function with multiple intervals

Dear all, why i cant plot and export the result of my equation ?

clc, clear all, close all
x = (0:0.4:4)'
%
if 0<= x && x< 1
   y = 2.*x
   disp('y1')
elseif 1<= x &&  x< 2
    y = 2
    disp('y2')   
elseif 2<= x &&  x<3
    y = (3/x).^(1/4)
    disp('y3')
elseif 3<= x &&  x<=4
    y = ((3/x).^(1/4))*((x/4).^(1/3))
    disp('y4') 
end
plot(x,y)
%
M = [x; y];
%
fileID = fopen('out.txt','w');
fprintf(fileID,'%6s %12s\n','x','y');
fprintf(fileID,'%6.2f %12.8f\n',M);
fclose(fileID);

Thanks

Answer 1

[Dave B]

When you write

x = (0:0.4:4)'
%
if 0<= x && x< 1
...
end

MATLAB sees: if the number zero is less than the list of values 0, 0.4, ...

It throws an error because && is for comparing two scalars, and you've given it a scalar and a vector.

I believe what you'd like to do is compare each of the values in x to 0 (and to 1, and later to two, etc.)

There are two options, the *good* approach is to do this in a vectorized way:

x = (0:0.4:4)';
y = nan(size(x)); % initializing y is smart because it helps you to catch cases where you forgot to define what should happen for some x

y(0 <= x & x < 1) = 2.*x(0 <= x & x < 1);
y(1 <= x & x < 2) = 2;
y(2 <= x & x < 3) = (3/x(2 <= x & x < 3)).^(1/4);
y(3 <= x & x <= 4) = ((3/x(3 <= x & x <= 4)).^(1/4))*((x(3 <= x & x <= 4)/4).^(1/3));
plot(x,y)

Here I read this as y, for those x where 0 is less than or equal to x and x is less than 1, should be set two 2 times x for those x where 0 is less than x and x is less than 1, etc.

Note use of one & instead of two for comparing two vectors.

In general you can make this code look cleaner by naming an 'index' variable to hold the bit that goes in the parentheses.

The second approach, which will look more similar to your existing code, is to loop over the values in x:

x = (0:0.4:4)'
y = nan(size(x)); % initializing for the same reason as before

for i = 1:length(x)
    xx = x(i);
    if 0 <= xx && xx < 1
        y(i) = 2.*xx;
    elseif 1<= xx &&  xx < 2
        y(i) = 2;
    elseif 2 <= xx &&  xx < 3
        y(i) = (3/xx).^(1/4);
    elseif 3<= xx &&  xx<=4
        y(i) = ((3/xx).^(1/4))*((xx/4).^(1/3));
    end
end

[Touts Touts]

@Dave B thank you, I prefer the second approach.

[Touts Touts]

@Dave B, please there is some different betwen the obtinaed x and y values and the out.txt file, what hapen

form workspace

x =

0

0.4000

0.8000

1.2000

1.6000

2.0000

2.4000

2.8000

3.2000

3.6000

4.0000

y =

0

0.8000

1.6000

2.0000

2.0000

1.1067

1.0574

1.0174

0.9135

0.9225

0.9306

from out.txt file

x y

0.00 0.40000000

0.80 1.20000000

1.60 2.00000000

2.40 2.80000000

3.20 3.60000000

4.00 0.00000000

0.80 1.60000000

2.00 2.00000000

1.11 1.05737126

1.02 0.91345989

0.92 0.93060486

[Dave B]

You created x and y as column vectors, and then put them into one long column. I think for fprintf you want a column for each row that it will write:

M = [x y]';
fileID = fopen('out.txt','w');
fprintf(fileID,'%6s %12s\n','x','y');
fprintf(fileID,'%6.2f %12.8f\n',M);
fclose(fileID);

x y

0.00 0.00000000

0.40 0.80000000

0.80 1.60000000

1.20 2.00000000

1.60 2.00000000

2.00 1.10670000

2.40 1.05740000

2.80 1.01740000

3.20 0.91350000

3.60 0.92250000

4.00 0.93060000

(alternatively, just make x as a row vector above):

x = (0:0.4:4);

[Touts Touts]

@Dave B thanks very much