Python Crash Course book it gives this example:

alien_0 = {'x-position': 0, 'y-position': 25, 'speed': 'medium'}
print(f'Original position: {alien_0["x-position"]}')

Move alien to the right.

Determine how far to move the alien based on it’s current speed.

if alien_0['speed'] == 'slow':
    x_increment = 1
elif alien_0['speed'] == 'medium':
    x_increment = 2
else:
    x_increment = 3

(else being fast)

The new position is the old position plus the increment

alien_0['x-position'] = alien_0['x-position'] + x_increment
print(f'New position: {alien_0['x-position']}')

The results of this is:
Original position: 0
New position: 2

At the end of this section he says I can change a medium-alien to a fast alien using this line:

alien_0['speed'] = 'fast'

"“The if-elif-else block would then assign a larger value to x-increment the next time the code runs.”

After I do this:

alien_0['speed'] = 'fast'
alien_0['x-position'] = alien_0['x-position'] + x_increment
print(f'New position: {alien_0['x-position']}')

The results are:
Original position: 0
New position: 2
New position: 4

I asked elsewhere why the increment isn’t 3, and was told in order to change the increment variable’s value (didn’t even know increment was a variable) I have to execute code to change it. I tried several things, but I don’t understand it at all. I would appreciate any help.

2

This code:

if alien_0['speed'] == 'slow':
    x_increment = 1
elif alien_0['speed'] == 'medium':
    x_increment = 2
else:
    x_increment = 3

sets x_increment according to the value of alien_0['speed'].

Then you change the speed of the alien with:

alien_0['speed'] = 'fast'

but this doesn’t magically change the value of x_increment.

You have to run the code that sets x_increment again.

3

Hi,

if I told you I would pay you $1000 dollars if you worked an 8 hour shift on Monday, $2000 on Wednesday, and $3000 on Friday, then you could say that your pay varies depending on the day that you work, correct? So, we can say that your pay is variable. The same holds for x_increment as highlighted by the if/elif/else conditional statements. It varies as a function of the alien_0[‘speed’] value.

Your friend is correct. Unless you explicitly change the value of alien_0[‘speed’], it will remain its original value the next time the if/elif/else conditional statement is encountered (either by way of a function call or some loop, etc.)

4

Others have already really answered your basic question, but you can avoid having to rerun the code each time, by changing the variable x_increment into a function (which seems more suitable instead of a fixed variable):

alien_0 = {'x-position': 0, 
           'y-position': 25, 
           'speed': 'medium'}
print(f'Original position: {alien_0["x-position"]}')

def x_increment():
    match alien_0["speed"]:
        case "slow": return 1
        case "medium": return 2
        case _: return 3

Then do

alien_0['speed'] = 'fast'
alien_0['x-position'] += x_increment()   # note: now called as function of the speed
print(f"New position: {alien_0['x-position']}")

Now the speed will be taken into account as if by magic :mage: :magic_wand:

5

A shorter alternative is to use a dict:

def x_increment():
    return {'slow': 1, 'medium': 2}.get(alien_0['speed'], 3)
2 Likes
6

Thanks for the reply. I don’t know how to actually set x-increment again though. Maybe it’s very simple, but I just don’t know. I had no idea I’d run into this problem lol.

7

Thank you for the reply. I haven’t gotten to “def” yet. I’ll get back to this as soon as I do!

8

Thanks. I’ll try this out soon too.

9

This might be a good time to re-evaluate the book you’re using. From the code that’s shown here, it looks like the author is expecting you to understand concepts like this already. It could be that you aren’t in the target audience for the book (teaching someone’s first programming language is different, because it’s necessary to teach the basic idea of programming, as well as the actual language). Or perhaps it will help to review previous exercises (assuming this isn’t the first one!) and make sure you understand them more fully.

10

Thanks for the reply. I was thinking if I forgot something, but I feel like I haven’t forgotten this, but I’ll take a look back. I think the book is fine for a beginner so far. Of course it’s not going to have all the answers I need. People on the internet have helped me (there’s no way I’d have understood some of the for loop stuff from the book without help from people on the internet). For this example I thought it was abrupt not to give the result.

I don’t mind much doing some research on my own/asking questions, I just don’t like the way this problem was presented if it’s a curveball.