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.

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.

[VBBV]

B=rand(randi([10 20]),randi([5 30]))

B = 14×17

0.2662 0.5350 0.7262 0.5919 0.2591 0.5229 0.0765 0.6560 0.0342 0.9492 0.7424 0.1078 0.2836 0.0978 0.4986 0.9076 0.4798 0.4739 0.7140 0.0168 0.5693 0.7115 0.6679 0.1618 0.4827 0.7419 0.1918 0.5744 0.6739 0.3099 0.5712 0.7473 0.4096 0.1138 0.5934 0.8406 0.1858 0.1633 0.9961 0.8446 0.9879 0.0102 0.7838 0.2878 0.2784 0.7965 0.8308 0.7435 0.0159 0.4783 0.8092 0.6353 0.2810 0.9106 0.0191 0.1172 0.3086 0.3049 0.4064 0.9442 0.5953 0.8699 0.7872 0.6546 0.4964 0.9400 0.1969 0.6890 0.7596 0.8566 0.5199 0.2754 0.9745 0.4120 0.9487 0.1730 0.6434 0.5738 0.9058 0.8732 0.8554 0.1654 0.7022 0.4140 0.4176 0.0532 0.6580 0.7334 0.4252 0.7397 0.1041 0.0820 0.7379 0.2164 0.2215 0.5551 0.8366 0.5806 0.8171 0.6491 0.7825 0.7886 0.6177 0.5449 0.1613 0.5434 0.0893 0.6318 0.2567 0.3719 0.9117 0.1364 0.5861 0.7521 0.4518 0.0895 0.3815 0.0456 0.4336 0.2658 0.0945 0.2259 0.5536 0.0123 0.9001 0.7797 0.7251 0.0081 0.0123 0.9932 0.7810 0.3892 0.4055 0.5873 0.7282 0.5138 0.0012 0.5678 0.4892 0.6592 0.3102 0.1620 0.0680 0.0266 0.8569 0.0633 0.7350 0.4684 0.2345 0.7571 0.6804 0.3015 0.5253 0.7078 0.0139 0.6540 0.2869 0.4237 0.5063 0.3109 0.2217 0.3522 0.4859 0.2493 0.4879 0.3163 0.8592 0.4764 0.8334 0.6707

H = 5*prod(B([1,9],[1,2]))

H = 1×2

0.0016 1.5189

[VBBV]

Do you mean multplying with 5* ? or product of 5 consecutive values

[Spaceman]

Variable H must be of size [2 2]. Otherwise this looks good. How do we get to the proper size for H?

Answer 1

[John D'Errico]

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.

[Spaceman]

Eureka! Learning coding is sometimes a mental struggle when you overthink concepts. This community is amazing and I will be using you all to grasp what I am learning with what little practice I get. This appears to be the answer, the questions wording is just funky. Thank you.

[John D'Errico]

You were already there. Just getting confused by the wording, as you said, overthinking it.

[Spaceman]

:)

Answer 2

[VBBV]

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

[Spaceman]

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].