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From: Alex on 8 Feb 2010 12:23 Hi, few beginner questions. in matlab are arrays and matrices ultimately the same thing? I want an array of three columns. The first column holds the time, 2nd holds a pressure, and the third a volume. I created an array pv = ones(1000, 3); i believe this will be an array of 1000 rows, and 3 columns. then in a for loop for t = 0:0.0001:10; i = i+1; pressure = [an equation]; volume = [an equation]; now i want to put those values for t, pressure and volume into the array pv. pv(i,i,i) = (t,pressure, volume); this doesnt seem to work, but i cant find a tutorial explaining something like this. Am i close? if i then wanted to plot the pressures against volumes, could i do plot (pressure,volume)? do i need to use a for loop at all? thanks alex
From: Luca Zanotti Fragonara on 8 Feb 2010 12:33 "Alex " <a.mlw.walker(a)googlemail.com> wrote in message <hkph9o$lg6$1(a)fred.mathworks.com>... > Hi, few beginner questions. > > in matlab are arrays and matrices ultimately the same thing? > > I want an array of three columns. The first column holds the time, 2nd holds a pressure, and the third a volume. > I created an array pv = ones(1000, 3); > > i believe this will be an array of 1000 rows, and 3 columns. > > then in a for loop > > for t = 0:0.0001:10; > i = i+1; > pressure = [an equation]; > volume = [an equation]; > > now i want to put those values for t, pressure and volume into the array pv. > > pv(i,i,i) = (t,pressure, volume); > > this doesnt seem to work, but i cant find a tutorial explaining something like this. Am i close? > > if i then wanted to plot the pressures against volumes, could i do > > plot (pressure,volume)? > > do i need to use a for loop at all? > > thanks > > alex t=0:0.0001:10; for ii=1:length(t) pv(:,1)=t; pv(ii,2)=an equation f(t) that defines pressure; pv(ii,3)=an equation f(t) that defines volume; end plot(pv(:,2),pv(:,3)) You can also avoid the for loop, if you can vectorize your equation: Imagine that a and b are constant, who defines the pressure and the volume: pv(:,2)=a*b*t; pv(:,3)=a/b*t; a and b can be either scalar or vectors. Good luck, Luca
From: Oleg Komarov on 8 Feb 2010 13:06 "Alex " > Hi, few beginner questions. > > in matlab are arrays and matrices ultimately the same thing? > > I want an array of three columns. The first column holds the time, 2nd holds a pressure, and the third a volume. > I created an array pv = ones(1000, 3); > > i believe this will be an array of 1000 rows, and 3 columns. > > then in a for loop > > for t = 0:0.0001:10; > i = i+1; > pressure = [an equation]; > volume = [an equation]; > > now i want to put those values for t, pressure and volume into the array pv. > > pv(i,i,i) = (t,pressure, volume); > > this doesnt seem to work, but i cant find a tutorial explaining something like this. Am i close? > > if i then wanted to plot the pressures against volumes, could i do > > plot (pressure,volume)? > > do i need to use a for loop at all? > > thanks > > alex A useful tutorial: http://www.mathworks.com/access/helpdesk/help/techdoc/math/f1-85462.html Oleg
From: Steven Lord on 8 Feb 2010 13:11 "Alex " <a.mlw.walker(a)googlemail.com> wrote in message news:hkph9o$lg6$1(a)fred.mathworks.com... > Hi, few beginner questions. Welcome. > in matlab are arrays and matrices ultimately the same thing? Usually the term 'matrix' refers to a 2-dimensional array of numeric data, while "blocks" of data with more than 2 dimensions or non-numeric data (like a cell array, struct array, char array, etc) are referred to as 'arrays'. > I want an array of three columns. The first column holds the time, 2nd > holds a pressure, and the third a volume. > I created an array pv = ones(1000, 3); > > i believe this will be an array of 1000 rows, and 3 columns. Yes, or you could also call it a 1000-by-3 matrix. > then in a for loop > for t = 0:0.0001:10; > i = i+1; > pressure = [an equation]; > volume = [an equation]; > > now i want to put those values for t, pressure and volume into the array > pv. > > pv(i,i,i) = (t,pressure, volume); > > this doesnt seem to work, but i cant find a tutorial explaining something > like this. Am i close? Close; you just need to make the right-hand side into a vector and tell MATLAB to assign into a rectangular piece of pv, rather than an individual element. pv(i, :) = [t, pressure, volume]; This assigns the 1-by-3 row vector created by [t, pressure, volume] into all the columns of the ith row of pv. For a tutorial that describes the basics of