interp1 returning NaN for values <1

Hey all,

I have a function that I am using as part of another program, and it relies on interp1, however, my interp1 function is not returning a mix of NaNs and numbers. Here is my code:

function [T] = thrust(t);

    thrust_time = xlsread('rocketdata.xlsx')
    e = length(thrust_time)
    time_reduced = thrust_time((1:e),2);
    thrust_reduced = thrust_time((1:e),1);
    A = interp1(time_reduced,thrust_reduced)
    T = A(t)

end

where the XLS file has 2 columns of 153 rows. The first column is Thrust, and the second column is time. All values are positive, but the first ~25 and last ~25 thrust values are decimals less than 1 but greater than zero, which return NaN. All thrust values greater than 1 return actual numbers.

Can anyone please tell me what I am doing wrong? Thank you!

Answer 1

[Star Strider]

I don’t know what the contents and ranges of time_reduced and thrust_reduced are. I suspect you may be asking interp1 to interpolate values not within the range of the data, but that’s only a guess. You will need to specify a third argument to interp1 if you want it to interpolate values for thrust_reduced at new times for time_reduced.

From the documentation on interp1:

vq = interp1(v,xq) returns interpolated values and assumes a default set of sample point coordinates. The default points are the sequence of numbers from 1 to n, where n depends on the shape of v:

• When v is a vector, the default points are 1:length(v) .
• When v is an array, the default points are 1:size(v,1) .

Use this syntax when you are not concerned about the absolute distances between points.

[Joey]

Thank you, i finally got it to work!

[Star Strider]

My pleasure!