I have a Pandas DataFrame of key-value pairs for a collection of IDs. The columns in the DataFrame are (ID, Key, Value).
data = {
"ID":{0:1,1:1,2:1,3:2,4:2,5:2,6:3,7:3,8:3,9:4,10:4,11:4},
"Key":{0:"A",1:"B",2:"B",3:"A",4:"B",5:"B",6:"A",7:"B",8:"B",9:"A",10:"B",11:"C"},
"Value":{0:28,1:94,2:107,3:67,4:70,5:70,6:24,7:77,8:87,9:24,10:83,11:83}
}
data = pd.DataFrame(data)
I am trying to create a new table where the columns are the unique Keys, and their associated value is the maximum value for each ID:
So far I am able to create a DataFrame that contains the desired maximum values:
max_data = data.loc[ data.groupby(["ID", "Key"])["Value"].idxmax() ]
However, I am not sure the best way to get a DataFrame where the columns are the unique Keys with their associated values. This is what I have so far, but I am trying to avoid a loop:
result = pd.DataFrame(max_data["ID"].unique(), columns=["ID"])
for key in max_data["Key"].unique():
result = result.merge(
max_data.loc[max_data["Key"] == key][["ID", "Value"]],
how="left",
on="ID"
)
Something like pivot_table
data.pivot_table(index='ID',columns='Key',values='Value',aggfunc='max')
Out[22]:
Key A B C
ID
1 28.0 107.0 NaN
2 67.0 70.0 NaN
3 24.0 87.0 NaN
4 24.0 83.0 83.0
Related
I have an input dataframe.
I have also a list, with the same len as the number of rows in the dataframe.
Every element of the list is a dictionary: the key is the name of the new column, and the value is the value to be inserted in the cell.
I have to insert the columns from that list in the dataframe.
What is the best way to do so?
So far, given the input dataframe indf and the list l, I came up with something on the line of:
from copy import deepcopy
outdf = deepcopy(indf)
for index, row in indf.iterrows():
e = l[index]
for key, value in e:
outdf.loc[index, key] = value
But it doesn't seem pythonic and pandasnic and I get performance warnings like:
<ipython-input-5-9dde586a9c14>:8: PerformanceWarning: DataFrame is highly fragmented. This is usually the result of calling `frame.insert` many times, which has poor performance. Consider joining all columns at once using pd.concat(axis=1) instead. To get a de-fragmented frame, use `newframe = frame.copy()`
If the sorting of the list and the data frame is the same, you can convert your list of dictionaries to a data frame:
mylist = [
{'a':1,'b':2,'c':3},
{'e':11,'f':22,'c':33},
{'a':111,'b':222,'c':333}
]
mylist_df = pd.DataFrame(mylist)
a
b
c
e
f
0
1
2
3
nan
nan
1
nan
nan
33
11
22
2
111
222
333
nan
nan
Then you can use pd.concat to merge the list to your input data frame:
result = pd.concat([input_df, mylist_df], axis=1)
In this way, there is always a column created for all unique keys in your dictionary, regardless of they exist in one dictionary and not the other.
I've got a pandas DataFrame filled mostly with real numbers, but there is a few nan values in it as well.
How can I replace the nans with averages of columns where they are?
This question is very similar to this one: numpy array: replace nan values with average of columns but, unfortunately, the solution given there doesn't work for a pandas DataFrame.
