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Matlab Onramp course notes - Part 4 Programming & Projects

Studying the self paced course in matlab academy for academic work - Part 4

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By Sadman Ishtiak
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Matlab Onramp course notes - Part 4 Programming & Projects

Logical Arrays

Rational Operators

  • Relational operators: >, <, ==, ~=. Comparisons yield 1 (true) or 0 (false).

Task Test if π is less than 4 by using the relational operator <. Assign the output to a variable named x.

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x = (pi < 4)
  • Comparing an array to a scalar returns a logical array of the same size.
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y = [5 10 15] > 12
y = 
    0    0    1

Task Test the vector v1 for elements that are less than 4. Assign the output to a variable named y.

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y = (v1 < 4)
  • Use = to assign and == to test equality.
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y = [5 10 15] == 10
y = 
    0    1    0

Task Test the vector sample for elements that are equal to 18. Assign the output to a variable named z.

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z = (sample == 18)
  • Combine comparisons with logical AND & and OR |:
    • Both conditions: v1 < 6 & v1 > 5
    • Either condition: v1 < 6 | v1 > 2

Try finding the values in sample that are between 10 and 20.

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v1;
v1 < 6 & v1 > 5
sample .* (sample >= 10 & sample <= 20)

Logical Indexing

  • Extract elements using a logical array as an index.
  • Logical index must match the indexed vector’s size.
  • MATLAB returns elements where the index is 1 (true).

Task — use isGreater to extract v1 > 6:

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load datafile
sample = data(:,1);
v1 = data(:,3)
isGreater = v1 > 6
x = v1(isGreater)

One-line extraction example:

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v = v1(v1 > 6)
v =
    6.6678
    9.0698

Task — one-line: elements of v1 < 4:

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y = v1(v1 < 4)

Use logical indexing across arrays:

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v = sample(v1 > 6)
v =
    18
    23

Task — sample elements where v1 < 4:

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z = sample(v1 < 4)

Reassign via logical indexing (example):

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x(x==999) = 1

Task — set v1 values < 4 to 0:

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v1(v1 < 4) = 0

Logical indexing with matrices — change all data < 4 to 0:

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data(data < 4) = 0

Programming

Conditional Statements

  • Scripts run top-to-bottom; use if to run code only when a condition is true.
  • Use else to run alternate code when the condition is false.
  • Use == for equality checks.
  • End an if/else block with end.
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% example: check nonnegative
if x >= 0
    % runs when x is nonnegative
end

Decision Branching

  • Goal: run plotting code only when doPlot == 1.
  • Add if on line 4 and end on line 9.
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doPlot = randi([0 1])
load datafile
density = data(:,2);
if doPlot == 1
    plot(density)
    title("Sample Densities")
    xticklabels(element)
    ylabel("Density (g/cm^3)")
end
  • To run alternate code when the condition is false, add else before end and put the disp command there.
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doPlot = randi([0 1])
load datafile
density = data(:,2);
if doPlot == 1
    plot(density)
    title("Sample Densities")
    xticklabels(element)
    ylabel("Density (g/cm^3)")
else
    disp("The density of " + element + " is " + density)
end

Repeat code with loop

  • Use for loops to repeat a block over index values.
  • MATLAB arrays often eliminate the need for loops, but use loops when required.
  • Loop syntax: for = ... end. The loop variable takes each value in turn.
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% example: loop over 1:5
for x = 1:5
    % body runs 5 times
end

Write a for loop

  • Task: iterate over density (7 elements).
  • Create idx = 1:7 then loop to display/plot each point.
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load datafile
density = data(:,2)
hold on
idx = (1:7)
plot(idx,density(idx),"*")
pause(0.3)
hold off
  • To animate points one at a time, wrap plotting in a for loop and end after pause.
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load datafile
density = data(:,2)
hold on
for c = (1:7)
    idx = c
    plot(idx,density(idx),"*")
    pause(3)
end
hold off
  • Use length(density) to loop over a vector of unknown length:
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for idx = 1:length(density)
    % process density(idx)
end
  • Note: pause(0.3) updates the plot between iterations.
  • Use hold on/off to overlay and clear plots as needed.

