For loop in python: Code issue

Question

I am somehow able to write the below code(taking help from various sources):

langs=['C','Java','Cobol','Python']
f1=open('a.txt','r')
f2=open('abc.txt','w')
for i in range(len(langs)):
 for line in f1:
 f2.write(line.replace('Frst languag','{}'.format(langs[i])))
f1.close()
f2.close()

Don't know why the the for loop is not running till the end. Because everytime i open the txt only 'C' gets stored in the txt. I want the script to run and at the end of the script's execution the txt should have the last value of the list (here python)

• ZSG: It is best to put all the code in one block so folks can copy and paste into an editor and reproduce your problem. Since you are reading from a.txt, you should also include a copy of the contents of a.txt.
• sbru: How many lines do you have in f1?
• Lukas Graf: When reading lines from a file in Python, you advance the file pointer. Once you've read all the lines the file pointer will be at the end, and the file iterator won't yield any more lines.
• abarnert: Well, your bug is obvious; there's no such language as Cobol, only COBOL, and you forgot to put all the code between columns 12 and 72. :)
• abarnert: A few side notes: '{}'.format(langs[i]) adds nothing to just langs[i]. You're taking a string, and saying to format it as a string and then place it into a string with nothing else. Also, instead of doing for i in range(len(langs)): and then using langs[i], just do for lang in langs: and use lang.

Answer 1

After the first pass of your inner for loop, f1 is pointing to the end of the file. So the subsequent passes don't do anything.

The easiest fix is to move f1=open('a.txt','r') to just before for line in f1:. Then the file will be re-read for each of your languages. (Alternatively, you might be able to restructure your logic so that you can handle all of the languages at the same time in one pass of the file.)