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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Spaceman 2024-2-19

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评论： Spaceman 2024-3-21

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.

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VBBV 2024-2-20

移动：VBBV 2024-2-20

在 MATLAB Online 中打开

B=rand(randi([10 20]),randi([5 30]))
B = 14×17
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
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
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
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
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
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
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
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
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
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
0016    1.5189

VBBV 2024-2-20

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

Spaceman 2024-2-20

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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Answer 1

John D'Errico 2024-2-20

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https://ww2.mathworks.cn/matlabcentral/answers/2084128-how-do-i-create-a-variable-h-which-contains-the-product-of-5-and-the-values-of-b-in-the-1st-and-9th#answer_1412158

编辑：John D'Errico 2024-2-20

在 MATLAB Online 中打开

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
5745    0.6058    0.5994    0.6935    0.0956    0.2848
8822    0.8884    0.9519    0.3293    0.0266    0.6770
4545    0.1755    0.2823    0.9036    0.6515    0.7482
7823    0.5347    0.4965    0.6685    0.5269    0.4783
8030    0.9700    0.4688    0.5584    0.8849    0.9196
7843    0.5541    0.7347    0.1885    0.2401    0.6638
4241    0.2129    0.2587    0.7451    0.0305    0.8609
9187    0.1319    0.1306    0.4830    0.0690    0.1599
5090    0.7634    0.5069    0.6934    0.1676    0.5085
2144    0.5469    0.5273    0.1173    0.4948    0.9835
B([1,9],[1,2])
ans = 2×2
5745    0.6058
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.

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John D'Errico 2024-2-20

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

Spaceman 2024-3-21

:)

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Answer 2

VBBV 2024-2-20

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https://ww2.mathworks.cn/matlabcentral/answers/2084128-how-do-i-create-a-variable-h-which-contains-the-product-of-5-and-the-values-of-b-in-the-1st-and-9th#answer_1412153

在 MATLAB Online 中打开

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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Spaceman 2024-2-20

编辑：Spaceman 2024-2-20

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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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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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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