Increment counter the first time a number is reached

Question

This is probably a very silly question. But, I'll still go ahead and ask. How would you increment a counter only the first time a particular value is reached?

For example, if I have step below as a column of the df and would want to add a counter column called 'counter' which increments the first time the 'step' column has a value of 6

Answer 1

You can use .shift() in pandas -

Notice how you only want to increment if value of df['step'] is 6 and value of df.shift(1)['step'] is not 6.

df['counter'] = ((df['step']==6) & (df.shift(1)['step']!=6 )).cumsum()
print(df)

Output

      step  counter
0      2        0
1      2        0
2      2        0
3      3        0
4      4        0
5      4        0
6      5        0
7      6        1
8      6        1
9      6        1
10     6        1
11     7        1
12     5        1
13     6        2
14     6        2
15     6        2
16     7        2
17     5        2
18     6        3
19     7        3
20     5        3

Explanation

a. df['step']==6 gives boolean values - True if the step is 6

0     False
1     False
2     False
3     False
4     False
5     False
6     False
7      True
8      True
9      True
10     True
11    False
12    False
13     True
14     True
15     True
16    False
17    False
18     True
19    False
20    False
Name: step, dtype: bool

b. df.shift(1)['step']!=6 shifts the data by 1 row and then checks if value is equal to 6.

When both these conditions satisfy, you want to increment - .cumsum() will take care of that. Hope that helps!

P.S - Although it's a good question, going forward please avoid pasting images. You can directly paste data and format as code. Helps the people who are answering to copy-paste