For loop in python - beginner

Question

I'm trying to solve a project where i have to count the years in a list but I`m having problems understanding the difference between

years = [n[1] for n in data]

and

for n in data:
         years = n[1]

here is the rest of the body

for m in years:
    if m not in year_counts:
        year_counts[m] = 0
    year_counts[m] += 1

print(year_counts)

So if I use the first sentence then the code will run normally and will show the proper answer but if i use the second code it will give me random numbers I don't know from where.

What does the data in years look like?

Patrick Haugh
– Patrick Haugh

2017-12-08 15:01:49 +00:00
Commented Dec 8, 2017 at 15:01 — Patrick Haugh
– Patrick Haugh, Commented Dec 8, 2017 at 15:01

Gal Ben-Haim · Accepted Answer · 2017-12-08 15:02:33Z

1

years = [n[1] for n in data] - years is now a list, each element is index [1] of an element in data

for n in data: years = n[1]

years is a single object and is always updated with n[1] and will finally be index [1] of the last element in data

answered Dec 8, 2017 at 15:02

Gal Ben-Haim

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1 Comment

Martin Bonner supports Monica Over a year ago

Might be helpful to show data = ['1922', '1999', '2017'], and that years will either be ['9', '9', '0'] or '0'

Ryan Deschamps · Accepted Answer · 2017-12-08 15:17:47Z

0

In

for n in data:
         years = n[1]

years will be replaced for each n1. The "random" entry you are seeing is probably the second letter of the last entry in data.

You could do this instead to match the comprehension:

years = list()
data = ['a1', 'a2', 'a3', 'a4']
for n in data:
  years.append(n[1])  # years = ['1', '2', '3', '4']

but

years = [n[1] for n in data]

will tend to be faster overall.

For your overall script, you might consider a using map.

edited Dec 8, 2017 at 15:17

answered Dec 8, 2017 at 15:05

Ryan Deschamps

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2 Comments

Mihai Over a year ago

I see. so years = [n[1] for n in data] could be translated to years = [] for n in data: years.append(n[1])

Ryan Deschamps Over a year ago

Right. But loops are slower than list comprehensions in general.

vmele · Accepted Answer · 2017-12-08 15:23:51Z

0

li = [1, 2, 3, 4, 5, 6, 7]
# For loop uses for list iteration.
# for loop iterate every item in a list.

for i in li: 
    print i

>>> 1
>>> 2
>>> 3
>>> 4
>>> 5
>>> 6
>>> 7

edited Dec 8, 2017 at 15:23

vmele

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answered Dec 8, 2017 at 15:08

Madhu Bolagani

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Comments

Majid · Accepted Answer · 2017-12-08 16:07:47Z

0

This code years = [n[1] for n in data] output is list. This code

for n in data:
     years = n[1]

output is a single variable that contains last value.

edited Dec 8, 2017 at 16:07

Majid

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answered Dec 8, 2017 at 15:01

Ahmad

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