How to extract the value of column 1 when column 2 changes?(python)-CodePudding

I have a pandas.DataFrame of the form.(It doesn't matter if you use numpy.) I want to output a value of 'moID' whenever the value of column 'time' changes. I'll show you a simple example below. I will mark the row that should be output as '<<<'.

index 'moID' 'time'
0       1      0     <<< 
1       25     0
2       3      1     <<< 
3       45     1
4       12     1
5       2      2     <<< 
6       34     1     <<< 
7       4      1
8       12     1
9       2      3     <<< 
10      5      3
11      37     3
12      85     0     <<< 
13      2      0
14      45     1     <<< 
15      55     1
16      2      3     <<< 
17      23     3
18      42     0     <<< 
19      1      0
20      42     1     <<< 
21      2      2     <<< 
22      41     2
23      3      1     <<< 
24      52     1
25      2      1
26      24     3     <<< 
27      3      3
28      5      3

result is :

    index 'moID'
            1
            3
            2
            34
            2
            85
            45
            2
            42
            42
            2
            3
            24

help me please.

CodePudding user response：

You can use shift ne to see if consecutive rows match and create a boolean Series (where it's False if the time is the same but True if it's different). Then use it as a mask to filter the desired items:

out = df.loc[df['time'].ne(df['time'].shift()), 'moID']

Output:

0      1
2      3
5      2
6     34
9      2
12    85
14    45
16     2
18    42
20    42
21     2
23     3
26    24
Name: moID, dtype: int64

CodePudding user response：

You can use boolean indexing the following way:

result = df.moID[df.time.diff() != 0]

df.time.diff() != 0 generates a Series of boolean and it is used to index moID column.

The result, for your source data, is:

0      1
2      3
5      2
6     34
9      2
12    85
14    45
16     2
18    42
20    42
21     2
23     3
26    24
Name: moID, dtype: int64

The left column is the index and the right one - actual values.