How do I populate a column with a constant value?

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balsip 2016-6-21

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评论： Star Strider 2016-6-21

I am trying to populate a column in a newly created empty variable (ConstantVector) with a constant value for the column length of an existing vector (Vector_A), which is 5904x1.

The following code populates a row (1x5904):

ConstantVector=[];
for i=1:length(Vector_A)
   ConstantVector(i)=((10^5)/((8.31434)*(298.15)))*(10^6);
end

How should I be coding this so that the ConstantVector is one column with the same value in the length of Vector_A?

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Answer 1

Star Strider 2016-6-21

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Use the ones function to create a vector of 1 and then multiply it by your constant value:

ConstantVector = ones(size(VectorA))*((10^5)/((8.31434)*(298.15)))*(10^6);

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balsip 2016-6-21

Thank you, Star Strider! That does the trick, and without a for loop, too.

Star Strider 2016-6-21

My pleasure!

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Answer 2

Steven Lord 2016-6-21

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Depending what you're planning to do with the resulting vector. One obvious possibility is repmat.

x = magic(5);
y = repmat(pi, size(x));
z = repmat(42, size(x, 1), 1);
w = repmat(-999, 2, size(x, 2));

If you need to add the constant to the original, like if I wanted to add to the vector z, take advantage of scalar expansion rather than creating a (potentially large) temporary array.

q1 = z + 1;
q2 = z + ones(size(z));
isequal(q2, q2) % returns true

The repelem and bsxfun functions may also be of interest, again depending on what you're trying to do.

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balsip 2016-6-21

Pretty sure your solution would help me, too, Steven. I have a four-term formula (not shown) where two of the terms are variable time series from my data set and the other two terms are constant values.

I'm a little surprised that I need to create a vector of a certain length for a constant value. Maybe you're showing me a way around this, but the first answer above is a bit easier to wrap my head around.

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