I am working with a big dataset and thus I only want to use the items that are most frequent. 

Simple example of a dataset:

1 2 3 4 5 6 7   

1 2   

 3 4 5   

4 5   

4   

8 9 10 11 12 13 14   

15 16 17 18 19 20   

4 has 4 occurrences,   

1 has 2 occurrences,   

2 has 2 occurrences,   

5 has 2 occurrences,   

I want to be able to generate a new dataset just with the most frequent items, in this case the 4 most common: 

The wanted result: 

1 2 3 4 5   

1 2   

3 4 5   

4 5   

4 

I am finding the first 50 most common items, but I am failing to print them out in a correct way. (my output is resulting in the same dataset)

Here is my code: 

```
 from collections import Counter

with open('dataset.dat', 'r') as f:
    lines = []
    for line in f:
        lines.append(line.split())
    c = Counter(sum(lines, []))
    p = c.most_common(50);

with open('dataset-mostcommon.txt', 'w') as output:
    ..............

```

Can someone please help me on how I can achieve it?