How to (not) repeat code lines with simple iterative variations?

Question

I'm checking some averages for different categories

cat_1 = []

for i, j, k in zip(is, js, ks):    
    if i == 1:
        cat_1.append(i)

avg_cat_1 = stats.mean(cat_1)
print("Avg for cat_1:", avg_cat_1)

If I want to include multiple categories I would like to avoid having to write a new line for each new category.

For the initialization of the lists I could do something like this:

for i in range(nr of categories): 
    a = "cat_%s" % (i)
    print (a)

but that of course only prints me a line and doesnt execute one and for the if statements thats even more complicated.

I wonder if there is a way to just do a loop like this:

for i in range(nr of categories): 
    cat_i = []

or

for i in range(nr of categories): 
    cat_%s = [] %(i)

where the "i" gets recognized not as part of the variable string but as the temporary loop variable and just produces a executable line of code and not just a print out

Why are you ignoring 2/3 of your loop variables in the first example? — Mad Physicist, Nov 04 '18 at 16:49
What is `stats`? Please provide a minimal example that I can paste directly into my interpreter and run, without having to supply external inputs. — Mad Physicist, Nov 04 '18 at 16:51
Your first code sample only appends the value of `i` to the list if it equals `1`. This means your `cat_1` list just contains entries that have the value `1` and the mean is going to be `1`. Please provide a code sample and/or data that does a better job of showing what you want to accomplish. — Craig, Nov 04 '18 at 16:52
If you ever find yourself wanting to use a variable like `cat_i`, just use an array instead and write `cat[i]`. — melpomene, Nov 04 '18 at 16:56

score 0 · Answer 1 · answered Nov 04 '18 at 17:06

There is a way to lookup variables by name. Say for global variables:

globals()[f'cat_{i}'] = 9

But that is convoluted. Given the limited info, your use case is the textbook use case of a lists of lists:

for i in range(num_of_categories):
    cat[i].append(some_operation(i))

Cory Nezin · Accepted Answer · 2018-11-04T17:25:25.533

You could use a dictionary, for example:

cat_dict = {'cat_1': cat_1, 'cat_2': cat_2, 'cat_3': cat_3, ... }
for i in range(1, number_of_categories + 1):
    cat_dict['cat_%s' % i] = []

There's also the NOT recommended way: you could use exec

for i in range(1, number_of_categories + 1):
    exec('cat_%s = []' % i)

One very simple reason this is bad is clarity. If a user wants to see where the variable cat_3 is created, they might search the file for cat_3, but it won't be found at this point.

For a list of reasons that this is poor practice, see Why is using 'eval' a bad practice?

How to (not) repeat code lines with simple iterative variations?

2 Answers2