Final Project

Steller Moon

  • Given: spectra sampled at evenly spaced wavelengths: lambdaStart, lambdaDelta, nObs.
  • Compute last wavelength lambdaEnd = lambdaStart + (nObs - 1) * lambdaDelta and vector lambda from start to end (nObs points).
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load starData
nObs = size(spectra,1)
lambdaStart = 630.02
lambdaDelta = 0.14
lambdaEnd = lambdaStart + (nObs - 1) * lambdaDelta
lambda = linspace(lambdaStart, lambdaEnd, nObs)
  • Each column is a star spectrum. Column 6 is HD 94028.
  • Extract column 6 to s.
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load starData
nObs = size(spectra,1)
lambdaStart = 630.02
lambdaDelta = 0.14

lambdaEnd = lambdaStart + (nObs - 1) * lambdaDelta
lambda = linspace(lambdaStart, lambdaEnd, nObs)

s = spectra(:,6)
  • Plot s vs lambda with point markers and solid line; label axes.
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load starData
nObs = size(spectra,1)
lambdaStart = 630.02
lambdaDelta = 0.14

lambdaEnd = lambdaStart + (nObs - 1) * lambdaDelta
lambda = linspace(lambdaStart, lambdaEnd, nObs)

s = spectra(:,6)

plot(lambda, s, ".-")
xlabel("Wavelength")
ylabel("Intensity")
  • Use min to find hydrogen-alpha line: sHa (min value) and idx (index). Get lambdaHa = lambda(idx).
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load starData
nObs = size(spectra,1)
lambdaStart = 630.02
lambdaDelta = 0.14

lambdaEnd = lambdaStart + (nObs - 1) * lambdaDelta
lambda = linspace(lambdaStart, lambdaEnd, nObs)

s = spectra(:,6)

plot(lambda, s, ".-")
xlabel("Wavelength")
ylabel("Intensity")

[sHa, idx] = min(s)
lambdaHa = lambda(idx)
  • Mark the hydrogen-alpha point on the plot as a red square, MarkerSize 8.
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load starData
nObs = size(spectra,1)
lambdaStart = 630.02
lambdaDelta = 0.14

lambdaEnd = lambdaStart + (nObs - 1) * lambdaDelta
lambda = linspace(lambdaStart, lambdaEnd, nObs)

s = spectra(:,6)

plot(lambda, s, ".-")
xlabel("Wavelength")
ylabel("Intensity")

[sHa, idx] = min(s)
lambdaHa = lambda(idx)

hold on
plot(lambdaHa, sHa, "rs", MarkerSize=8)
  • Observed lambdaHa ≈ 656.62 nm (vs lab 656.28 nm).
  • Compute redshift z = (lambdaHa/656.28) - 1 and speed = z * 299792.458 km/s.
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load starData
nObs = size(spectra,1)
lambdaStart = 630.02
lambdaDelta = 0.14

lambdaEnd = lambdaStart + (nObs - 1) * lambdaDelta
lambda = linspace(lambdaStart, lambdaEnd, nObs)

s = spectra(:,6)

plot(lambda, s, ".-")
xlabel("Wavelength")
ylabel("Intensity")

[sHa, idx] = min(s)
lambdaHa = lambda(idx)

hold on
plot(lambdaHa, sHa, "rs", MarkerSize=8)

z = (lambdaHa / 656.28 - 1)
speed = z * 299792.458
  • To run the calculation for star 2 instead of star 6: change the column index to 2.
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load starData
nObs = size(spectra,1)
lambdaStart = 630.02
lambdaDelta = 0.14

lambdaEnd = lambdaStart + (nObs - 1) * lambdaDelta
lambda = linspace(lambdaStart, lambdaEnd, nObs)

s = spectra(:,2)

plot(lambda, s, ".-")
xlabel("Wavelength")
ylabel("Intensity")

[sHa, idx] = min(s)
lambdaHa = lambda(idx)

hold on
plot(lambdaHa, sHa, "rs", MarkerSize=8)

z = (lambdaHa / 656.28 - 1)
speed = z * 299792.458

Compare Stellar Spectra

  • Background
    • Line 2 extracts star 2; lines 3–5 compute its speed.
    • To compute speed for all stars, delete (:,2) on line 2.
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load starData

[sHa,idx] = min(spectra(:,:));
lambdaHa = lambda(idx);
z = lambdaHa/656.28 - 1;
speed = z*299792.458

  • Note: speed is now a vector. Positive → away (redshift). Negative → toward (blueshift).

