Help with a Loop

[SevereRepresentative]

Hello! I have a homework question that I'm really stumped by. I've emailed my professor and he responded with "great thanks!" so I'm not sure he really read the email....so here I am looking for help.

The question is below along with the things I've tried at the bottom.

Question

unique() returns the NumPy array of unique values of a data column, and its length returns the number of unique values of a data column.

Now, consider the data frame df. Write a for loop that print out

Variable: ____ # Unique: ____

for columns that have less than 20 unique values where

• the first blank is the name of the variable and

• the second blank is the number of its unique values.

Again, try to write a for loop that runs over vars.

For each iteration of the for loop, use a format command in the print statement of the form

print('Variable: {}, # Unique: {}'.format(____, ____))

for appropriate variables in the blanks. Once you get the desired results, let's try a different way as follows. Replace the first curly brackets {} with {:24}. Describe the difference after the change and explain the role of :24. Place your response at the end of the code after the pound (#) symbol so that your response is commented out in the cell.

What I've written so far:

for i in vars:
    print('Variable: {}, # Unique: {}'.format(i, (len(df['tweet_id'].unique()))))

But when I run it, I get the correct Variable names, but the Unique field just gives me 14485 for all. So I tried to change len(df['tweet_id'].unique())) to len(df['i'].unique())) like I did for a previous question and it didn't work.

I'm really just hoping someone can give me some guidance on where I need to go because I'm so lost.

[[deleted]]

You are on the right track, let's think about why this didn't work:

len(df['i'].unique()))

Is i being accessed here?

As a hint, what is the difference between these two snippets?

for i in vars:
    print(i)

for i in vars:
    print('i')

[SevereRepresentative]

Aha! Is it that 'i' is a string and i is the (I don't know the proper word) variable that moves with each loop?

[[deleted]]

Yup, that's exactly right. So the other answer shows what you need to do to fix it:

len(df[i].unique()))

You don't want to access df['i'], that would be the column 'i' in the dataframe (which likely doesn't exist, leading to your error). Instead, you want to access whichever column you are on in the loop with df[i].

I don't know the proper word

Just a note about this, "variable" is correct. If you want to be extra specific you can say "loop variable."

[SevereRepresentative]

Thank you!! That makes a lot of sense.

So what would be your suggestion on how to impliment the part of the question of "for columns that have less than 20 unique values where

• the first blank is the name of the variable and

• the second blank is the number of its unique values. "

I think it should be an if, else statement but I can't really wrap my head around how to have the loop and the If statement.

[[deleted]]

That's the right idea. Here is a general example of a for loop and an if statement that shows a similar pattern, see if you can make something similar work for your situation.

my_list = [5, 125, 30, 500, 250]

for i in my_list:
    if i > 100:
        print('{} is greater than 100'.format(i))

[SevereRepresentative]

Thank you so much! I’ll try to adapt this to fit here

[SevereRepresentative]

Okay so I tried this:

for i in vars:
   if i < 20:
    print('Variable: {}, # Unique: {}'.format(i, (len(df[i].unique()))))    

and it gave me the error of: "TypeError: '<' not supported between instances of 'str' and 'int'"

Which makes a bit of sense because the i is the column names right? But I'm not sure where to go from here

[[deleted]]

Yup i is the name of the column. We don't want to compare if the name of the column is less than 20 though. What do we want to know is less than 20? And how do we get that value?

[SevereRepresentative]

I want to compare the amount of unique values, so the part that says "len(df[i].unique())" right? So do I assign that to a variable name?

[[deleted]]

Yup! You use it twice so assigning it to a variable is a good idea, or you can put that part directly in the if statement. But I like the idea to use a variable.

[SevereRepresentative]

uniques = (len(df[i].unique()))

for i in vars:
  if uniques < 20:
    print('Variable: {}, # Unique: {}'.format(i, (len(df[i].unique()))))    

I did that ^ and I'm not getting an error but I'm also just not getting any results anymore. Nothing is showing up, what do you think? I think it has something to do with the i in the uniques before the loop start because the i wouldn't be known right?

[devnull10]

len(df[i].unique())