# How do I create a variable H which contains the product of 5 and the values of B in the 1st and 9th rows, and the 1st and 2nd column.

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Kyle Weaver il 19 Feb 2024
Commentato: Kyle Weaver il 21 Mar 2024
Given: B=rand(randi([10 20]),randi([5 30]))
Find: Create a variable H which contains the product of 5 and the values of B in the 1st and 9th rows, and the 1st and 2nd column. Variable H must be of size [2 2].
Issue: I am unfamiliar with the wording, does it want 5 * the values of B in ([1,9],[1,2])?
My Solution: I got as far as to state >> H=prod(B([1,9],[1,2])); but I do not know what to do with "the product of 5" piece.
##### 3 CommentiMostra 1 commento meno recenteNascondi 1 commento meno recente
VBBV il 20 Feb 2024
Do you mean multplying with 5* ? or product of 5 consecutive values
Kyle Weaver il 20 Feb 2024
Variable H must be of size [2 2]. Otherwise this looks good. How do we get to the proper size for H?

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### Risposta accettata

John D'Errico il 20 Feb 2024
Modificato: John D'Errico il 20 Feb 2024
Ok, first, you do understand how to extract the elements from B. Good there. I think maybe the question wording is confusing you.
B=rand(randi([10 20]),randi([5 30]))
B = 15×6
0.5745 0.6058 0.5994 0.6935 0.0956 0.2848 0.8822 0.8884 0.9519 0.3293 0.0266 0.6770 0.4545 0.1755 0.2823 0.9036 0.6515 0.7482 0.7823 0.5347 0.4965 0.6685 0.5269 0.4783 0.8030 0.9700 0.4688 0.5584 0.8849 0.9196 0.7843 0.5541 0.7347 0.1885 0.2401 0.6638 0.4241 0.2129 0.2587 0.7451 0.0305 0.8609 0.9187 0.1319 0.1306 0.4830 0.0690 0.1599 0.5090 0.7634 0.5069 0.6934 0.1676 0.5085 0.2144 0.5469 0.5273 0.1173 0.4948 0.9835
B([1,9],[1,2])
ans = 2×2
0.5745 0.6058 0.5090 0.7634
So the result is a 2x2 array.
The question is not asking you to form the product of all those elements. So there is no reason to use the function prod.
The clue is the result itself must be a 2x2 array. So now all you need to do is multiply all elements of that 2x2 array by the number 5. So you are correct that
5*B([1,9],[1,2])
ans = 2×2
2.8726 3.0291 2.5449 3.8168
is what appears to have been requested.
##### 3 CommentiMostra 1 commento meno recenteNascondi 1 commento meno recente
John D'Errico il 20 Feb 2024
You were already there. Just getting confused by the wording, as you said, overthinking it.
Kyle Weaver il 21 Mar 2024
:)

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### Più risposte (1)

VBBV il 20 Feb 2024
Do you mean like this ?
B=rand(randi([10 20]),randi([5 30]));
H = 5*kron(B([1,9]),B([1,2]).')
H = 2×2
2.0647 1.3872 2.0548 1.3806
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Kyle Weaver il 20 Feb 2024
Modificato: Kyle Weaver il 20 Feb 2024
I don't think we have learned about kron, (Kronecker tensor product), however I do appreciate your efforts and it did return H with size [2 2].

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