Fastest way to make division between a number and elements in a list

Question

I have a list y={{a,b},{c,d},...} of dimension {10^5, 2}, and number x. I want to divide only the second part of each element of the list by x, and have as result a new list {{a,b/x},{c,d/x},...}. What is the fastest/best way to do it? This is what I have done:

{#[[1]], Divide[#[[2]], x]} & /@y

Thanks

List[[All,2]]=list[[All,2]]/x – Mike Honeychurch Jan 16 '15 at 09:50 — Mike Honeychurch, Jan 16 '15 at 09:50
Why are answers given as comments? – Philipp Jan 16 '15 at 10:38 — Philipp, Jan 16 '15 at 10:38
@Philipp I was looking for a duplicate :) – Kuba Jan 16 '15 at 11:32 — Kuba, Jan 16 '15 at 11:32

score 2 · Answer 1 · answered Jan 16 '15 at 16:31

2

If y is a packed array then Dot should be quite fast:

y = RandomReal[{0, 1}, {10^5, 2}];
x = 12.3;

result = y.{{1, 0}, {0, 1/x}};

This takes about a millisecond on my not-very-fast PC.

answered Jan 16 '15 at 16:31

Simon Woods

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Fastest way to make division between a number and elements in a list

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