How do I create a matrix from a file from reading the first row and then use that first number to control a "for" loop to build

Below is my assignment that I need help on.

" Read the data file, dragCoef06.txt, into a matrix. The first column will contain the year and the second column contains the drag coefficient for vehicles. Write a MATLAB script file that uses the velocity, horizontal force, and air density given above and the drag coefficient read from the data file to compute the frontal area for all the vehicles. Display and print title and column headers over the numbers of a three column matrix with the year, drag coefficient and frontal area of each vehicle. Note: you will need to add the frontal area as the third column of the original input data matrix. Sort the output table by the year column for output. Use fscanf() to read the data file. Read the first number (the number of cars in this file) then use it to control the for loop to build a matrix ONE row at time in the for loop saving one number at a time into the first and second columns of each car row. Use fprintf() title and column headers over the numbers of a three column table of the information in the matrix. Print the table ONE row at a time in a for loop. At the end of each line, mark each frontal area greater than the average frontal area with an asterisk *. Print a legion after the table. "

I have written the code:

clc, clear all
format short

% ***** CONSTANT *****
FILENAME = 'dragCoef06.txt';
HORIZONTAL_FORCE = 350;           % Newtons
AIR_DENSITY = 1.225;              % kg/m^3
VELOCITY = 25;                    % m/s
NUM_CARS = 10;
NUM_COLS = 2;

% does file exist?
if ~exist( FILENAME, 'file' )
    disp ( 'File not avaliable' )
else
    % file exist contiune
    
    % ***** INPUT *****
    % read year and drag coefficient from file
    fileID = fopen ( FILENAME );
    
    % ***** COMPUTE *****
    % compute the frontal area all cars
    for r = 1:NUM_CARS
        for c = 1:NUM_COLS
            dragMatrix06(r,c) = fscanf( fileID, '%f', 1 );
        end
    end
    
    frontalArea = ( 2.*HORIZONTAL_FORCE )./ ( AIR_DENSITY.*VELOCITY^2.*dragMatrix06 );
    
    % add frontal area as third column of matrix
    newMatrix = [ dragMatrix06, frontalArea ];
    
    
    % ***** OUTPUT *****
    % print table using fprintf()
    fprintf ( '%21s \n', 'All Cars' )
    fprintf ( '%9s', 'Year' )
    fprintf ( '%10s', 'Drag' )
    fprintf ( '%11s \n', 'Frontal' )
    fprintf ( '%19s', 'Coef' )
    fprintf ( '%13s \n', 'Area(m^2)' )
    
    fprintf ( '%9.0f %9.2f %9.4f \n', [newMatrix]' )
    fclose ( fileID );
end

this outputs the following:

             All Cars 
     Year      Drag    Frontal 
               Coef    Area(m^2) 
       10   2015.00    0.0914 
        0      0.24 2016.0000 
        4      0.00    0.2700 
     2016      3.39    0.0005 
        0   2016.00    2.5397 
        0      0.39 2017.0000 
        2      0.00    0.2200 
     2017      4.16    0.0005 
        0   2018.00    3.9752 
        0      0.26 2018.0000 
        4      0.00    0.3300 
     2018      2.77    0.0005 
        0   2019.00    2.0317 
        0 >> 

any help?

I was given a file...

10
2015    0.24
2016    0.27
2016    0.36
2016    0.39
2017    0.22
2017    0.23
2018    0.26
2018    0.33
2018    0.45
2019    0.28

I need it to output a table that looks like this. Thank you!

[BobH]

Yes, please provide input file

[engrCE1]

10
2015    0.24
2016    0.27
2016    0.36
2016    0.39
2017    0.22
2017    0.23
2018    0.26
2018    0.33
2018    0.45
2019    0.28

[engrCE1]

How do I add an astrerisk to the end of the rows where the frontal area is greater than the drag coef?

Answer 1

[MOHD AQUIB]

Hi,

May be you have to alter your code a little bit.

