returning value outside a for loop

Question

I have this text file:

[admin]# cat /etc/passwd
root:!:0:0::/:/usr/bin/ksh
daemon:!:1:1::/etc:
bin:!:2:2::/bin:
sys:!:3:3::/usr/sys: 
adm:!:4:4::/var/adm:
uucp:!:5:5::/usr/lib/uucp: 
guest:!:100:100::/home/guest:
nobody:!:4294967294:4294967294::/:
lpd:!:9:4294967294::/:
lp:*:11:11::/var/spool/lp:/bin/false 
invscout:*:200:1::/var/adm/invscout:/usr/bin/ksh
nuucp:*:6:5:uucp login user:/var/spool/uucppublic:/usr/sbin/uucp/uucico
paul:!:201:1::/home/paul:/usr/bin/ksh
jdoe:*:202:1:John Doe:/home/jdoe:/usr/bin/ksh

and some code here

with open(file) as f2:
            for lines in f2:
                if "cat /etc/passwd" in lines:
                    for i in range(3):
                        cat = f2.readline()
                        print(cat)

if it finds the string "cat /etc/passwd", it will store the next couple of lines inside the variable cat

output:

root:!:0:0::/:/usr/bin/ksh
daemon:!:1:1::/etc:
bin:!:2:2::/bin:

This is the case if I call cat from inside the for loop. If I call it outside of the for loop, I only get the last line :

bin:!:2:2::/bin:

I assume the line for i in range(3) is the reason for this. is there a way I can call cat outside the loops and have it return each of the lines I want printed?

This what you want? http://stackoverflow.com/questions/1767513/read-first-n-lines-of-a-file-in-python — , Apr 28 '16 at 13:38

score 3 · Accepted Answer · answered Apr 28 '16 at 13:38

3

cat = []
with open(file) as f2:
    for lines in f2:
        if "cat /etc/passwd" in lines:
            for i in range(3):
                cat.append(f2.readline())

print cat

Read about python lists.

answered Apr 28 '16 at 13:38

Alex Hall

34,833
5
57
89

I see my problem. I had done the same thing but had initiated cat as a list inside the loop as opposed to doing so outside of the loop. I keep forgetting the small things as usual. – Kevin R. Apr 28 '16 at 13:46

score 1 · Answer 2 · answered Apr 28 '16 at 13:40

1

For each iteration you overwrite the last value of cat.

Inside of the loop, you print each time before you overwrite. Outside of the loop, you only print its last value.

You could store your results in an array

with open(file) as f2:
        for lines in f2:
            if "cat /etc/passwd" in lines:
                cat = []
                for i in range(3):
                    cat.append(f2.readline())
                    print cat[i] 

for x in cat:
    print x

answered Apr 28 '16 at 13:40

Peter1807

184
9

-1 This will not work. `cat` will be out of scope at that point. Also, if for some reason the file had multiple lines with `"cat /etc/passwd"` you would lose all previous data collected – Tom Myddeltyn Apr 28 '16 at 14:14

score 0 · Answer 3 · answered Apr 28 '16 at 13:41

0

Have you tried this ?

cat = ''
with open(file) as f2:
            for lines in f2:
                if "cat /etc/passwd" in lines:
                    for i in range(3):
                        cat += f2.readline()
print(cat)

answered Apr 28 '16 at 13:41

Kodain

1
1

score 0 · Answer 4 · answered Apr 28 '16 at 14:18

The mixing of iteration (for lines in f2) and read method (f2.readline) is not nice. I have a other solution with itertools.islice, which only uses iteration

from itertools import islice

with open(file) as f2:
    for line in f2:
        if "cat /etc/passwd" in lines:
            cat = list(islice(f2, 3))

print cat

cat is a list like the answer of Alex

returning value outside a for loop

4 Answers4