You can simply use DataFrame.fillna to fill the nan's directly:
In [27]: df
Out[27]:
A B C
0 -0.166919 0.979728 -0.632955
1 -0.297953 -0.912674 -1.365463
2 -0.120211 -0.540679 -0.680481
3 NaN -2.027325 1.533582
4 NaN NaN 0.461821
5 -0.788073 NaN NaN
6 -0.916080 -0.612343 NaN
7 -0.887858 1.033826 NaN
8 1.948430 1.025011 -2.982224
9 0.019698 -0.795876 -0.046431
In [28]: df.mean()
Out[28]:
A -0.151121
B -0.231291
C -0.530307
dtype: float64
In [29]: df.fillna(df.mean())
Out[29]:
A B C
0 -0.166919 0.979728 -0.632955
1 -0.297953 -0.912674 -1.365463
2 -0.120211 -0.540679 -0.680481
3 -0.151121 -2.027325 1.533582
4 -0.151121 -0.231291 0.461821
5 -0.788073 -0.231291 -0.530307
6 -0.916080 -0.612343 -0.530307
7 -0.887858 1.033826 -0.530307
8 1.948430 1.025011 -2.982224
9 0.019698 -0.795876 -0.046431
The docstring of fillna says that value should be a scalar or a dict, however, it seems to work with a Series as well. If you want to pass a dict, you could use df.mean().to_dict().
Try:
sub2['income'].fillna((sub2['income'].mean()), inplace=True)
In [16]: df = DataFrame(np.random.randn(10,3))
In [17]: df.iloc[3:5,0] = np.nan
In [18]: df.iloc[4:6,1] = np.nan
In [19]: df.iloc[5:8,2] = np.nan
In [20]: df
Out[20]:
0 1 2
0 1.148272 0.227366 -2.368136
1 -0.820823 1.071471 -0.784713
2 0.157913 0.602857 0.665034
3 NaN -0.985188 -0.324136
4 NaN NaN 0.238512
5 0.769657 NaN NaN
6 0.141951 0.326064 NaN
7 -1.694475 -0.523440 NaN
8 0.352556 -0.551487 -1.639298
9 -2.067324 -0.492617 -1.675794
In [22]: df.mean()
Out[22]:
0 -0.251534
1 -0.040622
2 -0.841219
dtype: float64
Apply per-column the mean of that columns and fill
In [23]: df.apply(lambda x: x.fillna(x.mean()),axis=0)
Out[23]:
0 1 2
0 1.148272 0.227366 -2.368136
1 -0.820823 1.071471 -0.784713
2 0.157913 0.602857 0.665034
3 -0.251534 -0.985188 -0.324136
4 -0.251534 -0.040622 0.238512
5 0.769657 -0.040622 -0.841219
6 0.141951 0.326064 -0.841219
7 -1.694475 -0.523440 -0.841219
8 0.352556 -0.551487 -1.639298
9 -2.067324 -0.492617 -1.675794
Although, the below code does the job, BUT its performance takes a big hit, as you deal with a DataFrame with # records 100k or more:
df.fillna(df.mean())
In my experience, one should replace NaN values (be it with Mean or Median), only where it is required, rather than applying fillna() all over the DataFrame.
I had a DataFrame with 20 variables, and only 4 of them required NaN values treatment (replacement). I tried the above code (Code 1), along with a slightly modified version of it (code 2), where i ran it selectively .i.e. only on variables which had a NaN value
#------------------------------------------------
#----(Code 1) Treatment on overall DataFrame-----
df.fillna(df.mean())
#------------------------------------------------
#----(Code 2) Selective Treatment----------------
for i in df.columns[df.isnull().any(axis=0)]: #---Applying Only on variables with NaN values
df[i].fillna(df[i].mean(),inplace=True)
#---df.isnull().any(axis=0) gives True/False flag (Boolean value series),
#---which when applied on df.columns[], helps identify variables with NaN values
Below is the performance i observed, as i kept on increasing the # records in DataFrame
DataFrame with ~100k records
Code 1: 22.06 Seconds
Code 2: 0.03 Seconds
DataFrame with ~200k records
Code 1: 180.06 Seconds
Code 2: 0.06 Seconds
DataFrame with ~1.6 Million records
Code 1: code kept running endlessly
Code 2: 0.40 Seconds
DataFrame with ~13 Million records
Code 1: --did not even try, after seeing performance on 1.6 Mn records--
Code 2: 3.20 Seconds
Apologies for a long answer ! Hope this helps !
