finding the gradient of a graph between two points.

Hi I have imported some data from excell and have plotted a graph from 2 columns of data. I need to find the gradient of the graph between two points, not the gradient of the whole graph because I am using data from semiconductors which properties change as the voltage increases.

Here is my code

%%%%%%%%%%%%%%%  variables   %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
voltage = data (:,1);
current = data (:,2);
current_density = data (:,3);
LED_power = data (:,4);
LEDPower_density = data (:,5);
Corrected = data (:,6);
droop_model = data (:,7);
electron_charge= 1.6E-19;
boltzman_constant =  1.3806503E-23;
temperature = 300;
constant = (electron_charge / (boltzman_constant*temperature));

F = plot(voltage,current);

[Luffy]

 Then between which two points do you want to calculate slope as it should vary on the basis of points u choose

[Phillip Addison]

between 3 and 3.5 on x axis.

Answer 1

[Luffy]

 i=find(voltage==3); % this gives index of 3 in voltage vector to calculate corresponding current value at voltage = 3.
 j=find(voltage==3.5);
 slope = 2*(current(j)-current(i));

[Phillip Addison]

when i put in this code it says that the second = on the slope = current (j) line is invalid .

??? Error: File: analysis_of_data.m Line: 29 Column: 40 The expression to the left of the equals sign is not a valid target for an assignment.

[Luffy]

 Yeah do not directly enter the code that i wrote.
 In the last line just enter this ,
 slope = 2*(current(j)-current(i)); %I wrote it so that u could understand

[Phillip Addison]

it has solved the error, however the program is finding i and j as [ ] so the slope is being calculated as [ ].

[Luffy]

 Can you enter a sample of your data(to see what type it is) if this does not work
 instead of i,j directly use, 
 current(find(voltage==3)) & current(find(voltage==3.5))

[Phillip Addison]

the two columns i am using are 101x1 double

i put in the line you said and it said this

ans =

   Empty matrix: 0-by-1

ans =

   Empty matrix: 0-by-1

[Luffy]

 Check your data,if it is a empty in between

[Phillip Addison]

data is all full there are no empty places

[Ryan]

It looks like there are no voltage values equal to 3 or 3.5. This method relies on there being a value in voltage exactly equal to the values you set.

[Phillip Addison]

found my problem your answer was right.