In[1]:= list1 = Range[5];
In[5]:= list2 = RandomReal[{0, 1}, 5]
Out[5]= {0.616207, 0.0909475, 0.963306, 0.63963, 0.714089}
In[6]:= Do[f[list1[[i]]] = list2[[i]], {i, 5}]
In[7]:= ?f
Global`f
f[1]=0.616207
f[2]=0.0909475
f[3]=0.963306
f[4]=0.63963
f[5]=0.714089
