Replace characters in a list by column rather than by row

Question

Currently I have the following lists:

counter = [13]
instruments = ['3\t     ---', '2\t    /   \\', '1\t   /     \\', '0\t---       \\       ---', '-1\t           \\     /', '-2\t            \\   /', '-3\t             ---']
score = ['|*************|']

What I am trying to do is to replace the characters in the instruments list with the characters from the score list (excluding the |).

I am currently experiencing the following issues

The characters are being replaced row by row, rather than column by column.

Instrument List:

3        ---
2       /   \
1      /     \
0   ---       \       ---
-1             \     /
-2              \   /
-3               ---

Score List:

|*************|

EXPECTED OUTPUT:

3        ***
2       *   *
1      *     *
0   ***       *       
-1             *     
-2              *   
-3               

Current Output:

3        ***
2       *   *
1      *     *
0   ***       *       **
-1                  
-2                 
-3               

This is how I am currently replacing the characters in the instruments list:

for elements in counter:
    current_counter = elements
    count = 0
    for elements in instrument_wave:
        amplitude, form = elements.split('\t')
        for characters in form:
            if characters in ['-', '/', '\\']:
                form = form.replace(characters, '*', 1)
                count += 1
            if count == current_counter:
                break
        for characters in form:
            if characters in ['-', '/', '\\']:
                form = form.replace(characters, '')
        if '-' not in amplitude:
            amplitude = ' ' + amplitude
        new_wave = amplitude + "\t" + form
        waveform.append(new_wave)

Any help would be appreciated, especially with regards to how I should fix my replace character to make it go column by column rather than row by row.

Answer 1

To solve your first issue, you need to iterate via columns.

If you zip the lists (via itertools.zip_longest(), as they are not all the same length), you can then go through them in order and truncate the result:

import itertools

cols = list(itertools.zip_longest(*lst, fillvalue=" "))
for i in range(3, 17):  # skip negative signs
    cols[i] = "".join(cols[i]).replace('-', '*', 1)
    cols[i] = "".join(cols[i]).replace('/', '*', 1)
    cols[i] = "".join(cols[i]).replace('\\', '*', 1)
fixed = map("".join, zip(*cols[:17]))  # no need to zip longest

for l in fixed:
    print(l)

See a working example on repl.it, which outputs:

3        ***     
2       *   *    
1      *     *   
0   ***       *  
-1             * 
-2              *
-3   

Note it does pad the lists out with spaces, so you may want to .strip() the results if it isn't just for printing. Adapting that to your score input I'll leave up to you.

Another option, which is probably clearer:

def convert_and_truncate(lst, cutoff):
    result = []
    for str in lst:
        str = str[0] + str[1:].replace('-', '*')  # skip the negative signs
        str = str.replace('/', '*')
        str = str.replace('\\', '*')
        result.append(str[:cutoff])  # truncate
    return result

Because we're truncating the rest of the list, it doesn't matter that replace is changing them all.