How can I calculate the product of elements in even positions in a vector using a for loop, without using sum or prod built-in functions?
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I'm having trouble calculating the product of even positions in a given matrix using a for loop. The vector is 100,000 uniformly randomly generated numbers between 0 and 50, and I know that is uniform=(50)*rand(1,100000). I don't know how to get the product of the even positions using for loops, without using prod function. Any help would be appreciated
回答（1 个）
Lessmann
2015-11-4
Hi,
you can use the modulo function to detect the even positions and then simply multiply the values.
z = (50)*rand(1,100000);
r = 0
for ii = 1:n
if ~mod(ii,2)
r = r*z(ii)
end
end
4 个评论
@Lessmann: An initial r value of zero means that the output is also going to be zero, no matter how many times you multiply it by some finite random number.
Hers is that code corrected and simplified:
V = 50 * rand(1,100000);
X = 1;
for k = 2:2:numel(V)
X = X*V(k);
end
But the concept is still flawed for the task given: the task is to calculate the product of 100000 random numbers between 0 and 50. Consider the maximum size of a double is
>> realmax
ans = 1.7977e+308
or ~10^308 whereas the average expected answer of this task will be approximately
>> 25^100000
ans = Inf
How can a numeric data class with a maximum value of ~10^308 be used to calculate a value ~25^100000 ?
Lessmann
2015-11-4
Thanks for spotting the mistake.
EaglesFan
2015-11-5
Torsten
2015-11-5
Expected value of the result is 25^100000/(100000*25).
Still much too large in magnitude.
Best wishes
Torsten.
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