python - how to compare string in one line with string in next line? -

i have file 16,000 lines in it. of them have same format. here simple example if it:

atom    139  c1  dppc   18      17.250  58.420  10.850  1.00  0.00  <...>  atom    189  c1  dppc   19      23.050  20.800  11.000  1.00  0.00 

i need check if lines contains string dppc , has identifier 18 forms 50 line block before identifier switches 19, etc.

so now, have following code:

cnt = 0 open('test_file.pdb') f1:     open('out','a') f2:         lines = f1.readlines()         i, line in enumerate(lines):              if "dppc" in line:                    = line.strip()[22:26]                    if a[i] == [i+1]:                        cnt = cnt + 1                    elif a[i] != a[i+1]:                        cnt = 0 

and here stuck. found examples how compare subsequent lines similar approach did not work here. still cannot figure out how compare value of a in line[i] value of a in line[i+1].

try (explanations in comments).

data = """atom    139  c1  dppc   18      17.250  58.420  10.850  1.00  0.00 atom    139  c1  dppc   18      17.250  58.420  10.850  1.00  0.00 atom    139  c1  dppc   18      17.250  58.420  10.850  1.00  0.00 atom    139  c1  dppc   18      17.250  58.420  10.850  1.00  0.00 atom    139  c1  dppc   18      17.250  58.420  10.850  1.00  0.00 atom    139  c1  dppc   18      17.250  58.420  10.850  1.00  0.00 atom    139  c1  dppc   18      17.250  58.420  10.850  1.00  0.00 atom    139  c1  dppc   18      17.250  58.420  10.850  1.00  0.00 atom    139  c1  dppc   18      17.250  58.420  10.850  1.00  0.00 atom    189  c1  dppc   19      23.050  20.800  11.000  1.00  0.00 atom    189  c1  dppc   19      23.050  20.800  11.000  1.00  0.00 atom    189  c1  dppc   19      23.050  20.800  11.000  1.00  0.00 atom    189  c1  dppc   19      23.050  20.800  11.000  1.00  0.00 atom    189  c1  dppc   19      23.050  20.800  11.000  1.00  0.00 atom    189  c1  dppc   19      23.050  20.800  11.000  1.00  0.00 atom    189  c1  dppc   19      23.050  20.800  11.000  1.00  0.00 atom    189  c1  dppc   19      23.050  20.800  11.000  1.00  0.00 atom    189  c1  dppc   19      23.050  20.800  11.000  1.00  0.00 atom    189  c1  dppc   19      23.050  20.800  11.000  1.00  0.00"""  # last code seen in 5th column. code = none  # count of lines of current code. count = 0  line in data.split("\n"):     # 5th column.     c = line.split()[4]      # code in 5th column changed.     if c != code:         # if aren't @ start of file, print count         # code ended.         if code:             print("{}: {}".format(code, count))          # rember new code.         code = c      # count line     count = count + 1  # print count last code. print("{}: {}".format(code, count)) 

output:

18: 9 19: 19