Let's say I have a text file in this format:
***a
foo bar
lorem ipsum
dolor
---a
I want to print the lines between ***a and ---a I'm trying to do it with this:
def printlines():
pattern = open('text.txt').read().splitlines()
for line in pattern:
if line == "***a":
pass
while line != "---a":
print line
else:
pass
But it prints ***a in an infinite loop. How can I solve this?
Use a state machine. That means, once you see your opening pattern, set a state so you know that the following lines are now relevant to you. Then keep looking out for the ending pattern to turn it off:
def printlines():
# this is our state
isWithin = False
with open('text.txt') as f:
for line in f:
# Since the line contains the line breaking character,
# we have to remove that first
line = line.rstrip()
# check for the patterns to change the state
if line == "***a":
isWithin = True
elif line == "---a":
isWithin = False
# check whether we’re within our state
elif isWithin:
print line
Since we only print once we’re in the isWithin state, we can easily skip any part out side of the ***a/---a pattern. So processing the following file would correctly print out Hello and World and nothing else:
Foo
***a
Hello
---a
Bar
***a
World
---a
Baz
Also, you should use the with statement to open your file, and iterate over the file object directly instead of reading it and calling splitlines(). That way you make sure that the file is properly closed, and you only ever read one line after another, making this more memory efficient.
def printlines():
pattern = open('text.txt').read().splitlines()
for line in pattern:
if line == "***a":
continue
if line == "---a":
break
print line
Break
The break statement, like in C, breaks out of the smallest enclosing for or while loop.
Continue
The continue statement, also borrowed from C, continues with the next iteration of the loop.
If you have multiple occurrences you can start an inner loop when you hit the start line, which is equivalent to what your while is trying to do:
with open("test.txt") as f:
for line in f:
if line.rstrip() == "***a":
print("")
for line in f:
if line.rstrip() == "---a":
break
print(line.rstrip())
Which for:
***a
foo bar
lorem ipsum
dolor
---a
***a
bar bar
foobar
foob
---a
Would output:
foo bar
lorem ipsum
dolor
bar bar
foobar
foob
If you want to have lines without the newlines we can map them off and still iterate line by line:
with open("test.txt") as f:
# itertools.imap python2
f = map(str.rstrip, f)
for line in f:
if line == "***a":
print("")
for line in f:
if line == "---a":
break
print(line)
