How can I add linspace in a For Loop

Hello good day, I wish to have a 'for' or 'while' loop which keeps increasing the value of 'x' as the value of 'phi' keeps increasing from 0 - 77.3049 with 100 numbers. I attempted a few times but kept getting errors. Any idea how this can be implemented?

[Dyuman Joshi]

Which x?

Show us what you have attempted.

[Light]

Sorry, X as the value which changes as the value of phi increases. I just cant figure out how to complete the for or while loop at the bottom.

Answer 1

[Star Strider]

There is not enough information (such as a description of what you want to do).

However one possibility is something like this —

phi = linspace(0, 77.3049, 100);

for k = 1:numel(phi)

x(k) = some_calculation_using(phi(k));

end

.

[Light]

Okay thanks very much. I would like to declare phi as 0 to 77.3049 with 100 values.

I would also like to have a variable called X() for each of the 100 values from 0 to 77.3049.

[Dyuman Joshi]

@Edson, this is what you are looking for.

I would suggest you to pre-allocate x

phi = linspace(0, 77.3049, 100);
%pre-allocation
x=zeros(size(phi))
for k = 1:numel(phi)
    x(k) = some_calculation_using(phi(k)); 
end

[Light]

Okay, thanks very much. Could you explain lines 3 with the zeros function and lines 5 with the (phi(k)); ? I dont fully understand it

[Star Strider]

The zeros call creates a consecutive memory range that will store the resutls of the calculation. This speeds up a loop that creates new results about 20% over not prealllocating, due to the increased memory efficiency.

The ‘x(k)’ assignment is just an expression that uses the value of ‘phi(k)’ to create some result using it. It is just there for illustration purposes.

[Light]

Okay, thank you, I understand now. I also have another question. I edited the code to have 50 values instead of 100 and I got 50 values for x, y and z. Is there any way to use the formula for l and b to calculate the values (for l and b) and plot the various values received?

[Dyuman Joshi]

1 - A good practice is to club all the for loop together, as their loop indices are same i.e. 1:numel(phi).

for k=1:numel(phi)
    X(k)=%code;
    Y(k)=%code;
    Z(k)=%code;
end

2 - Use semi-colon, ; at the end of a line to suppress the outputs (such as after X, Y and Z), as I've done above.

3 - Use plot to make a plot of data, for plot in polar coordinates use polarplot

[Star Strider]

I am not going to type those in, since they should be provided as text, not an image.

That aside, the loops are likely not necessary. Just do something like this —

phi = linspace(0, 77.3049, 100);

x1 = ...;

x3 = ...;

y1 = ...;

y3 = ...;

z1 = ...;

z3 = ...;

X = x1*cos(phi) + x3*sin(phi);

Y = ...;

Z = ...;

b = asin(Z);

l = atan2(Y,X);

That should work, eliminating the loops and the need for preallocation. All of those results should be vectors of the appropriate dimensions. The ‘phi’ vector can be any length that produces the desired results.

.

[Light]

@Dyuman Joshi okay I will edit the answers, thanks very much.

@Star Strider Wow okay, I got the same values as if I used the loops as well. Thank you. I used the loops because there were many values for phi.

[Star Strider]

My pleasure!

My approach there is called Vectorization and it has its own documentation section.

It is one of MATLAB’s strengths as a language.

[Light]

Awesome, im still getting adjusted to Matlab but I learnt alot from the replies posted.

[Star Strider]

Thank you!

[Light]

I have another question if its okay? We were given this code to edit to plot the path between two cities using the longitude and latitude values. I edited the code but I still keep getting errors. I dont necessarily want the answer but maybe a guide to properly edit the code?

[Star Strider]

Copy that line exactly as written in the instructions. Changing it threw the error.

[Light]

I copied it exactly the way it was in the instructions and there still was an error

[Star Strider]

No, you didn’t.

The 'XTick' and 'YTick' name-value pairs are on different lines in your code, and not within the parentheses.

Look again!

[Light]

Sorry I should have stated, initially I copied the code exactly the way it was in the instructions, then I saw an error and edited the code the way it is now with the 'Xtick' and 'Ytick' in separate lines. But I also got an error after attempting to edit it.

[Star Strider]

That original set call looks correct to me. It should work as written.

What error did it throw?

[Light]

It stated: "Invalid expression. When calling a function or indexing a variable, use parentheses. Otherwise, check for mismatched delimiters."

I also tried correcting it but there was still an error.

[Star Strider]

I only have an image of the file, not the actual text, so I can do nothing with it.

Copy the text and paste it to a Comment here.

[Dyuman Joshi]

You need to either write the whole command in the same line or use three dots (..., Ellipsis) called at the end of each line break.

[Light]

Okay no problem:

load('topo.mat', 'topo', 'topomap1'); % load earth topography whos topo topomap1; % set coordinates of points Longitude = 0:15:360; Latitude = -72:6:72; % set background parameters contour(0:359, -89:90, topo, [0,0], 'w'); axis equal box on set(gca, 'Color', [0 0 1], 'XLim', [0 360], 'YLim', [-90 90], 'XTick', [0:180:360], 'YTick', [-90:90:90]); %set(gca, 'Color', [0 0 1], 'XLim', [0 360], 'YLim', [-90 90]); grid; hold on % wait for more plot commands plot(Longitude, Latitude, 'y.', 'linewidth', 1); % enter plot hold off % no more plot commands to come after this line

[Star Strider]

Please use one of the options that saves the code in a readable format. I have no strong desire to format this myuself. I will let you put in the line breaks and any other necesary formatting —

load('topo.mat', 'topo', 'topomap1'); % load earth topography whos topo topomap1; % set coordinates of points Longitude = 0:15:360; Latitude = -72:6:72; % set background parameters contour(0:359, -89:90, topo, [0,0], 'w'); axis equal box on set(gca, 'Color', [0 0 1], 'XLim', [0 360], 'YLim', [-90 90], 'XTick', [0:180:360], 'YTick', [-90:90:90]); %set(gca, 'Color', [0 0 1], 'XLim', [0 360], 'YLim', [-90 90]); grid; hold on % wait for more plot commands plot(Longitude, Latitude, 'y.', 'linewidth', 1); % enter plot hold off % no more plot commands to come after this line

.