  • Compute and plot each spectrum (loop over columns)

    • Loop skeleton: extract vth column to s.
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load starData

[sHa,idx] = min(spectra(:,:));
lambdaHa = lambda(idx);
z = lambdaHa/656.28 - 1;
speed = z*299792.458


for v = 1:7
        s = spectra(:, v)
end
  • Plot blueshifted spectra with dashed lines (hold on to overlay) ```matlab load starData

[sHa,idx] = min(spectra(:,:)); lambdaHa = lambda(idx); z = lambdaHa/656.28 - 1; speed = z*299792.458

for v = 1:7 s = spectra(:, v) if speed(v) <= 0 plot(lambda, s, “–”) hold on end end

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- Plot redshifted spectra with thick lines; then hold off
```matlab
load starData

[sHa,idx] = min(spectra(:,:));
lambdaHa = lambda(idx);
z = lambdaHa/656.28 - 1;
speed = z*299792.458


for v = 1:7
        s = spectra(:, v)
        if speed(v) <= 0
                plot(lambda, s, "--")
                hold on
        else 
                plot(lambda, s, LineWidth=3)
        end
end
hold off
  • Add legend using starnames ```matlab load starData

[sHa,idx] = min(spectra(:,:)); lambdaHa = lambda(idx); z = lambdaHa/656.28 - 1; speed = z*299792.458

for v = 1:7 s = spectra(:, v) if speed(v) <= 0 plot(lambda, s, “–”) hold on else plot(lambda, s, LineWidth=3) end end hold off

legend(starnames)

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- Find redshifted star names without a loop (logical indexing)
```matlab
load starData

[sHa,idx] = min(spectra(:,:));
lambdaHa = lambda(idx);
z = lambdaHa/656.28 - 1;
speed = z*299792.458


for v = 1:7
        s = spectra(:, v)
        if speed(v) <= 0
                plot(lambda, s, "--")
                hold on
        else 
                plot(lambda, s, LineWidth=3)
        end
end
hold off

legend(starnames)

moveaway = starnames(speed > 0)
  • Plot all spectra at once (no loop)
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    plot(lambda,spectra)
legend(starnames)

Conclusion

Summary

Basic Syntax

  • Example — Description
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    x = pi      % Create variables; left = name, right = value.
y = sin(-5) % Call functions with parentheses.

Desktop Management

  • Function — Example — Description
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    save        % save data.mat       % Save workspace to a MAT-file.
load        % load data.mat       % Load variables from a MAT-file.
clear       % clear               % Clear all variables.
clc         % clc                 % Clear Command Window text.
format      % format long         % Change numeric display precision.

Array Types

  • Example — Description
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    4                 % scalar
[3 5]             % row vector
[1;3]             % column vector
[3 4 5; 6 7 8]    % matrix

Evenly Spaced Vectors

  • Example — Description
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    1:4               % vector 1 to 4 step 1
1:0.5:4           % vector 1 to 4 step 0.5
linspace(1,10,5)  % 5 evenly spaced elements from 1 to 10

Matrix Creation

  • Example — Description
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    rand(2)        % 2×2 random matrix
zeros(2,3)     % 2×3 zeros
ones(2,3)      % 2×3 ones

Array Indexing

  • Example — Description
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    A(end,2)   % element in 2nd column of last row
A(2,:)     % entire 2nd row
A(1:3,:)   % all cols of first 3 rows
A(2) = 11  % set 2nd element to 11

Array Operations

  • Example — Description ```matlab [1 2;3 4] + 1 % ans = % 2 3 % 4 5 % array addition

[1 1;1 1]*[2 2;2 2] % ans = % 4 4 % 4 4 % matrix multiplication

[1 1;1 1].*[2 2;2 2] % ans = % 2 2 % 2 2 % element-wise multiplication

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Multiple Outputs
- Example — Description
```matlab
[xrow,xcol] = size(x)   % rows and cols to two variables
[xMax,idx] = max(x)     % max value and its index

Documentation

  • Example — Description
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    doc randi    % Open randi documentation

Plots

  • Example — Description
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    plot(x,y,"ro--",LineWidth=5) % red dashed line, circle markers, thick line
hold on                       % overlay next plot
hold off                      % reset axes for next plot
title("My Title")             % add title
xlabel("x")                   % x label
ylabel("y")                   % y label
legend("a","b","c")           % add legend

Tables

  • Example — Description
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    data.HeightYards                      % extract table variable
data.HeightMeters = data.HeightYards*0.9144 % derive new table variable

Logical Indexing

  • Example — Description
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    [5 10 15] > 12    % compare vector elements to 12
v1(v1 > 6)        % extract v1 elements > 6
x(x==999) = 1     % replace 999 with 1 in x

Programming

  • Example — Description ```matlab if x > 0.5 y = 3 else y = 4 end % If x > 0.5 set y=3; else y=4.

for c = 1:3 disp(c) end % Loop c over 1,2,3 and display each. ```

next step

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