Question

I would like to use a "match with" function to compare two matrix. I think I don t understand something about this function. Let s take a trivial example. Consider 2 simple matrix:

let a  = matrix [[1.0; 1155.0; 1.0]
                 [1.0; 1156.0; 1.0]
                 [2.0; 1157.0; 1.0]
                 [3.0; 1157.0; 1.0]]
let b  = matrix [[1.; 0.; 1.0]
                 [1.0; 0.; 1.0]
                 [2.0; 0.; 1.0]
                 [3.0; 0.; 1.0]]

Now I would like to copy the second column of matrix "a" in matrix "b". But with the following code, I have the error which say "error FS0010: unexpected symbole [ in the expression. use ( , () ..."*. Here is the code :

for i = 0 to a.NumCols -1 do
    match b.[i,0] with
    | a.[i,0] -> (b.[i,1]<-a.[i,1])
    | _ -> (b.[i,1]<-a.[i,1])

The problem seems to come from a.[i,0] which is not recognized or unexpected. I don t understand why as I just ask to match a float (b.[i,0]) with another float (a.[i,0]). Can you give me an explanation?

Answer 1

我认为<代码>match的关键词在这种情况下是有用的。只有当你需要根据某种表述的价值来做不同的事情时,这一点才会有用。可以通过书面方式轻易解决这一问题:

b.[0 .. b.NumCols, 1 .. 1] <- a.[0 .. b.NumCols, 1 .. 1]

进行大宗操作——使用slice,即<代码>的整整第二栏/code>,并将其分配给代表整个第二栏的斜体。

请通过<条码>填写<>。 lo,你可以使用:

for i in 0 .. b.NumCols - 1 do 
  b.[i, 1] <- a.[i, 1]

关于使用<代码>matrix的更多信息您可以读到矩阵<>,代表图表,但解释了你可以与矩阵做的所有基本合成物和操作。

EDIT To answer the additional question - if you want to check whether the matched value is the same as some other value (such as element in another matrix), then you need to use the when clause:

match a.[0,0] with
| v when v = b.[0,0] -> () // Do nothing, they are the same
| v -> a.[0,0] <- v        // Set a.[0,0] to b.[0,0] because they are different

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