[Light]

Okay, do you mean save it as an m file or just structure it in the comment so it is easier to read?

[Star Strider]

THe last: ‘structure it in the comment so it is easier to read’.

[Light]

Okay

load('topo.mat', 'topo', 'topomap1');
% load earth topography whos topo topomap1; % set coordinates of points
Longitude = 0:15:360; Latitude = -72:6:72;
% set background parameters
contour(0:359, -89:90, topo, [0,0], 'w');
axis equal
box on
set(gca, 'Color', [0 0 1], 'XLim', [0 360], 'YLim', [-90 90], 'XTick', [0:180:360], 'YTick', [-90:90:90]);
%set(gca, 'Color', [0 0 1], 'XLim', [0 360], 'YLim', [-90 90]);
grid;
hold on % wait for more plot commands
plot(Longitude, Latitude, 'y.', 'linewidth', 1); % enter plot
hold off % no more plot commands to come after this line

[Star Strider]

As posted, it runs without error —

load('topo.mat', 'topo', 'topomap1');
% load earth topography whos topo topomap1; % set coordinates of points
Longitude = 0:15:360; Latitude = -72:6:72;
% set background parameters
contour(0:359, -89:90, topo, [0,0], 'w');
axis equal
box on
set(gca, 'Color', [0 0 1], 'XLim', [0 360], 'YLim', [-90 90], 'XTick', [0:180:360], 'YTick', [-90:90:90]);
%set(gca, 'Color', [0 0 1], 'XLim', [0 360], 'YLim', [-90 90]);
grid;
hold on % wait for more plot commands
plot(Longitude, Latitude, 'y.', 'linewidth', 1); % enter plot
hold off % no more plot commands to come after this line

Now, use that with the rest of your code.

.

[Light]

Wow, amazing thanks very much. Could I ask what caused the errors? Also in line 8 with with set(gca, 'Color' etc. Is a color supposed to be inputted there?

[Star Strider]

My pleasure!

The errors were caused by breaking up that line so that all the name-value pair arguments were not within the argument parentheses of the set call. The original code was, as I mentioned previously, correct. The name-value pair arguments that I saw were all correct, so definitely yes.

See the documentation on set for details.

[Light]

Okay, alright, thanks. I understand now. Thanks for taking the time to explain.

[Dyuman Joshi]

@Edson, if it solved your queries, please accept Star Strider's answer.

[Star Strider]

@Dyuman Joshi — Thank you!

[Light]

Okay thanks very much, I will. Thank you for all your help. Its just one little error I get whenever I run the code, I dont know why. I didnt make any changes to it.

[Star Strider]

You have to use continuation ellipses (...) to continue an expression over more than one line —

set(gca, 'Color', [0 0 1], 'XLim', [0 360], 'YLim', [-90 90], 'XTick', ...

[0:180:360], 'YTick', [-90:90:90]);

Testing it —

load('topo.mat', 'topo', 'topomap1');
% load earth topography whos topo topomap1; % set coordinates of points
Longitude = 0:15:360; Latitude = -72:6:72;
% set background parameters
contour(0:359, -89:90, topo, [0,0], 'w');
axis equal
box on
set(gca, 'Color', [0 0 1], 'XLim', [0 360], 'YLim', [-90 90], 'XTick', ... 
    [0:180:360], 'YTick', [-90:90:90]);
%set(gca, 'Color', [0 0 1], 'XLim', [0 360], 'YLim', [-90 90]);
grid;
hold on % wait for more plot commands
plot(Longitude, Latitude, 'y.', 'linewidth', 1); % enter plot
hold off % no more plot commands to come after this line

See MATLAB Operators and Special Characters for details.

.

[Light]

Oh okay, thanks alot again 🤦♂

[Star Strider]

As always, my pleasure!

[Light]

I have another question pertaining to plotting a graph if its okay. Should I ask it here or post it in a new thread?

[Star Strider]

You can ask it here.

What is it?

[Light]

Okay so I copied the code and implemented some values but if I wish to have a line shown passing through the different points on the plot, do I have to use the polyfit function?

[Star Strider]

I am slightly lost, since I’m not certain what you want to plot.

If you have the (x,y) coordinates of the line (whatever ‘x’ and ‘y’ actually are), just plot them using either the plot or line functions.

I believe the hold function is already implemented in the code you’re using. You will need to put any additional plot calls between the hold on and hold off calls that already exist.

[Light]

I would like to plot a path between two cities using longitude and latitude (l and b). In the last image I attached, I plotted l vs b but there is no path or line passing through the datapoints between the two cities. I would like to have a path passing through the datapoints from one city to the next using l and b.

[Justin]

@Edson am doing something similiar and hopefully if you got through you can share what you ended up doing. Thanks in advance

[Star Strider]

@Justin — See: How to properly create for loop with if statement