Task : Find unique elements in an array. Count their occurrences. Find the numbers that occur less than 10 times in an array of 5000 elements - python-3.x

Question

I tried a few solutions :
1.)
uniqueValues, indexList,occurCount = np.unique(desired_array,
return_index=True, return_counts=True)
print(uniqueValues,indexList,occurCount)
However the indexList only gives first occurrence of a number. For example : if num 33 occurred at 20,56,3000, indexList would only show that it occurred at 20. Since 33 occurs less than 10 times, i.e 3 times, I need all the locations.
2.) I decided to use dictionary to find all the index locations. But this is not working.
for i in range(5000):
...: if not d.get(i):
...: d[desired_array[i]]=[i]
...: else:
...: indices = d[desired_array[i]]
...: indices.append(i)

Answer 1

This jobs screams for collections.Counter:
from collections import Counter
desired_array = [1, 2, 3, 1, 3, 5, 3]
result = Counter(desired_array)
print(result)
This will print out the unique elements and the count of occurrences:
Counter({3: 3, 1: 2, 2: 1, 5: 1})
You can replace
for i in range(1250):
var = desired_array[i]
if not d.get(var):
d[var] = []
# print(var)
s = d[var]
s.append(i)
with
for i in range(1250):
var = desired_array[i]
d.setdefault(var, []).append(i)
According to the documentation dict.setdefault(key, default):
If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None.
To write a csv file it's best to use the standard csv.Writer class:
import csv
with open('some.csv', 'w', newline='') as f:
writer = csv.writer(f)
writer.writerows(someiterable)
If you want to write the key/value pairs of your dict to the csv file you need to write something like:
with open('some.csv', 'w', newline='') as f:
writer = csv.writer(f)
for k, v in desired_array.items():
writer.writerow((k, v))