Not entering the for loop in python 2.7.2

Question

Here is the code I was using to manipulate a CSV file. It is not entering the 2nd and 3rd for loops.

import csv
f=open("/Users/shivendraagrawal/Desktop/Training Dataset.csv","rU")
cr = csv.reader(f)
mapping={}
out={}
i=0
for row in cr:
      i=i+1
      mapping[row[0]]=0
      out[row[0]]=0
for row in cr:
      print "hi"
      mapping[row[0]]=mapping[row[0]]+row[5]
      if row[6]=="TRUE":
            out[row[0]]=out[row[0]]+1

for row in cr:
      print mapping[row[0]] +'  '+out[row[0]]

print i

you need to rewind it to initial position after first and second loop — Grijesh Chauhan, Sep 21 '13 at 07:29
I think my iterator was defined only for one For Loop's scope. Isn't it? If not, can you suggest me how to do that. — Shivendra, Sep 21 '13 at 07:31
Read below I have posted an answer @Vladimir Sinenko also posted — Grijesh Chauhan, Sep 21 '13 at 07:36

score 2 · Answer 1 · edited May 23 '17 at 12:06

2

You're using the iterator more than once, but the iterators are single-shot only.

Just make a copy of the cr contents when reading it, and iterate through that:

cr_copy = [c for c in cr]
for row in cr_copy:

Or, there is a more Pythonic way to do that: you can clone the iterator three times by using the itertools.tee (see this post):

import itertools

cr1, cr2 = itertools.tee(cr)
for row in cr1:
     # first loop

for row in cr2:
     # secondloop

edited May 23 '17 at 12:06

Community

1
1

answered Sep 21 '13 at 07:31

Vladimir Sinenko

4,629
1
27
39

I tried with a different iterator too. (row1, row2) but the result was the same. – Shivendra Sep 21 '13 at 07:42
I just wished there was a way to accept more than one answers. Got it. Awesomely presented :) – Shivendra Sep 21 '13 at 07:47

score 2 · Accepted Answer · answered Sep 21 '13 at 07:31

2

use f.seek(0) to rewind back after first and second loop e.g. before second loop:

f.seek(0)
for row in cr:
    # loop code

Read: from Python doc code

Give it a try!!

answered Sep 21 '13 at 07:31

Grijesh Chauhan

57,103
20
141
208

Why are you rewinding back the 'f' object while I am iterating over 'cr'? And thanks a lot, it is working now. – Shivendra Sep 21 '13 at 07:43
@Shivendra I just learn the trick from linked page. through `cr` not reset. – Grijesh Chauhan Sep 21 '13 at 08:07
@Shivendra It's part of the way the `csv` module is designed. The position of the `reader` is linked to the position of the file pointer. Advancing your `reader` moves the file pointer forwards; moving the file pointer back again resets the `reader`. – Benjamin Hodgson Sep 21 '13 at 13:51
@poorsod Thanks as learner I posted this answer. Can you provide some link to read further. – Grijesh Chauhan Sep 21 '13 at 13:53
@GrijeshChauhan It's basically because CSV readers work on any iterable. Here are the docs on [file objects](http://docs.python.org/2/library/stdtypes.html#file-objects) and the way they iterate; here are the docs on [CSV readers](http://docs.python.org/2/library/csv.html#reader-objects). If you're using Python 3, file objects are implemented somewhat differently, though they expose the same interface. – Benjamin Hodgson Sep 21 '13 at 14:05

Not entering the for loop in python 2.7.2

2 Answers2