add string at the beging of each row

I would like to add a string at the beginning of each row- either positive or negative – depending on the value in the columns:
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I keep getting ValueError, as per screenshot

>Solution :

Use Series.map for prefixes by conditions and add to index:

df.index = df['positive'].eq(1).map({True:'positive_', False:'negative_'}) + df.index

Or use numpy.where:

df.index = np.where(df['positive'].eq(1), 'positive_','negative_') + df.index

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