Why does the subscript indices error mean in my code?

Nassetti

2017 12 6

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Hi, I'm a new user. My apologies if the answer is devastatingly simple, but I can't seem to understand why I keep getting this error 'Subscript indices must either be real positive integers or logicals.

Error in CWIIIQ1 (line 13)

in my code. The total code is as follows:

t = [0, 10, 15, 20, 32, 59, 62, 125]; %denotes values of time
v = [0, 56.4, 97.23, 136.25, 226.16, 403.86, 440.44, 1265.23]; %denotes velocity measurements in m/s
format short e; % changes output values to scientific notation with 5 significant figures
p = polyfit(t,v,4); %plots coefficients of a 4th degree polynomial that approximates a function to the data set, choose 4th degree as it results in most accurate fit
%equation polyfit gives is v(t) = (1.14*10e-05)*(x^4)-(1.94e-03)*(x^3)+
%0.10*x^2 + 5.42*x -1.54 with t replacing x in reality
pvx = polyval(p,t); %creates time values for approximated quartic function in array form
v(t) = (1.14*10^-5)*(pvx.^4)-(1.94*10^-3)*(pvx.^3) + 0.10*pvx.^2 + 5.42*pvx -1.54; %this is the velocity equation polyfit calculated
s(t) = int(v(t)); %this is the position equation of the rocket,which is equal to distance travelled as it always is in positive direction
a(t) = diff(v(t),x); %this is the acceleration of the rocket 
figure
subplot(2,2,1) %creates first subplot
plot(x, v(t))
title('Velocity of Rocket versus Time')
subplot(2,2,2)
plot(x, s(t))
title('Position of Rocket versus Time')
subplot(2,2,3)
plot(x, a(t))
title('Acceleration of Rocket versus Time')

Thanks a lot for the help.

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Roger Stafford 2017-12-6

The "devastatingly simple answer" is that all matlab indices must be real positive integers, and yours aren't. You have defined both t and v as vectors, so writing v(t) violates that rule, because t is being interpreted, unavoidably, as a vector of indices to v. You will have to express your computations in some other way.

Walter Roberson 2017-12-6

If you could assign to v(t) with t being non-integer, you would have the problem on the next line with int(v(t)) : int() is for symbolic integration of a symbolic expression or symbolic function but you are trying to int() a numeric vector.

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