working with matrices and arrays in MATLAB, look in the Getting Started guide: http://www.mathworks.com/access/helpdesk/help/techdoc/learn_matlab/bqr_2pl.html Once you've read through that, if you are interested in learning more, check out this section from the User's Guide: http://www.mathworks.com/access/helpdesk/help/techdoc/math/f1-84787.html > if i then wanted to plot the pressures against volumes, could i do > > plot (pressure,volume)? Not the way you've written the example above; the variables pressure and volume will only contain the last entry calculated. Since you stored the data in pv, though, you could use: plot(pv(:, 2), pv(:, 3)) to plot using the 2nd and 3rd columns for all the rows of pv. > do i need to use a for loop at all? That depends. Is your expression vectorized or can you make it vectorized? http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_prog/f8-784135.html#br8fs0d-1 -- Steve Lord slord(a)mathworks.com comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ
From: Alex on 23 Feb 2010 11:16 Thank you very much that helped alot, I am just looking at getting rid of my for loops because matlab runs so much faster without them if everything is vectorized. i had a for loop like so for t = 0:0.01:10; equation1 = ... equation2 = ...[something including equation1] eqn3 = ...[something including equation2] eqn4=...[something inlcuding eqn3] etc end plot (pv(eqn(n)), pv(eqn(j)) if i want to vectorize this so to get rid of the for loop can i write t = 0:0.01:10; equation1 = ... equation2 = ...[something including equation1] eqn3 = ...[something including equation2] eqn4=...[something inlcuding eqn3] etc plot (pv(eqn(n)), pv(eqn(j)) because if i can it doenst seem to work, what if those equations are time dependant but only by the step size and not the actual time now, although i do want it to be simulated over time? "Steven Lord" <slord(a)mathworks.com> wrote in message <hkpk58$h3f$1(a)fred.mathworks.com>... > > "Alex " <a.mlw.walker(a)googlemail.com> wrote in message > news:hkph9o$lg6$1(a)fred.mathworks.com... > > Hi, few beginner questions. > > Welcome. > > > in matlab are arrays and matrices ultimately the same thing? > > Usually the term 'matrix' refers to a 2-dimensional array of numeric data, > while "blocks" of data with more than 2 dimensions or non-numeric data (like > a cell array, struct array, char array, etc) are referred to as 'arrays'. > > > I want an array of three columns. The first column holds the time, 2nd > > holds a pressure, and the third a volume. > > I created an array pv = ones(1000, 3); > > > > i believe this will be an array of 1000 rows, and 3 columns. > > Yes, or you could also call it a 1000-by-3 matrix. > > > then in a for loop > > for t = 0:0.0001:10; > > i = i+1; > > pressure = [an equation]; > > volume = [an equation]; > > > > now i want to put those values for t, pressure and volume into the array > > pv. > > > > pv(i,i,i) = (t,pressure, volume); > > > > this doesnt seem to work, but i cant find a tutorial explaining something > > like this. Am i close? > > Close; you just need to make the right-hand side into a vector and tell > MATLAB to assign into a rectangular piece of pv, rather than an individual > element. > > pv(i, :) = [t, pressure, volume]; > > This assigns the 1-by-3 row vector created by [t, pressure, volume] into all > the columns of the ith row of pv. > > For a tutorial that describes the basics of working with matrices and arrays > in MATLAB, look in the Getting Started guide: > > http://www.mathworks.com/access/helpdesk/help/techdoc/learn_matlab/bqr_2pl.html > > Once you've read through that, if you are interested in learning more, check > out this section from the User's Guide: > > http://www.mathworks.com/access/helpdesk/help/techdoc/math/f1-84787.html > > > if i then wanted to plot the pressures against volumes, could i do > > > > plot (pressure,volume)? > > Not the way you've written the example above; the variables pressure and > volume will only contain the last entry calculated. Since you stored the > data in pv, though, you could use: > > plot(pv(:, 2), pv(:, 3)) > > to plot using the 2nd and 3rd columns for all the rows of pv. > > > do i need to use a for loop at all? > > That depends. Is your expression vectorized or can you make it vectorized? > > http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_prog/f8-784135.html#br8fs0d-1 > > -- > Steve Lord > slord(a)mathworks.com > comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ >
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