Why doesn't my script work. I try to create a vector in dependecy of variable without a loop. I'm not sure how i have to define the vector before the if cases

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Sebastian
Sebastian 2017-11-16
评论： Jan 2017-11-25
if true
% code
z=0:1:4000;
g_1=0;
g_2=0;
v(z)=NaN(length(z));
for b=1:length(z)
if z<2000
v(z)=1500+z*g_1;
elseif z>=2000
v(z)=2000+z*g_2;
end
end
end

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Birdman
Birdman 2017-11-16
z=0:1:4000;
g_1=0;
g_2=0;
v=NaN(1,length(z));
for b=1:length(z)
if z(b)<2000
v(b)=1500+z(b)*g_1;
elseif z(b)>=2000
v(b)=2000+z(b)*g_2;
end
end
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Stephen23
Stephen23 2017-11-16
@Sebastian: you would be better off learning how to use MATLAB effectively. See KL's answer for a much neater solution.
Jan
Jan 2017-11-25
I mention this, because it occurs in many code posted in the forum:
After
if z(b)<2000
it is useless to check for:
elseif z(b)>=2000
A simple else is sufficient already: It runs faster and the simpler code reduces the chance for typos.
This is not really important for Sebastian's question, but recommending to omit redundant code will make it easier to debug and maintain the code in general.

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更多回答（1 个）

KL
KL 2017-11-16
No need for a loop, use logical indexing. Here's a better way to write that,
indx = z<2000;
v(indx) = 1500+z(indx)*g_1;
v(~indx)=2000+z(~indx)*g_2;
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Jan
Jan 2017-11-25
+1. Or shorter:
idx = (z >= 2000);
v = 1500 + 500 * idx + z .* (g_1 .* (~idx) + g_2 .* idx);
With reordering the terms this is running 3 times faster:
v = (1500 + z .* g_1) .* ~idx + (2000 + z.* g_2) .* idx;

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