If you want to impute missing values with mean and you want to go column by column, then this will only impute with the mean of that column. This might be a little more readable.
sub2['income'] = sub2['income'].fillna((sub2['income'].mean()))
# To read data from csv file
Dataset = pd.read_csv('Data.csv')
X = Dataset.iloc[:, :-1].values
# To calculate mean use imputer class
from sklearn.impute import SimpleImputer
imputer = SimpleImputer(missing_values=np.nan, strategy='mean')
imputer = imputer.fit(X[:, 1:3])
X[:, 1:3] = imputer.transform(X[:, 1:3])
Directly use df.fillna(df.mean()) to fill all the null value with mean
If you want to fill null value with mean of that column then you can use this
suppose x=df['Item_Weight'] here Item_Weight is column name
here we are assigning (fill null values of x with mean of x into x)
df['Item_Weight'] = df['Item_Weight'].fillna((df['Item_Weight'].mean()))
If you want to fill null value with some string then use
here Outlet_size is column name
df.Outlet_Size = df.Outlet_Size.fillna('Missing')
Pandas: How to replace NaN (nan) values with the average (mean), median or other statistics of one column
Say your DataFrame is df and you have one column called nr_items. This is: df['nr_items']
If you want to replace the NaN values of your column df['nr_items'] with the mean of the column:
Use method .fillna():
mean_value=df['nr_items'].mean()
df['nr_item_ave']=df['nr_items'].fillna(mean_value)
I have created a new df column called nr_item_ave to store the new column with the NaN values replaced by the mean value of the column.
You should be careful when using the mean. If you have outliers is more recommendable to use the median
Another option besides those above is:
df = df.groupby(df.columns, axis = 1).transform(lambda x: x.fillna(x.mean()))
It's less elegant than previous responses for mean, but it could be shorter if you desire to replace nulls by some other column function.
using sklearn library preprocessing class
from sklearn.impute import SimpleImputer
missingvalues = SimpleImputer(missing_values = np.nan, strategy = 'mean', axis = 0)
missingvalues = missingvalues.fit(x[:,1:3])
x[:,1:3] = missingvalues.transform(x[:,1:3])
Note: In the recent version parameter missing_values value change to np.nan from NaN
I use this method to fill missing values by average of a column.
fill_mean = lambda col : col.fillna(col.mean())
df = df.apply(fill_mean, axis = 0)
You can also use value_counts to get the most frequent values. This would work on different datatypes.
df = df.apply(lambda x:x.fillna(x.value_counts().index[0]))
Here is the value_counts api reference.
Let us say I have two dataframes: df1 and df2. Assume the following initial values.
df1=pd.DataFrame({'ID':['ASX-112','YTR-789','ASX-124','UYT-908','TYE=456','ERW-234','UUI-675','GHV-805','NMB-653','WSX-123'],
'Costperlb':[4515,5856,3313,9909,8980,9088,6765,3456,9012,1237]})
df2=df1[df1['Costperlb']>4560]
As you can see, df2 is a proper subset of df1 (it was created from df1 by imposing a condition on selection of rows).
I added a column to df2, which contains certain values based on a calculation. Let us call this df2['grade'].
df2['grade']=[1,4,3,5,1,1]
df1 and df2 contain one column named 'ID' which is guaranteed to be unique in each dataframe.
I want to:
Create a new column in df1 and initialize it to 0. Easy. df1['grade']=0.
Copy df2['grade'] values over to df1['grade'], ensuring that df1['ID']=df2['ID'] for each such copy.
The result should be the grade values for the corresponding IDs copied over.
Step 2 is what is perplexing me a bit. A naive df1['grade']=df2['grade'].values does not work obviously as the lengths of the two dataframes is different.
Now, if I think hard enough, I could possibly come up with a monstrosity like:
df1['grade'].loc[(df1['ID'].isin(df2)) & ...] but I am uncomfortable with doing that.
