Python: loop index issue

Question

Idea: I want to replace parts of data1 with newer price data.

Q1: Why are 'laptop' and 'printer' updated, but not 'chair'?

Q2: 'bed' in data2 does not exist for data1. In that case, there can't be an update to data1. I'm just wondering why there is no error like "'bed' has no match in data1"?

import pandas as pd

data1 =pd.DataFrame({'product_name': ['laptop', 'printer', 'tablet', 'desk', 'chair'],
        'price': [0, 4, 6, 7, 9]
        })
data2 =pd.DataFrame({'product_name': ['laptop', 'printer','chair','bed'],
        'price': [89,32,34,355]
        })

indi = data2['product_name']

for i in indi:
    temp = data2.loc[data2['product_name'] == '%s'%i,'price']
    data1.loc[data1['product_name'] == '%s'%i,'price'] = temp

Yashar Ahmadov · Accepted Answer · 2021-11-07 13:21:12Z

1

You need to do this: (temp.iloc[0])

for i in indi:
    temp = data2.loc[data2['product_name'] == '%s'%i,'price']
    data1.loc[data1['product_name'] == '%s'%i,'price'] = temp.iloc[0]

answered Nov 7, 2021 at 13:21

Yashar Ahmadov

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4 Comments

Fabian Over a year ago

Thanks a lot! I'd like to understand why I have to add ".iloc[0]. I thought temp was a "number".

Yashar Ahmadov Over a year ago

Glad it helped you! If you type print(temp), you will see that it is not really a number. It has name and index attached to it.

Yashar Ahmadov Over a year ago

Please upvote if this answer was useful.

Fabian Over a year ago

Thanks! I just wonder why updating works for the first two entries, but then fails for the last one (when there are some entries in data 2 that are not in data1).

navneethc · Accepted Answer · 2021-11-07 13:23:00Z

I believe this may have something to with non-matching indexes for the chair entries. When you loop through indi, you get not only the product_name from data but also its corresponding index. Here's a better way to deal with situations like this:

In [53]: data1
Out[53]:
  product_name  price
0       laptop      0
1      printer      4
2       tablet      6
3         desk      7
4        chair      9

In [54]: data2
Out[54]:
  product_name  price
0       laptop     89
1      printer     32
2        chair     34
3          bed    355

In [55]: for row in data2.itertuples():
    ...:     data1.loc[data1['product_name']==row.product_name, 'price'] = row.price
    ...:

In [56]: data1
Out[56]:
  product_name  price
0       laptop     89
1      printer     32
2       tablet      6
3         desk      7
4        chair     34

In [57]: data2
Out[57]:
  product_name  price
0       laptop     89
1      printer     32
2        chair     34
3          bed    355

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