MATLAB Practice questions solution

Hello all,

I am new to MATLAB and I am still trying to learn it by myself. I was trying to solve the questions posted by MATLAB. I know I didn't use the double function as suggested so please enlighten me with your ideas and let me know if i was at least close to what the problem is asking for:

Problem(link: https://www.mathworks.com/services/training/certification/ml-professional-exam/practice-test.html)

The provided text file (readings.txt) contains a timestamp broken up into year, month, day, hour, minute, second, and timezone components, as well as a reading from a sensor. Write a script that reads the data from the file using the textscan function.

The script must:

• Convert the timestamps into single numeric serial date numbers stored in a variable named dates
• Ignore the timezone component of the timestamp by not reading it into the workspace
• Place numeric values for the readings in a single array of type double with a variable name of readings

clc
clear all
fileID1 = fopen('readings.txt');
dates = textscan(fileID1, '%s %s %s %s %s %s %s %s');
fclose(fileID1);
whos dates;
celldisp(dates)

fileID2=fopen('readings.txt')
C=textscan(fileID2, '%q %q %q %q %q %q %*q %q');
fclose(fileID2);
whos C;
celldisp(C)

fileID3=fopen('readings.txt');
D=textscan(fileID3,'%*q %*q %*q %*q %*q %*q %*q %q');
fclose(fileID3);
whos D;
celldisp(D)

[DEEPAK RAJ B]

add one section to the end of the script

Answer 1

[Image Analyst]

Perhaps datenum()?

>> d=datenum(now)

d =

738159.473416505

[Ali Awada]

How does that answer the problem. please elaborate

[Image Analyst]

It turns complicated date formats into a floating point number, which is easier to sort. So it does this:

• Convert the timestamps into single numeric serial date numbers stored in a variable named dates

If you want more specific help, then attach the 'readings.txt' file which you forgot to attach. Because otherwise we really don't know what format your "timestamps" are in. There is a dizzying variety of time and date formats and functions. It can get really confusing.

[Ali Awada]

The readings file is in the link i posted. Here it is for ease of access

[Image Analyst]

You should have tried datenum like I suggested. I did and it seems to work fine:

t = readtable('readings.txt')
d = datenum(t.Year, t.Month, t.Day, t.Hour, t.Minute, t.Second)
readings = t.Reading
plot(d, readings, 'b.-', 'LineWidth', 2, 'MarkerSize', 30);
grid on;
title('Alis Homework', 'FontSize', 20);
xlabel('Date/Time Stamp', 'FontSize', 20);
ylabel('Reading', 'FontSize', 20);

[Ali Awada]

Hey. Thanks for a fast reply. But the MATTLAB practice test does not ask to graph the stored value (please see above problem description) . I think you misubderstood what the problem is asking for but thanks for helping anyway.

[Image Analyst]

Then just use the first 3 lines and skip the rest of the lines that have to do with plotting.

[Ali Awada]

the problem asks us to use the function textscan

Answer 2

[Kazem Gheysari]

clear
fileID = fopen('readings.txt');
C_text = textscan(fileID,'%s',8,'Delimiter',' ');
C = textscan(fileID,'%f %f %f %f %f %f EST %f','Delimiter',' ','EmptyValue',-Inf);
fclose(fileID);

Y = C{1,1};
M = C{1,2};
D = C{1,3};
H = C{1,4};
MN= C{1,5};
S = C{1,6};

dates = datenum(Y,M,D,H,MN,S)

readings = C{1,7};

matlab1.com

[Image Analyst]

What is matlab1.com? (I did not click your link because there is no explanation)

Answer 3

[Abhiram Rayidi]

syms y(t); 
dsolve(diff(y)==y, y(0)==1, 'Expansion Point', 0)

Error using symengine
Unexpected 'identifier'.

Error in mupadengine/evalin_internal

Error in dsolve>mupadDsolve (line 333)
sys = [sys_sym reshape(evalin_internal(symengine, sys_str), 1, [])];

Error in dsolve (line 203)
sol = mupadDsolve(args, options);

[Walter Roberson]

syms y(t); 
dsolve(diff(y)==y, y(0)==1, 'ExpansionPoint', 0)

ans =