I am a newbie with python, and furthermore, the indices of df1 are being used elsewhere after this assignment, and I do not want drop indices, reset indices as some of the solutions are suggested in some of the search results I found.
I just want to find out rows in df1 where the 'ID' row matches the 'ID' row in df2, and then copy the 'grade' column value in that specific row over. How do I do this?
Your code:
df1=pd.DataFrame({'ID':['ASX-112','YTR-789','ASX-124','UYT-908','TYE=456','ERW-234','UUI-675','GHV-805','NMB-653','WSX-123'],
'Costperlb':[4515,5856,3313,9909,8980,9088,6765,3456,9012,1237]})
df2=df1[df1['Costperlb']>4560]
df2['grade']=[1,4,3,5,1,1]
You can use merge with "left". In this way the indexing of df1 is preserved:
new_df = df1.merge(df2[["ID","grade"]], on="ID", how="left")
new_df["grade"] = new_df["grade"].fillna(0)
new_df
Output:
ID Costperlb grade
0 ASX-112 4515 0.0
1 YTR-789 5856 1.0
2 ASX-124 3313 0.0
3 UYT-908 9909 4.0
4 TYE=456 8980 3.0
5 ERW-234 9088 5.0
6 UUI-675 6765 1.0
7 GHV-805 3456 0.0
8 NMB-653 9012 1.0
9 WSX-123 1237 0.0
Here I called the merged dataframe new_df, but you can simply change it to df1.
EDIT
If instead of 0 you want to replace the NaN with a string, try this:
new_df = df1.merge(df2[["ID","grade"]], on="ID", how="left")
new_df["grade"] = new_df["grade"].fillna("No transaction possible")
new_df
Output:
ID Costperlb grade
0 ASX-112 4515 No transaction possible
1 YTR-789 5856 1
2 ASX-124 3313 No transaction possible
3 UYT-908 9909 4
4 TYE=456 8980 3
5 ERW-234 9088 5
6 UUI-675 6765 1
7 GHV-805 3456 No transaction possible
8 NMB-653 9012 1
9 WSX-123 1237 No transaction possible
I have a pandas dataframe which need to group by a text column to obtain sum of duplicated values along that column. But when I run the groupby method it drop many columns mysteriously. Can anyone help me on this?
Try to check your column dtypes , sum will only for numeric value.
For example you have df as below :
df=pd.DataFrame({'V1':[1,2,3],'V2':['A','B','C'],'KEY':[1,2,2]})
df.dtypes
Out[159]:
KEY int64
V1 int64
V2 object
dtype: object
Then you groupby key and do sum for whole dataframe it will only return the result of numeric columns
df.groupby('KEY').sum()
Out[160]:
V1
KEY
1 1
2 5
If you need string type to join together you can
df.groupby('KEY',as_index=False).apply(lambda x : x.sum())
Out[164]:
KEY V1 V2
0 1 1 A
1 4 5 BC
I have a dictionary that looks like this
dict = {'b' : '5', 'c' : '4'}
My dataframe looks something like this
A B
0 a 2
1 b NaN
2 c NaN
Is there a way to fill in the NaN values using the dictionary mapping from columns A to B while keeping the rest of the column values?
You can map dict values inside fillna
df.B = df.B.fillna(df.A.map(dict))
print(df)
A B
0 a 2
1 b 5
2 c 4
This can be done simply
df['B'] = df['B'].fillna(df['A'].apply(lambda x: dict.get(x)))
This can work effectively for a bigger dataset as well.
Unfortunately, this isn't one of the options for a built-in function like pd.fillna().
Edit: Thanks for the correction. Apparently this is possible as illustrated in #Vaishali's answer.
However, you can subset the data frame first on the missing values and then apply the map with your dictionary.
df.loc[df['B'].isnull(), 'B'] = df['A'].map(dict)