plot a function having two variables with unequal elements

hello

i have a fuction as follows

XT=k*L*pitch;

where L contains 100 elements and pitch contains 5 elements. k also contains 5 elements.

i want to create this function and also do plot(L,XT).

it will be a great help if anyone can help me in this.

[Walter Roberson]

With k having 5 elements and pitch having 5 elements and L having 100 elements, then what output array size are you hoping to get in XT ?

[Debasish Mishra]

i need output such that for pitch(1) i ll have XT to be having 100 output values.

same for pitch(2) and so on.

so basically for each value of pitch i must get XT being a 1 by 100 array

[Walter Roberson]

Assuming L is 1 x 100 and k is 1 x 5, then k.' * L would be (1 x 5 transposed) * (1 x 100) = 5 x 1 * 1 x 100 -> 5 x 100.

Now you want to multiply that by 1 x 5 pitch. If you pre-multiply, pitch * (k.' * L) then that would be 1 x 5 * 5 x 100 which would give 1 x 100, but you want 5 x 100 or 100 x 5 output. If you post-multiply then that would be 5 x 100 * 1 x 5 and you can only get that to work with some transposes, but you are going to end up with 100 x 1 or 1 x 100 again.

You will need to show us the mathematical form of the calculation you want.

[Debasish Mishra]

thanks walter for your timely response.

here i am attaching my matlab syntax for easy understanding.

pitch=[30 40 50 60 70] ;
L=linspace(1,1000);
N11=sqrt(delta1).*U1.^2.*sqrt((pi*a)./(pitch.*W1)).*exp(-pitch.*W1/a);
D=a*V^3.*K1.^2;
kpq1=N11./D;
XT1=(2.*kpq1.^2*Rb.*L)./(pitch.*B1);
plot(L,XT1)

so basically except L and pitch all are constants.For each element of pitch the code calculates 100 values of XT1.

The plot function should plot 5 different curves for each element of pitch.I tried using for loop but failed.

1st curve should denote for pitch =30,second for pitch =40 and so on.

thanks in advance.

Answer 1

[the cyclist]

Merging the code you've provided with Walter's comments:

pitch=[30 40 50 60 70]' ;
L=linspace(1,1000);

delta1 = 1;
W1 = 1;
U1 = 1;
a = 1;
V = 1;
K1 = 1;
Rb = 1;
B1 = 1;

N11=sqrt(delta1).*U1.^2.*sqrt((pi*a)./(pitch.*W1)).*exp(-pitch.*W1/a);
D=a*V^3.*K1.^2;
kpq1=N11./D;
XT1=(2.*kpq1.^2*Rb.*L)./(pitch.*B1);

figure
h = plot(L,XT1,'.-');
set(h,'MarkerSize',16)

A couple things to note.

First, there are five curves there. Presumably they will be separated when you put in your actual constants.

As Walter suggested, I have reoriented pitch so that it is 5x1 instead of 1x5. This is the crucial element you need to understand. If you try to multiply

rand(1,5) .* rand(1,100)

then you are going to get a dimension mismatch error. But if you do

rand(5,1) .* rand(1,100)

then MATLAB is going to use implicit expansion, and effectively make both of those arrays into 5x100 arrays before multiplying them. That is what happens in the calculation of XT1 in the code I posted.

Then, the syntax of the plot command will plot five lines (with 100 points each) when you input the 5x100 array. (I used large dots to illustrate this.)