# More fun with APL

Note: see the rest of the series so far.

I stumbled across this post the other day and problem 2 seems like something I can easily do in APL since it essentially requires you to interleave two arrays.

The problem is:

Write a function that combines two lists by alternatingly taking elements. For example: given the two lists [a, b, c] and [1, 2, 3], the function should return [a, 1, b, 2, c, 3].

Here’s the solution I have come up with:

since it uses both $\omega$ (right argument) and $\alpha$ (left argument) so it’s a dyadic function, let’s test it out:

$'a' \ 'b' \ 'c' \ p2 \ 1 \ 2 \ 3$

=> a 1 b 2 c 3

Here’s how it works:

• concatenate the two arguments together, with the left argument first $(\alpha, \omega)$
• reshape $\rho$ the concatenated vector into 2 rows, so that you have effectively placed $\alpha$ and $\omega$ into a matrix, i.e.

$a \ b \ c\\* 1 \ 2 \ 3$

• transpose that matrix

$a \ 1\\* b \ 2\\* c \ 3$

• reshape $\rho$ the transposed matrix into a vector, and that’s it!

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Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.