CODEWITHSUNDEEP
pythonif-statement
Bogdan Janiszewski
Bogdan Janiszewski

Reputation: 2883

Trying to modify text file with if statements

I have a text file that I am reading in, and based on certain conditions modifying specific lines and rewriting the file to a new text file. My present code mostly works but one of the elif statements seems to be simply ignored by Python as there are no run time errors. A MWE is as follows:

energy = .1
source = str('source  x y z energy=%f' %energy)
c = energy - 0.001
c_string = str("1010 %f %f" %(c, energy))


f = open("file.txt", "r+")
with open ("newfiletxti", 'w') as n:
    d = f.readlines()
    for line in d:
        if not line.startswith("source"):
            if not line.startswith("xyz"):
                n.write(line)
        elif line.startswith("source"):
            n.write(source + "\n")
        elif line.startswith("xyz"):
            n.write(c_string + "\n")
    n.truncate()
    n.close()

The code:

elif line.startswith("source"):
    n.write(source + "\n")

Works as expected where the line in the text file is replaced with the string titled "source" however the next block:

elif line.startswith("xyz"):
    n.write(c_string + "\n")

Has no effect. The new text file is simply missing the line that starts with xyz. My guess is my syntax for multiple elif statements is incorrect but I am uncertain as to why.

Upvotes: 1

Views: 1415

Answers (3)

Barmar
Barmar

Reputation: 781068

The first if and elif handle all the cases -- either the line starts with source or it doesn't. I think you need to combine the first if and its nested if into a single condition:

if not line.startswith("source") and not line.startswith("xyz"):
    n.write(line)

or the equvivalent (by de Morgan's Laws):

if not(line.startswith("source") or line.startswith("xyz")):
    n.write(line)

Or you can make it clearer by reordering your conditions:

if line.startswith("source"):
    n.write(source + "\n")
elif line.startswith("xyz"):
    n.write(c_string + "\n")
else:
    n.write(line)

Upvotes: 2

Gabe
Gabe

Reputation: 169

The third elif will never be reached. Here is the code reduced for clarity:

if not line.startswith("source"):
# suff
elif line.startswith("xyz"):
# stuff

Something that starts with "xyz" does not start with "source".

Upvotes: 0

DaLord
DaLord

Reputation: 149

Try your if block like this:

    if line.startswith("source"):
        n.write(source + "\n")
    elif line.startswith("xyz"):
        n.write(c_string + "\n")
    else:
        n.write(line)

Upvotes: 2

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