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

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

[Stephen23]

See KL's answer for a much simpler solution.

Answer 1

[Birdman]

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

[Sebastian]

thank you very much.

[Stephen23]

@Sebastian: you would be better off learning how to use MATLAB effectively. See KL's answer for a much neater solution.

[Jan]

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.

Answer 2

[KL]

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;

[Stephen23]

+1 simple, and very good use of MATLAB.

[KL]

Thanks Stephen:)

[Sebastian]

thank you KL :)

[Jan]

+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;