Keep rows if difference between two values in column 2 exceeds certain amount; do this for each category given in column 1 - subset

Question

I want to subsample a file by keeping as many entries as possible whose difference in values in column 2 are at least 500 units, for each name in column 1. The full file is ~200,000 lines long, sorted by column 1 then column 2, tab-separated, and looks something like this:
name1 107
name1 110
name1 472
name1 509
name1 599
name1 679
name1 710
name2 36
name2 179
name2 391
name2 696
name2 1427
name2 1583
name2 1722
name2 2090
name2 2136
name2 2235
name3 687
name3 933
name4 43
name4 207
name4 384
name4 439
name4 447
name4 603
name4 774
name4 802
name4 876
name4 988
I would like an output that looks like this:
name1 107
name1 679
name2 36
name2 696
name2 1427
name2 2090
name3 687
name4 43
name4 603
I think one way to do it is to keep the first entry for each name and then keep the next entry for that name that is at least 500 units larger, and then the next entry that is at least 500 larger than that, etc. Then, repeat for each name. It would also be fine if it was in reverse starting with the last entry for each name, or it would be fine if it started elsewhere as long as it maximized the number of entries retained for each name that are greater 500 units apart.
However, I have no idea how to code this, as I am a novice! Thank you for your help!

Answer 1

I chose to do it in Python, which is turning into the lingua franca of bioinformatics.
(Learn enough Python for your biology needs here: http://learnpythonthehardway.org/book/)
Copy the following into a file and run it with python script_name.py input_textfile.txt
(If you do not know enough python to do that chapters 0 and 1 in the book referred to above will help you)
import sys
name_column = 0
number_column = 1
last_name = "dummy variable"
last_number = -1
min_difference = 500
with open(sys.argv[1], 'r') as input_file:
for line in input_file:
name = line.split()[name_column]
number = int(line.split()[number_column])
if name != last_name:
print(line.strip())
last_number = number
last_name = name
continue
if (number-last_number) >= min_difference:
print(line.strip())
last_number = number
Output using data above:
name1 107
name1 679
name2 36
name2 696
name2 1427
name2 2090
name3 687
name4 43
name4 603
If you want the output in a file, use python script_name.py input_textfile.txt > output_file