# Liu Sida's Homepage

Machine Learning & Human Learning

Yesterday, I saw tf.contrib.layers.sparse_column_with_hash_bucket in a tutorial. That’s a very useful function! I thought. I never met such a function in Keras or TFLearn.

Basically, the function do something like this:

hash(category_string) % dim


Let’s say the text “the quick brown fox”. If we want to put them into 5 buckets, we can get result like this:

hash(the) % 5 = 0
hash(quick) % 5 = 1
hash(brown) % 5 = 1
hash(fox) % 5 = 3


This example is metioned by Luis Argerich

That’s really easy for preprocessing, but there are disadvantages of that, metioned by Artem Onuchin also in that page.

So, the common way to do this feature engineering thing is metioned by Rahul Agarwal:

• Scaling by Max-Min
• Normalization using Standard Deviation
• Log based feature/Target: use log based features or log based target function.
• One Hot Encoding

Anyway, if we want to do hash_bucket without tensorflow, we can do it in Pandas which is metioned here:

import pandas as pd
import numpy as np

'year': [2000, 2001, 2002, 2001, 2002],
'pop': [1.5, 1.7, 3.6, 2.4, 2.9]}

data = pd.DataFrame(data)

def hash_col(df, col, N):
cols = [col + "_" + str(i) for i in range(N)]
print(cols)
def xform(x): tmp = [0 for i in range(N)]; tmp[hash(x) % N] = 1; return pd.Series(tmp,index=cols)
df[cols] = df[col].apply(xform)
return df.drop(col,axis=1)

print(hash_col(data, 'state',4))


result:

   pop  year  state_0  state_1  state_2  state_3
0  1.5  2000        1        0        0        0
1  1.7  2001        1        0        0        0
2  3.6  2002        1        0        0        0
3  2.4  2001        1        0        0        0
4  2.9  2002        1        0        0        0


[Edited] Actually, we can use pandas.get_dummies to do this directly:

import pandas as pd

'year': [2000, 2001, 2002, 2001, 2002],
'pop': [1.5, 1.7, 3.6, 2.4, 2.9]}

data = pd.DataFrame(data)
print(pd.get_dummies(data, columns=['state','year']))


result:

   pop  state_Nevada  state_Ohio  year_2000  year_2001  year_2002
0  1.5           0.0         1.0        1.0        0.0        0.0
1  1.7           0.0         1.0        0.0        1.0        0.0
2  3.6           0.0         1.0        0.0        0.0        1.0
3  2.4           1.0         0.0        0.0        1.0        0.0
4  2.9           1.0         0.0        0.0        0.0        1.0


After all,

Coming up with features is difficult, time-consuming, requires expert knowledge. “Applied machine learning” is basically feature engineering.

Said Andrew Ng.