Question

Please correct the MATLAB code under the problem, which is attempting to solve the following problem:

Sally is trying to figure out how far she walked to Taco Bell to pick up food for her favorite professor, who is coughing so hard that they may have a cold. To get to her local Taco Bell, she has to walk the following distances at reasonable paces for her, especially in these hills of East LA.
1) Sally walked at 3.0 miles per hour for 20 minutes.
2) Sally then slowed down and climbed the hill, which slowed her down to 2.3 miles per hour for 5 minutes.
3) Encountering a small downslope, Sally sped up to 3.5 miles per hour for 5 minutes.
4) Some flat terrain comes, so Sally walked at 2.8 miles for 10 more minutes.
5) Around the corner from Taco Bell, Sally stopped to tie her shoes for 2 minutes.
6) Finally, Sally skipped to Taco Bell at 4.0 miles per hour for 1 minute.
How far did Sally walk to Taco Bell? Skipping is included in the term "walking".

Broken MATLAB code:

hours_per_minute = 1/60;  % 1 hr / 60 mins for conversion
walk_normal = 3.0*hours_per_minute;  % miles per minute
walk_climb = 2.3*hours_per_minute;  % miles per minute
walk_downhill = 3.5*hours_per_minute;  % miles per minute
walk_tired = 2.8*hours_per_minute;  % miles per minute
walk_skipping = 4.0*hours_per_minute;  % miles per minute
walk_stopped = 0;  % miles per minute

walking_speeds = [walk_normal, walk_climb, walk_downhill, walk_tired, 0, walk_skipping];  % vector of miles per min
walking_times = [20, 5, 5, 10, 2, 1];  % vector of minutes
total_time = zeros(1, length(walking_times)+1);  % minutes, getting the total time Sally's walked so far
distance = zeros(1, length(walking_times)+1);  % miles, pre-initializing distance traveled so far

for i=1:length(walking_times)
    distance(i) = distance(i-1) + walking_speeds(i-1)*walking_times(i-1);
    total_time(i) = total_time(i-1) + walking_times(i-1);
end

plot(total_time, distance);

Answer 1

The problem was with 'i' values in for loop it was going from 1 to length(walking_times) but it should be from 2 to length(walking_times)+1. Since Matlab takes only positive integers as array indices, i-1=1-1 makes array index value zero which raises an error. Length of total_time and distance is length(walking_times)+1 that is why the last value of 'i' should be length(walking_times)+1.

Otherwise, the code is correct.

Corrected Code:

hours_per_minute = 1/60; % 1 hr / 60 mins for conversion
walk_normal = 3.0*hours_per_minute; % miles per minute
walk_climb = 2.3*hours_per_minute; % miles per minute
walk_downhill = 3.5*hours_per_minute; % miles per minute
walk_tired = 2.8*hours_per_minute; % miles per minute
walk_skipping = 4.0*hours_per_minute; % miles per minute
walk_stopped = 0; % miles per minute

walking_speeds = [walk_normal, walk_climb, walk_downhill, walk_tired, 0, walk_skipping]; % vector of miles per min
walking_times = [20, 5, 5, 10, 2, 1]; % vector of minutes
total_time = zeros(1, length(walking_times)+1); % minutes, getting the total time Sally's walked so far
distance = zeros(1, length(walking_times)+1); % miles, pre-initializing distance traveled so far

for i=2:length(walking_times)+1
    distance(i) = distance(i-1) + walking_speeds(i-1)*walking_times(i-1);
    total_time(i) = total_time(i-1) + walking_times(i-1);
end

plot(total_time, distance);
xlabel('Time(minutes)')
ylabel('Distance(miles)')