# Extract a decimal from a vector in maple

Not sure if this question belongs in this form but I don't know where else to go. I'm trying to index a matrix and do a < comparison on it. Every time I try something like A[1] it says:

Error, a Vector is not valid lhs to < or <=.

Can anyone help?

EXAMPLE:

A:=<1,2,3>:

b:=A[2]:

if b < 2 then .....

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A matrix is a two-dimensional array of numbers. Why are you providing only one index? – Henning Makholm Sep 30 '11 at 3:50
sorry. I reduce it down to a 1 dimensional 1x4 matrix (vector). – NSjonas Sep 30 '11 at 4:04
And what do you expect < to do on a vector? That's not usually defined. – Henning Makholm Sep 30 '11 at 5:24
What exactly did you feed Maple? If you don't show your exact input, none of us could be more helpful... – J. M. Sep 30 '11 at 8:00
Adding to Henning's comment, you need to properly define < on vectors. For example, you can compare the n^th norms of vectors by norm(A, n). Or you could compare the two vectors component-wise as follow. C := zip((x,y) -> evalb(x < y), A, B); and then andmap(=, C, true); – user2468 Oct 1 '11 at 18:30

Which version of Maple are you using? For me, using Maple 12, the following works just fine:

A := <1,2,3>;


$$A := \begin{bmatrix}1 \\ 2 \\ 3\end{bmatrix}$$

b := A[2];


$$b := 2$$

is(b < 2);


$$false$$

is(b <= 2);


$$true$$

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yea well thats what i was trying to do but for for some reason my if statement wouldn't work. give you credit anyways – NSjonas Oct 4 '11 at 17:05