Why does this not vectorize
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Simply this sort of loop evaluates my function to the reference correctly:
for t = 2:length(x)
x(t) = x(t-1)+x(t);
end
tf = isequal(x,y);
tf
tf =
logical
1
While, surprisngly, this does not:
t = 2:length(x);
x(t) = x(t-1)+x(t);
tf = isequal(x,y);
tf
tf =
logical
0
Is there something obvious about Matlab I'm missing here?
回答（1 个）
You are overwriting x values with the loop, so when you then try the vectorized code you have different x values. Clearly different input x values will give different output x values.
It works perfectly for me, when I do NOT overwrite the x vector values:
>> x = randi(9,1,7)
x =
1 8 9 7 7 7 4
>> for t = 2:length(x), y(t) = x(t-1)+x(t); end % assign to y, not x
>> y
y =
0 9 17 16 14 14 11
>> t = 2:length(x);
>> z(t) = x(t-1)+x(t); % assign to z, not x
>> z
z =
0 9 17 16 14 14 11
5 个评论
Oliver Dechant
2019-1-25
"... except that I need to overwrite the values. That behaviour is desired."
>> x = [1,8,9,7,7,7,4];
>> for t = 2:length(x), x(t) = x(t-1)+x(t); end, x
x =
1 9 18 25 32 39 43
>> x = [1,8,9,7,7,7,4];
>> x = cumsum(x)
x =
1 9 18 25 32 39 43
And, in case you were wondering, vectorizing cumsum withthout the cumsum function is certainly possible, but it would not be particularly simple or particularly efficient. cumsum is the way to go (better than using a loop too).
Torsten
2019-1-25
I think the given recursion was just an example. "cumsum" won't be applicable in a more general case, I guess.
Jan
2019-1-25
@Torsten: If cumsum does not match the needs, filter is more powerful. E.g. an emultaion of cumsum:
x = randi(10, 1, 10)
a = cumsum(x)
b = filter(1, [1,-1], x)
isequal(a,b)
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2019-1-25
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