Python If vs. While?

Question

I have a small file read routine and I want only the 1st 200 records I have it working but along the way I could not figure out what was wrong with using the "while" construct. This code works:

import csv, sys, zipfile
sys.argv[0] = "/home/tom/Documents/REdata/AllListing1RES.zip"
zip_file    = zipfile.ZipFile(sys.argv[0])
items_file  = zip_file.open('AllListing1RES.txt', 'rU')
rows = []
for row_index, row in enumerate(csv.DictReader(items_file, dialect='excel', delimiter='\t')):
    if (row_index < 200):
        rows.append(row)
    else : break

This code runs until it fails with an out of memory condition I would have thought it was equivalent?

import csv, sys, zipfile
sys.argv[0] = "/home/tom/Documents/REdata/AllListing1RES.zip"
zip_file    = zipfile.ZipFile(sys.argv[0])
items_file  = zip_file.open('AllListing1RES.txt', 'rU')
rows = []
for row_index, row in enumerate(csv.DictReader(items_file, dialect='excel', delimiter='\t')):
    while (row_index < 200):
        rows.append(row)
    else : break

so what would be the right construct using while? –

Please don't add comments to your own question. Please update the question to be complete. Then you can delete the confusing and redundant comment. — S.Lott
– S.Lott, Commented Apr 19, 2011 at 14:53

Mike Lewis · Accepted Answer · 2011-04-19 15:26:24Z

6

They are not equivalent because in your while loop, it has the condition of row_index < 200, which will never be false because row_index will never change while you are in that loop.

This is why you are getting a memory conditional because you are probably running into an infinite loop.

You are essentially saying:

Psuedo Code:

stay in block one as long as row_index < 200:

block_one:
   rows.append(row)
   goto block_one

You can see that row_index will never change, thus you are going to be in block_one forever.

Whereas the if statement has the following psuedo code:

if row_index < 200 goto block_one otherwise break:

  block_one:
    rows.append(row)

You can see that block_one is not going back to itself, like you see in the while loop.

edited Apr 19, 2011 at 15:26

answered Apr 19, 2011 at 14:12

Mike Lewis

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3 Comments

Mike Lewis Over a year ago

When ever you want a block of code to run repeatedly while some condition holds true.

ncoghlan Over a year ago

Python has no construct named where. The filtering statement is if.

dartdog Over a year ago

So "For" and "While" are each creating loops and the "if" is simply testing...but no simple way to use "While" since the "for" is required for the iterator declaration? – @Ncoughlan, thanks for the complete explanation!

ncoghlan · Accepted Answer · 2011-04-19 15:38:01Z

The more traditional way of writing that loop would be:

for row_index, row in enumerate(csv.DictReader(items_file, dialect='excel', delimiter='\t')):
    if (row_index >= 200):
        break
    rows.append(row)

As soon as the row counter hits 200, we bail out of the loop.

To use a while loop instead of a for loop (note that, as a looping construct, while is an alternative to for rather than to if) it is necessary to step through the iterator manually:

itr = enumerate(csv.DictReader(items_file, dialect='excel', delimiter='\t'))
row_index = -1
while row_index < 199:
    try:
        row_index, row = next(itr) # Python 3. Use itr.next() in Python 2
    except StopIteration:
        break # Ran out of data
    rows.append(row)

All that said, there's actually a superior alternative to both of these options available in the itertools module:

from itertools import islice
itr = csv.DictReader(items_file, dialect='excel', delimiter='\t')
rows = list(islice(itr, 200))

vartec · Accepted Answer · 2011-04-19 14:15:29Z

0

In the second case you're stuck forever in the while loop appending the same row over and over...

answered Apr 19, 2011 at 14:15

vartec

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Comments

SHKT · Accepted Answer · 2011-04-19 14:19:13Z

0

They can't be equivalent because in your first code, only one loop is iterating (the for loop) which checks the if-else statements at each iteration of row_index. In your second code, the while loop is a nested loop in which the condition isn't being reached (since there is nothing iterating the row_index). that makes it go into an infinite loop, there by giving the memory error.

answered Apr 19, 2011 at 14:19

SHKT

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Comments

Matthis Kohli · Accepted Answer · 2016-04-14 08:16:06Z

So I was curious what would be faster and did a quick example in VB.NET. I don't know if the code I came up with has a logical errors but when doing only 100000 loops the while loops is faster.

When having large numbers of data the time difference is hugh.

Has nothing to do with the topic but somehow it fits the IF vs WHILE.

Public Class Form1
        Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim watch As New Stopwatch
        Dim i As Integer = 0

        For loops As Integer = 0 To 100000000

            watch.Start()
            If True Then
                i += 1
            End If
            watch.Stop()
        Next
        MessageBox.Show(watch.ElapsedMilliseconds) ' 2740
    End Sub
    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        Dim watch As New Stopwatch
        Dim loops As Integer = 0

        watch.Start()
        While loops < 100000000
            loops += 1
        End While
        watch.Stop()

        MessageBox.Show(watch.ElapsedMilliseconds) ' 300
    End Sub
End Class

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