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If you want to mix data types you are going to want <a href="http://docs.scipy.org/doc/numpy/user/basics.rec.html" rel="nofollow">structured arrays</a>. If you are going to want the index of matching values in a lookup array you want <code>searchsorted</code> Your example goes like this: <pre><code>>>> import numpy as np >>> PS = np.array([ ('A', 'LABEL1', 20), ('B', 'LABEL2', 15), ('C', 'LABEL3', 120), ('D', 'LABEL1', 3),], dtype=('a1,a6,i4')) >>> FACTOR = np.array([ ('LABEL1', 0.1), ('LABEL2', 0.5), ('LABEL3', 10)],dtype=('a6,f4')) </code></pre> Your structured arrays: <pre><code>>>> PS array([('A', 'LABEL1', 20), ('B', 'LABEL2', 15), ('C', 'LABEL3', 120), ('D', 'LABEL1', 3)], dtype=[('f0', '|S1'), ('f1', '|S6'), ('f2', '<i4')]) >>> FACTOR array([('LABEL1', 0.10000000149011612), ('LABEL2', 0.5), ('LABEL3', 10.0)], dtype=[('f0', '|S6'), ('f1', '<f4')]) </code></pre> And you can access individual fields like this (or you can give them names; see the docs): <pre><code>>>> FACTOR['f0'] array(['LABEL1', 'LABEL2', 'LABEL3'], dtype='|S6') </code></pre> How to perform the lookup of FACTOR on PS (FACTOR must be sorted): <pre><code>>>> idx = np.searchsorted(FACTOR['f0'], PS['f1']) >>> idx array([0, 1, 2, 0]) >>> FACTOR['f1'][idx] array([ 0.1, 0.5, 10. , 0.1], dtype=float32) </code></pre> Now simply create a new array and multiply: <pre><code>>>> newp = PS.copy() >>> newp['f2'] *= FACTOR['f1'][idx] >>> newp array([('A', 'LABEL1', 2), ('B', 'LABEL2', 7), ('C', 'LABEL3', 1200), ('D', 'LABEL1', 0)], dtype=[('f0', '|S1'), ('f1', '|S6'), ('f2', '<i4')]) </code></pre>
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