How can i read one specific line and 7 more lines under that one

Question

i have this big txt file

G29 2019 07 08 02 00 00 1.571122556925e-04-9.777068044059e-12 0.000000000000e+00
     6.100000000000e+01 6.809375000000e+01 3.670867192067e-09 2.120402980941e+00
     3.512948751450e-06 1.192553318106e-03 9.380280971527e-06 5.153736095428e+03
     9.360000000000e+04 4.656612873077e-08 1.565794672787e+00 8.195638656616e-08
     9.854740594187e-01 2.112812500000e+02 1.817507992857e+00-7.755323040334e-09
     2.692969315838e-10 1.000000000000e+00 2.061000000000e+03 0.000000000000e+00
     2.000000000000e+00 0.000000000000e+00-9.778887033463e-09 6.100000000000e+01
     9.999000000000e+08 0.000000000000e+00                                      
G30 2019 07 08 02 00 00-1.387689262629e-04-7.503331289627e-12 0.000000000000e+00
     4.300000000000e+01 3.143750000000e+01 5.236289541049e-09-1.114281617593e+00
     1.536682248116e-06 4.139962256886e-03 5.565583705902e-06 5.153780742645e+03
     9.360000000000e+04 3.166496753693e-08-5.518871386231e-01-4.097819328308e-08
     9.406574455640e-01 2.614687500000e+02-2.937938331210e+00-8.484639133562e-09
    -4.607334771129e-11 1.000000000000e+00 2.061000000000e+03 0.000000000000e+00
     2.000000000000e+00 0.000000000000e+00 3.725290298462e-09 4.300000000000e+01
     9.999000000000e+08 0.000000000000e+00                                      
J02 2019 07 08 01 00 00-9.192153811455e-07-3.410605131648e-13 0.000000000000e+00
     9.700000000000e+01-2.437500000000e+00 1.713285650939e-09-1.895301564152e+00
     1.378357410431e-07 7.528792973608e-02-1.499429345131e-06 6.493121416092e+03
     9.000000000000e+04 4.768371582031e-07-9.331615762110e-02-2.438202500343e-06
     7.607943375436e-01 2.135937500000e+02-1.583245983476e+00-1.648282943315e-09
     7.928901699153e-11 2.000000000000e+00 2.061000000000e+03 0.000000000000e+00
     2.900000000000e+00 0.000000000000e+00 1.396983861923e-09 8.650000000000e+02
     9.999000000000e+08 0.000000000000e+00

and i need to read all lines that start with G30 and all lines below (in this sample 7 lines) until next fragment starts. I managed to read only first line and number of this line but can't add other lines.

with open("filename", "r") as f:

    str1 = 'G30'
    for i,ln in enumerate(f):
        if  str1 in ln:
            print(ln[0:],i)

score 1 · Answer 1 · answered Oct 12 '19 at 17:31

Since you know the index of the line at which "G30" starts, just pick the next 7 lines.

with open("filename", "r") as f:
    str1 = 'G30'
    all_lines = f.readlines()
    for i,ln in enumerate(all_lines):
        if  str1 in ln:
            print(ln[0:])
            for l in range(1,8):
                print(all_lines[i+l])

score 0 · Accepted Answer · answered Oct 12 '19 at 17:31

To process the file content as a stream, without reading the entire content into memory, I think your best option will be to use a flag variable to decide whether it's time to print the current line or not. Consider this algorithm:

If the line matches the start pattern, set the flag
If the line matches the end pattern, clear the flag
If the flag is set, print the current line

For example, like this:

with open("filename") as fh:
    pattern = 'G30'
    should_print = False

    for line in fh:
        if line.startswith(pattern):
            should_print = True
        elif not line.startswith(' '):
            should_print = False

        if should_print:
            print(line, end='')

Thanks, that really help, to use flag variable. – Aureliano Buendia Oct 12 '19 at 17:41 — Aureliano Buendia, Oct 12 '19 at 17:41

rnso · Answer 3 · 2019-10-12T17:51:32.067

Try:

fname = "testing.txt"
with open(fname) as f: 
    lines = f.readlines()
    listlen = len(lines)
    i = 0
    while i<listlen: 
        line = lines[i]
        if line.startswith("G30"): 
            print(line, end="")
            count =0
            while True: 
                i=i+1
                line = lines[i]
                print(line, end="")
                count=count+1
                if count>=7: break
        i=i+1

score 0 · Answer 4 · answered Oct 12 '19 at 17:37

0

while filename.readline()[:3] ==‘G30’:
    exit()

As soon as the line starts with the program stops.

answered Oct 12 '19 at 17:37

rnso · Answer 5 · 2019-10-12T17:48:06.023

You can create a separate function to print required number of lines:

# this fn prints n number of lines
# it also removes printed lines from list
# and return remaining list: 
def printNlines(linelist, n):
    i=0
    while i<n: 
        if len(linelist)==0: break  # return if list becomes empty
        print(linelist[0], end="")  
        linelist = linelist[1:]     # remove printed line from list;
        i=i+1
    return linelist                 # return remaining list;

Now use above function to go through your file list:

fname = "testing.txt"
with open(fname) as f: 
    lines = f.readlines()       # get list of lines from file;
while len(lines)>0:             # loop till the list is empty
    line = lines[0]             # read first line
    if line.startswith("G30"):  # test it
        lines = printNlines(lines, 1+7) # print current line and seven more; 
    else: lines = lines[1:]   # remove this line from list without printing;

How can i read one specific line and 7 more lines under that one

5 Answers5