# Help with Plotting Data & Function in Matlab

• jdawg
In summary, the student attempted to plot data but ran into problems because t had more elements than Q. Linspace may be the source of the problem. The student was able to get a plot to come up but is not confident it is correct.f

## Homework Statement

I'm a little lost on how to plot this data and function. I included the homework question and my attempt at plotting in the attached picture. I'm pretty sure what I have is completely wrong and I honestly don't have much of an understanding of matlab, so the more you dumb it down the better. Any help is greatly appreciated! :)

## The Attempt at a Solution

#### Attachments

• matlabhw1.png
60.6 KB · Views: 435
• matlab_question.png
8.8 KB · Views: 423
Your t array has more elements than your Q array and so MATLAB can't pair them together for an (x,y) point to plot.

As an example:

Matlab:
x=[0:1:10]
y=x.*x
plot(x,y)

produces two vectors ##x## and ##y## where ##y=x^2## and so the plot(x,y) draws a simple parabola.

In your case, MATLAB can't match up each value in t with one in Q and hence issues the error message you got.

Ask yourself why does t have a different length from Q? Isn't Q dependent on t somehow?

Here's the definition of linspace which may be the source of your problem:

http://www.mathworks.com/help/matlab/ref/linspace.html?s_tid=gn_loc_drop

Notice you used a 3 argument version where you want to create a vector t from 0 to 10 and you want 100 points and that's what you got but the Q is only eight points.

So one solution is to modify the linspace arguments.

jdawg
You can use the length function to evaluate vector lengths.
Then make sure they are the same size

jdawg
Ok, I tried changing a few things and managed to get a plot to come up, but I'm not super confident that its right. I was thinking that I was supposed to use linspace somehow to plug t values into my q function and then plot the actual Q and t vectors as data points on the same graph.. I hope that makes sense.

Or could I just say something like plot q and then somehow add the Q and T vectors onto the same plot as data points?

Also, did I type the q function correctly? Matlab keeps giving me errors when I try to run it.

#### Attachments

• mlab2.png
36.2 KB · Views: 410
Ok, I tried changing a few things and managed to get a plot to come up, but I'm not super confident that its right. I was thinking that I was supposed to use linspace somehow to plug t values into my q function and then plot the actual Q and t vectors as data points on the same graph.. I hope that makes sense.

Or could I just say something like plot q and then somehow add the Q and T vectors onto the same plot as data points?

Also, did I type the q function correctly? Matlab keeps giving me errors when I try to run it.

A few things...
1. that plot looks correct.
2. you can use the following commands to make your plot look better
xlabel('string');
ylabel('string');
title('string')
legend('plot1','plot2',...)

3. exp is a function, get rid of the ^
exp(stuff) will execute the math function e^stuff

4. just do this

plot( t , vector1 , t , vector2 );

jdawg
Ok, I tried changing a few things and managed to get a plot to come up, but I'm not super confident that its right. I was thinking that I was supposed to use linspace somehow to plug t values into my q function and then plot the actual Q and t vectors as data points on the same graph.. I hope that makes sense.

Or could I just say something like plot q and then somehow add the Q and T vectors onto the same plot as data points?

Also, did I type the q function correctly? Matlab keeps giving me errors when I try to run it.
in general the MATLAB forums are VERY good.

when you get an error, just toss it in google.
when you want to say plot two vectors, type "matlab plot two vectors" into google. You'll be surprised by what has already been answered and the guides mathworks have put together.

jdawg
You were so much help! Thanks a bunch!