pd.Series to pd.DataFrame while multiplying each element with each other element-CodePudding

Assuming I have a vector of the form | a | b | c | d |, e.g.

vec = pd.Series([0.3,0.2,0.2,0.3])

What's a quick and elegant way to build a pd.DataFrame of the form:

| a*a | a*b | a*c | a*d |
| b*a | b*b | b*c | b*d |
| c*a | c*b | c*c | c*d |
| d*a | d*b | d*c | d*d |

CodePudding user response：

One option is to use dot:

fr = vec.to_frame()
out = fr.dot(fr.T)

Output:

      0     1     2     3
0  0.09  0.06  0.06  0.09
1  0.06  0.04  0.04  0.06
2  0.06  0.04  0.04  0.06
3  0.09  0.06  0.06  0.09

CodePudding user response：

Use numpy broadcasting:

vec = pd.Series([0.3,0.2,0.2,0.3])

a = vec.to_numpy()

df = pd.DataFrame(a * a[:, None], index=vec.index, columns=vec.index)
print (df)
      0     1     2     3
0  0.09  0.06  0.06  0.09
1  0.06  0.04  0.04  0.06
2  0.06  0.04  0.04  0.06
3  0.09  0.06  0.06  0.09

Or numpy.outer:

df = pd.DataFrame(np.outer(vec, vec), index=vec.index, columns=vec.index)
print (df)
      0     1     2     3
0  0.09  0.06  0.06  0.09
1  0.06  0.04  0.04  0.06
2  0.06  0.04  0.04  0.06
3  0.09  0.06  0.06  0.09