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list manipulation - How to understand the usage of Inner and Outer figuratively?


Description:


In Mathematica the functions like Thread, Inner, Outer etc. are very important and are used frequently.


For the function Thread:


Thread Usage1:


Thread[f[{a, b, c}]]



{f[a], f[b], f[c]}

Thread Usage2:


Thread[f[{a, b, c}, x]]


{f[a, x], f[b, x], f[c, x]}

Thread Usage3:



Thread[f[{a, b, c}, {x, y, z}]]


{f[a, x], f[b, y], f[c, z]}

And I understand the Usage1, Usage2, Usage3 easily as well as I use them masterly.


However I always cannot master the usage of Inner and Outer so that I must refer to the Mathematica Documentation every time when I feel I need using them.


I find that I cannot master them owing to that I cannot understand the results of Inner and Outer clearly. Namely, I always forget what construct they generate when executed.


The typical usage cases of Inner and Outer shown as below:




Inner Usage:


Inner[f, {a, b}, {x, y}, g]


g[f[a, x], f[b, y]]

Inner[f, {{a, b}, {c, d}}, {x, y}, g]


{g[f[a, x], f[b, y]], g[f[c, x], f[d, y]]}


Inner[f, {{a, b}, {c, d}}, {{x, y}, {u, v}}, g]


{{g[f[a, x], f[b, u]], g[f[a, y], f[b, v]]}, 
 {g[f[c, x], f[d, u]], g[f[c, y], f[d, v]]}}

Outer Usage:


Outer[f, {a, b}, {x, y, z}]



{{f[a, x], f[a, y], f[a, z]}, {f[b, x], f[b, y], f[b, z]}}

Outer[f, {{1, 2}, {3, 4}}, {{a, b}, {c, d}}]


{{{{f[1, a], f[1, b]}, {f[1, c], f[1, d]}}, 
  {{f[2, a], f[2, b]}, {f[2, c], f[2, d]}}}, 
 {{{f[3, a], f[3, b]}, {f[3, c], f[3, d]}}, 
  {{f[4, a], f[4, b]}, {f[4, c], f[4, d]}}}}


Questions:




  1. How to master the usage Inner and Outer? Namely, how can I use them without referring to the Mathematica Documentation?




  2. How to understand the result of Out[3],Out[4],Out[5] figuratively? Namely, by using graphics or other way.






Answer



I think of Outer just like nikie showed.


Inner is a generalization of matrix multiplication. I like the picture from the Wikipedia page.


Matrix Multiplication


To calculate an entry of matrix multiplication, you first pair list entries (a11,b12) and (a12,b22). You "times/multiply" those pairs (a11*b12) and (a12*b22), and then you "plus/add" all the results (a11*b12)+(a12*b22). Note that you "times" before you "plus" in matrix multiplication which helps me remember the order of arguments for Inner.


listL={{a11,a12},{a21,a22},{a31,a32},{a41,a42}};
listR={{b11,b12,b13},{b21,b22,b23}};
Inner[times,listL,listR,plus]

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