This can be one of its version:

clc, clear all

format short

% ***** CONSTANT *****

FILENAME = 'dragCoef06.txt';

HORIZONTAL_FORCE = 350; % Newtons

AIR_DENSITY = 1.225; % kg/m^3

VELOCITY = 25; % m/s

NUM_CARS = 10;

NUM_COLS = 2;

% does file exist?

if ~exist( FILENAME, 'file' )

disp ( 'File not avaliable' )

else

% file exist contiune

% ***** INPUT *****

% read year and drag coefficient from file

fileID = fopen ( FILENAME );

% ***** COMPUTE *****

% compute the frontal area all cars

dragtemp=fscanf(fileID,'%f');

k=1;

for r = 1:NUM_CARS

for c = 1:NUM_COLS

k=k+1;

dragMatrix06(r,c) = dragtemp(k);

end

end

frontalArea = ( 2.*HORIZONTAL_FORCE )./ ( AIR_DENSITY.*VELOCITY^2.*dragMatrix06 );

% add frontal area as third column of matrix

newMatrix = [ dragMatrix06, frontalArea ];

newMatrix(:,3)=[];

% ***** OUTPUT *****

% print table using fprintf()

fprintf ( '%21s \n', 'All Cars' )

fprintf ( '%9s', 'Year' )

fprintf ( '%10s', 'Drag' )

fprintf ( '%11s \n', 'Frontal' )

fprintf ( '%19s', 'Coef' )

fprintf ( '%13s \n', 'Area(m^2)' )

fprintf ( '%9.0f %9.2f %9.4f \n', [newMatrix]' )

fclose ( fileID );

end

[engrCE1]

I appreciate this, however I'm afraid this didn't achieve my goal...

[MOHD AQUIB]

BobH has commented about the part that I have missed, I hope including that part in the code that I have shared will serve your purpose.

Answer 2

[BobH]

MOHD AQUIB is on the right track, reading one data point first before the loop, but left out the part within the loop that continues to pull pairs of data points from the file, as you had in your original question.

So after the open, but just before your loop, add

NUM_CARS = fscanf( fileID, '%f', 1 );

The second problem you have was also caught by MOHD AQUIB, your newmatrix has four columns but your final fprintf only has three data items per line. This gives the staggered look to your output data. Try

newMatrix = [ dragMatrix06, frontalArea ];
newMatrix(:,3)=[];  % MOHD eliminates the unwanted column before print

Or instead, never bring the unwanted column in

newMatrix = [ dragMatrix06, frontalArea(:,2) ];

[BobH]

To print asterisks, you will have an additional field in your final fprintf, %c and give it a number that represents a blank (32) or an asterisk (42). You can confirm the proper numbers in any ASCII table.

First, consider changing your frontalArea computation to use only the interesting column of input data; as it stands, your first column of frontalArea is the computation done on the year value, which is nonsense. The interesting input column is dragMatrix06(:,2)

Compute the mean

Define an array (for ex, named 'ast') to hold blanks or asterisks, and assign the initial value of 32 (blank) to all

https://www.mathworks.com/matlabcentral/answers/352280-create-array-with-same-number

Change those elements of the array whose area is greater than the mean. "frontalArea > faMean" returns a logical array (trues or falses) which can be used to pick locations in the array.

% for those locations where area > mean, replace blank with '*'
ast( frontalArea > faMean ) = 42;  % 42 is ASCII asterisk

Append 'ast' to your newmatrix

Add a %c to the final fprintf statement

[Thomas Smith]

I'm trying this for myself, but I don't really understand how to do the part of appending an asterisk, can you share the full code for how to do this?

[BobH]

    % Only compute using second column of dragMatrix06; frontalArea will be a single column
    frontalArea = ( 2.*HORIZONTAL_FORCE )./ ( AIR_DENSITY.*VELOCITY^2.*dragMatrix06(:,2) );
        
    % compute mean
    faMean = mean(frontalArea);
        
    % define an array of the correct size, and init with blanks
    %  NOTE using numbers not strings, to keep fprintf simple
    ast = zeros( length(frontalArea), 1 ) + 32;  % 32 is ASCII blank
    
    % for those locations where area > mean, replace blank with '*'
    ast( frontalArea > faMean ) = 42;  % 42 is ASCII asterisk
    		
    % add frontal area as third column of matrix
    newMatrix = [ dragMatrix06, frontalArea, ast ];

the extra character to the screen via " %c"

    fprintf ( '%9.0f %9.2f %9.4f %c\n', [newMatrix]' )

[engrCE1]

Is there a way to do this with a for loop, and not using the 'ast = ' and what not?

Thank you for your help though.

[BobH]

Of course. After you compute the mean, then create a new loop, using r and NUM_CARS again, and examine each frontalArea element, fprintf most of your row but don't use the \n yet. If greater than mean then print an asterisk. Then have a separate fprintf statement just for the newline \n