CODEWITHSUNDEEP
python-3.x
Christopher
Christopher

Reputation: 1

subtract values from different lines

I have a csv file that I am reading in, I have a column of numerical strings and I'm trying to get the difference between the two subsequent rows. The numbers were in depths with "ft" following the values (ex. 4.23ft), I was able to get rid of the "ft" (ex. 4.230), but can't figure out how to assign the values so I can do the math.

    depth = float(depth)
    rate=0
    '''Need to find a way to subtract next line from current line to find 
    rate of change over 15 minute period'''
    for i, data in enumerate(depth):
        d1=i
        d2=i+1
        while rate == 0:
            rate = d1-d2                 
            print(rate)

This gives me a TypeError of " 'float' object is not iterable". when I have the "depth = float(depth)" line commented out, I only get -1 values, which I understand the issue there.

first few lines of raw data first few lines of result data

second row first value minus second value equals first value in third row.

Upvotes: 0

Views: 496

Answers (1)

Ankur Sinha
Ankur Sinha

Reputation: 6639

Since you have already stripped the "ft" part from your column and assuming you have converted the remaining part of the string to float type, I will jump into the next part directly.

If I understand what you want to achieve correctly, you can also use pandas.DataFrame.shift:

df = pd.DataFrame()
df['D1'] = [1.0, 2.0, 3.0, 4.0, 5.0]

Your D1 the value from current row, D2 will be the column from D1 by performing shift operation. 

df['D2'] = df['D1'].shift(-1)

Your dataframe will now look like:

    D1   D2
0  1.0  2.0
1  2.0  3.0
2  3.0  4.0
3  4.0  5.0
4  5.0  NaN

In short, you have the values from the subsequent row of the current row into a new column. You can now perform subtraction/difference operation between the two columns as usual. For example:

df['D3'] = df['D1'] - df['D2']

or 

df['D3'] = df['D1'].sub(df['D2'])

Upvotes: 1

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