How to extract non-consecutive submatrix in a easier way

Shawn Fan wrote: Hi, everyone, If I have a 100 by 100 matrix and I want to extract its 10 to 20, 40 to 50 and 70 to 80 rows and columns, Is there a easier way rather than submatrix the rows first and then the columns and use stack or augment to assemble the submatrice? Such as define a index of the rows and columns I want to extract and then extract the index from the matrix in Matlab? E.g, index= [10:20 40:50 70:80] K_BC=K(index, index)

I'm not sure there's an easy way in Prime (or in Mathcad 15). Improvements to array indexing to allow such things have been on my (submitted) wishlist for a long time. To my mind, the "natural" Mathcad way to do this would be by using a range variable to store ranges of the form "(10..20),(40..50),(70..80)". Whilst Mathcad 15 won't let you assign such a range to a range variable, it will let you enter it as the list of elements in a for loop, so you could just write "for r e (10..20),(40..50),(70..80) <code to extract row r and build the result matrix>", not as neat as Matlab, but it will work. The downside is that you can't pass such a range as an argument to a function or assign it to a range variable.

The even bigger downside to Prime is that you can't even enter such expressions or enter standard ranges as arguments to functions! This prevents several of my Mathcad 15 worksheets from working in Prime.

However, all is not total gloom as it's possible to write a function that will interpret a string like "10:20 40:50 70:80" and explicitly expand the implied range. The Big Black Cloud that accompanies this thin silver lining is that I've got the trial version of Prime 3.1 and you can't read it. I could write a Mathcad 15 version, which you could convert (provided you've got Mathcad 15), but my Prime version takes advantage of Prime's matrix row extraction operator to simplify things and Mathcad 15 only has the equivalent column operator. Unfortunately, I can't create the Mathcad 15 version at the moment.

Stuart

Here's an example of a function pickrows that will interpret a string as a range and pick out the desired rows. R is the matrix of interest. slist is a range that uses "," to separate ranges (as per normal Mathcad usage) and slist2 uses spaces to separate the ranges; both use the colon as per Matlab normal usage).

It makes use of my function str2arr that converts a string into an array, using a delimiter string "cr" where c is the column delimiter character and r is the row delimiter character.

Functions needed to support pickrows ...