Array indices must be positive integers or logical values.

I am trying to ploy a graph of speed against time. But I keep getting this error.

hold on

for t = 0:0.1:25

      if t < 5
          speed(t) = 0.1553567.*(t.^6) - 2.0416.*(t.^5) + 9.1837.*(t.^4) - 14.829.*(t.^3)  - 1.3703.*(t.^2) + 32.821.*t - 1.3155;

      elseif t >= 5 && t < 15.4 
          speed(t) = 0.003980879.*(t.^5) - 0.2247.*(t.^4) + 4.8682.*(t.^3) - 50.442.*(t.^2) + 254.67.*t - 430.66;

      else
          speed(t) = -0.073.*(t.^2) + 6.1802.*t + 40.423;
      end

end
plot(t,speed(t),'-')
hold off

Answer 1

[Image Analyst]

Your t is not an index - an integer - and it needs to be. The FAQ has a thorough discussion: https://matlab.wikia.com/wiki/FAQ#How_do_I_fix_the_error_.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22.3F

[Harry Brown]

So how would I get it to range from 0 to 25?

[Image Analyst]

tVector = 0:0.1:25
for k = 1 : length(t)
    t = tVector(k);
    speed(k) = same as before, using t (not tVector).....

Use a loop iterator k which is an integer. Then compute all the t before in a vector and extract the one single t you need for that particular iteration inside the loop and use it.

[Harry Brown]

Ah I see, cheers.

Answer 2

[Walter Roberson]

Learn to use this design pattern:

vals = ... appropriate vector
num_vals = length(vals);
output = zeros(1, num_vals);
for idx = 1 : num_vals
  this_val = vals(idx);
  ... calculation involving this_val goes here ...
  output(idx) = ... appropriate value
end
...
plot(vals, output)

Alternately, use

    for T = 1:1:251
      t = (T-1)/10;
      if t < 5
            speed(T) = 0.1553567.*(t.^6) - 2.0416.*(t.^5) + 9.1837.*(t.^4) - 14.829.*(t.^3)  - 1.3703.*(t.^2) + 32.821.*t - 1.3155;

        elseif t >= 5 && t < 15.4 
            speed(T) = 0.003980879.*(t.^5) - 0.2247.*(t.^4) + 4.8682.*(t.^3) - 50.442.*(t.^2) + 254.67.*t - 430.66;

        else
            speed(T) = -0.073.*(t.^2) + 6.1802.*t + 40.423;
        end

end

plot((T-1)/10, speed)

or

for t = 0:0.1:25
  T = round(t*10) + 1;
      if t < 5
          speed(T) = 0.1553567.*(t.^6) - 2.0416.*(t.^5) + 9.1837.*(t.^4) - 14.829.*(t.^3)  - 1.3703.*(t.^2) + 32.821.*t - 1.3155;

        elseif t >= 5 && t < 15.4 
            speed(T) = 0.003980879.*(t.^5) - 0.2247.*(t.^4) + 4.8682.*(t.^3) - 50.442.*(t.^2) + 254.67.*t - 430.66;

        else
            speed(T) = -0.073.*(t.^2) + 6.1802.*t + 40.423;
        end

end
plot(0:0.1:25, speed)

The first design pattern can be used even when the values are irregularly spaced or when it is difficult to calculate an index given a value. The control value associated with any one location is always the same because the control values are pre-calculated and recalled as needed.

The second one, using integer indices and calculating values from there, can produce more precise control values than you might get from using a floating point increment such as for 0:0.1:25 . For example, the 4th entry of 0:0.1:25 is not exactly the same as you would get from coding 0.3 but the 4th entry of (0:250)/10 is exactly the same as you would get from coding 0.3 .

The third one, using floating point increment for the 0:0.1:25 and calculating indices from it, is likely to be the least accurate, and can get you in trouble with floating point comparisons, but might be the most convenient to code... until, that is, you encounter the subtle problems with floating point accuracy, at which point it becomes a nuisance.

[Harry Brown]

Okay, thanks for the help

[Harry Brown]

I have edited my code to this. But when I try to plot the values, I get zero for speed up until the final time, where it jumps up to the correct value.

hold on
Vector_t = 0:0.1:25;
for k = length(Vector_t)
  t = Vector_t(k);
  if t < 5
      speed(k) = 0.1553567.*(t.^6) - 2.0416.*(t.^5) + 9.1837.*(t.^4) - 14.829.*(t.^3)  - 1.3703.*(t.^2) + 32.821.*t - 1.3155;

    elseif t >= 5 && t < 15.4 
        speed(k) = 0.003980879.*(t.^5) - 0.2247.*(t.^4) + 4.8682.*(t.^3) - 50.442.*(t.^2) + 254.67.*t - 430.66;

    else
        speed(k) = -0.073.*(t.^2) + 6.1802.*t + 40.423;
    end
    output(k) = speed(k);
end
plot(Vector_t,output,'*')
hold off

[Walter Roberson]

You missed the 1: in the for loop