Index chebyshev and jackson coefs by integers. Fixes #39 (#40)

* Index chebyshev and jackson coefs by integers. Fixes #39
* initialize jackson chebyshev coefs by np.empty

pygsp/filters/approximations.py

@@ -202,16 +202,16 @@ def compute_jackson_cheby_coeff(filter_bounds, delta_lambda, m):
     filter_bounds[1] = (filter_bounds[1]-a2)/a1
 
     # First compute cheby coeffs
-    ch = np.arange(float(m+1))
+    ch = np.empty(m+1, dtype=float)
     ch[0] = (2/(np.pi))*(np.arccos(filter_bounds[0])-np.arccos(filter_bounds[1]))
-    for i in ch[1:]:
+    for i in range(1, len(ch)):
         ch[i] = (2/(np.pi * i)) * \
             (np.sin(i * np.arccos(filter_bounds[0])) - np.sin(i * np.arccos(filter_bounds[1])))
 
     # Then compute jackson coeffs
-    jch = np.arange(float(m+1))
+    jch = np.empty(m+1, dtype=float)
     alpha = (np.pi/(m+2))
-    for i in jch:
+    for i in range(len(jch)):
         jch[i] = (1/np.sin(alpha)) * \
             ((1 - i/(m+2)) * np.sin(alpha) * np.cos(i * alpha) +
              (1/(m+2)) * np.cos(alpha) * np.sin(i * alpha))