Please see How do I post a question effectively? and provide a Short, Self-Contained, Correct Example with a small matrix (say, 10x10) that demonstrates what you are trying to do.

If you're running out of memory just reading the file, then how you might optimize the operations you want to perform depends on what those operations are, which you haven't fully described.

In general, PDL might be of interest.

In reply to Re: upper or lower triangular matrix to full by haukex
in thread upper or lower triangular matrix to full by savita

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