onrammp exercise 14.2 stellar motion part 2, task 6 movaway

For task 6 I don't understand how matlab connects starname to a particular graph.

Also I'm not sure how this piece of code works...

"movaway = starnames(speed < 0) "

how does this line of code use a variable that is in a loop before this line of code?

Sorry in advance if this is an obvious question.

Task 1

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

Tasks 2 - 4

for c = 1:7
    s = spectra(:,c);
    if speed(c) >= 0
        loglog(lambda,s,"--")
    else
       loglog(lambda,s,"LineWidth",3) 
    end
    hold on
end
hold off

Task 5

legend(starnames)

Task 6

movaway = starnames(speed < 0) 

[Nikita]

Add an if statement to the for loop body. If speed(v) is less than or equal to 0, create a plot of s against lambda using a dashed line (--).

Add the command hold on between the two end keywords so that you only create one plot.

Answer 1

[KALYAN ACHARJYA]

See the example, lets suppose starnames is variable having all types of numbers

starnames=[-2 3 45 7 2 0 -4 5 7 7 6 9];
speed=[2 3 -4 5 6 7 8  0 5 2 -30] ; % Just random example 

## speed<0 is an conditional assignment, its return a logical array (with 0 & 1), when condition true, its 1, otherwise 0

>> speed < 0
ans =
  1×11 logical array

   0   0   1   0   0   0   0   0   0   0   1

## starnames(speed < 0) return an array, where the 1 presence in the above logical array

>> movaway = starnames(speed < 0)
movaway =
    45     6

This is just an example to understand the issue (This approach is good way for efficient code, conditional & array indexing)

[Darnell Gawdin]

Thank you for your time and expertise to answer this question. I see how it works now.

Answer 2

[Bongani Tavengwa]

Task

Add an if statement to the for loop body. If speed(c) is less than or equal to 0, create a loglog plot of s against lambda using a dashed line (--).

After the if statement, add the command hold on so that only one plot is created.

for c = 1:7
    s = spectra(:,c);
    if speed(c) >= 0
       loglog(lambda,s,"--")
    else
       loglog(lambda,s,"LineWidth",3) 
    end
    hold on
end

please help me where do i go wrong

[Steven Lord]

The instructions state you should create the dashed line loglog plot if speed( c) is less than or equal to 0.

Your code does not contain the less than or equal to operator <=.

Your code does contains the greater than or equal to operator >=.

[Bongani Tavengwa]

so who do i write it then ?? because i m not coming write

[Bongani Tavengwa]

please do advise

[Steven Lord]

Use the less than or equal to operator instead of the greater than or equal to operator.

[Bongani Tavengwa]

Thanks Steven manage to crack it right.

[Sergio Andres Cuellar Oliveros]

I get an error, having all the code fine, then he asks me: Is there a graph with seven lines?, please help me

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

[MOHAMMED EL KASMI ALAOUI]

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

[Meifeng]

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

[Narmadha]

Add an else statement. If speed(v) is greater than 0, create a plot of s against lambda using a line width of 3.

After the for loop, enter hold off.