CODEWITHSUNDEEP
python
Mihai
Mihai

Reputation: 11

For loop in python - beginner

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.

Upvotes: 0

Views: 87

Answers (4)

Ahmad
Ahmad

Reputation: 910

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.

Upvotes: 0

Madhu Bolagani
Madhu Bolagani

Reputation: 1

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

Upvotes: 0

Ryan Deschamps
Ryan Deschamps

Reputation: 385

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.

Upvotes: 0

Gal Ben-Haim
Gal Ben-Haim

Reputation: 17803

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

Upvotes: 1

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