For loops and If statements not working correctly

Hello,

I'm trying to plot a time to velocity graph where the velocity is calculated from 3 equations. The three equations are:

This is what my code looks like:

clc
clear
time = [0:0.1:25];
velocity = zeros(length(time),1);

for t = time
    
    if t <= 5 
        velocity = 0.1553567*time.^6 - 2.0416*time.^5 + 9.1837*time.^4 - 14.829*time.^3 - 1.3703*time.^2 + 32.821*time - 1.3155;
    elseif (5<t) && (t<=15.4)
        velocity = 0.003980879*time.^5 - 0.2247*time.^4 + 4.8682*time.^3 - 50.442*time.^2 + 254.67*time - 430.66;
    else 
        velocity = -0.073*time.^2 + 6.1802*time + 40.423;
    end
    
end
plot(time,velocity)

For some reason all the time seems to go through the else statement. Can anyone please help me out, I've been struggling on this simple code for a while now. The graph should also look something similar to this:

Answer 1

[Turlough Hughes]

First of all, when you use element-wise operators (.* .^ ./) like you have been trying you don't necessarily need to run the for loop

Your answer could be:

time =[0:0.1:25];
t1=time(time<5);
velocity1=0.1553567*t1.^6 - 2.0416*t1.^5 + 9.1837*t1.^4 - 14.829*t1.^3 - 1.3703*t1.^2 + 32.821*t1 - 1.3155;

t2=time(5<=time & time<=15.4)
velocity2 = 0.003980879*t2.^5 - 0.2247*t2.^4 + 4.8682*t2.^3 - 50.442*t2.^2 + 254.67*t2 - 430.66;

t3=time(time>15.4)
velocity3=-0.073*t3.^2 + 6.1802*t3 + 40.423

velocity=[velocity1 velocity2 velocity3]
plot(time,velocity)

However, when you run the for loop you are solving for one value of time on each iteration of the loop and so element-wise operators aren't necessary. Though you will need to include an index to indicate the position to store your newly determined velocity on each iteration of the loop.

I included an index c which increments by 1 each iteration of the loop and the new value for velocity is stored in velocity(1,c)

So, the first three iteration of your loop will do the following

velocity (1,1) = answer when you plug t=0 into equations

velocity(1,2) = answer when t=0.1

velocity(1,3) = answer when t=0.2

and so on.

See the following:

clc
clear
time = [0:0.1:25];
velocity = zeros(size(time)); % time is a row vector, so best to keep velocity as a row vector.
c=0;
for t = time
    c=c+1;    % index
    if t < 5 
        velocity(1,c) = 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 5<=t && t<=15.4
        velocity(1,c) = 0.003980879*t^5 - 0.2247*t^4 + 4.8682*t^3 - 50.442*t^2 + 254.67*t - 430.66;
    else 
        velocity(1,c) = -0.073*t^2 + 6.1802*t + 40.423;
    end
end
plot(time,velocity)

[Deepak Singh]

Thank you very much for your help correcting the code and clearing up the mistakes I had made in your response.

Answer 2

[Walter Roberson]

https://www.mathworks.com/matlabcentral/answers/480004-need-help-with-taking-an-array-inside-a-for-loop-and-plotting-it#answer_391484