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Hi,

I have created two functions in Matlab. One is supposed to produce a concentration profile for a given location and time, but the current code gives this error "Subscript indices must either be real positive integers or logicals.". The second function, called testing, recalls the first function to do the calculation and uses loops instead, but still the same error is appearing.

1. Concentration profile code

function [c,t] = Disp_Conc(x,y,z,t)

%This function calculates the concentration of a an agent at a particular time and location

% Data generated through Dispersion model (This is a 1x61 vector)

[x,cc_x,b_x,betac_x,zc_x,sig_x,t,xc_t,bx_t,betax_t] = Read_Predict('predict');

% Time averaged volume concentration: concentration contour parameters

%erf = error function

sr2 = sqrt(2.0);

xa=(x-xc_t(t)+bx_t(t))/(sr2*betax_t(t));

xb = (x-xc_t(t)-bx_t(t))/(sr2*betax_t(t));

ya = (y+b_x(x))/(sr2*betac_x(x));

yb = (y-b_x(x))/(sr2*betac_x(x));

za = (z-zc_x(x))/(sr2*sig_x(x));

zb = (z+zc_x(x))/(sr2*sig_x(x));

c = (cc_x(x) * (erf(xa)-erf(xb)) * (erf(ya)-erf(yb)) * (exp(-za*za)+exp(-zb*zb)));

time=tm(t);

end

**Running this function, I get an error saying "Subscript indices must either be real positive integers or logicals." due to the fact that values of t and x are both fractional (both x and t are read from a 1x61 vector). To overcome this, I created another .m file that recalls the concentration function and uses loops as below:

2. Function "testing":

function [c, t] = testing( x,y,z,t )

%The values of x and t are already being read from a saved 1x61 vector. z and y can vary and must be specified by the user for a given location. The value of z should be set equal to zero for all cases and doesn't change.

for ts=1:length(t)

for xs=1:length(x)

z=0;

for y=0:100; %(k index?)

X=x(ts);

T=t(xs);

[c(ts),time(ts)]=Disp_Conc(X,y,z,T);

end

end

end

end

**Running "testing" for a particular parameters say testing(5,0,0,8.79), I get the below errors:

"Subscript indices must either be real positive integers or logicals.

Error in Disp_Conc (line 12)

xa=(x-xc_t(t)+bx_t(t))/(sr2*betax_t(t));

Error in testing (line 10)

[c(ts),time(ts)]=Disp_Conc(X,y,z,T);"

I don't know what is wrong this time? The original problem was because of indexing and now I still get the same error with the new function.

Any help would be greatly appreciated. Thx.

Star Strider
on 10 Jan 2015

In the line that throws the error:

xa=(x-xc_t(t)+bx_t(t))/(sr2*betax_t(t));

what is the value of ‘t’?

Stephen Cobeldick
on 10 Jan 2015

Typically there are two methods for looping over non-integer values in MATLAB:

- derive the values within each loop, using a function or calculating the values from the loop variables.
- use the loop variables to look up the values in a predefined array.

Simple examples for each of these, where we want the non-integer value 1/k in each iteration:

for k = 1:4

disp(1/k)

end

A = 1./(1:4);

for k = 1:4

disp(A(k))

end

While the second method looks more complicated, it actually has many advantages in MATLAB: it helps memory management , and makes it easier to write vectorized code. Note how the second example uses indexing, and the index value is an integer!

As you have arrays of data, you should consider using the look-up method. You will probably need these function to help you: size, numel.

Stephen Cobeldick
on 10 Jan 2015

Edited: Stephen Cobeldick
on 10 Jan 2015

In MATLAB all array indices must be logical or positive numeric integers. This means that the following is permitted:

>> A = [123,456,789];

>> A(2) % 2 is an integer

ans = 456

But the following produces an error:

>> A(3.14159) % 3.14159 is not an integer

Subscript indices must either be real positive integers or logicals.

Note that negative integers and zero are not permitted indices.

You need to review your code and check all of the array indexing. In particular it seems that you are using some data values (e.g. t, x) as indices.

Image Analyst
on 10 Jan 2015

Stephen, very good explanation (+ a vote). Since we answer this at least once or twice a day, I don't know why it's not on the FAQ yet. So I used your excellent explanation as the basis for a new FAQ entry: http://matlab.wikia.com/wiki/FAQ#How_do_I_fix_the_error_.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22.3F

By the way, empty seems to be okay

>> emptyvector = []

emptyvector =

[]

>> A(emptyvector)

ans =

[]

Stephen Cobeldick
on 10 Jan 2015

True, [] simply returns an empty array. I have edited my answer to reflect this.

Luca
on 21 Aug 2017

Edited: Walter Roberson
on 21 Aug 2017

clc; clear;

teta=linspace(0,pi/2,100);

R=1;

g=9.81;

v_quadro = g*R*cos(teta.*2) + R*g(2^0.5-1)-4*R^3*(1-cos(teta))*cos(teta)/((2*R-(2^0.5)*R)^2);

Where is the error, please? Thanks.

Walter Roberson
on 21 Aug 2017

Your g is not a vector, but you are trying to index it at (2^0.5-1).

MATLAB does not have any implicit multiplications. You need to add ".*" or "*" in g(2^0.5-1)

christina fayez
on 11 Jun 2020

hi guys I do the code for white noise of 3LDFT algorithm but give me error

Subscript indices must either be real positive integers or logicals.

Error in threeDFT (line 37)

x=v(((j-1):(j+ N0-2))*(dt));

- function [t_est,f_est]=threeDFT(v,fs,tmax,N0)
- % v : volt as function of time
- % fs : sampling frequency (Hz)
- % tmax : time of final estimation
- % N0 : number of samples in the window
- % to test: [t,f]=threeDFT(@(t)(220*sin(2*pi*50.1*t+pi/2)),50*512,1,512)
- % to test: [t,f]=threeDFT(@(t)(220*sin(2*pi*50.1*t+pi/2)+randn(size(t))*.1),50*512,1,512)
- fs=50*512; %sampling freq.
- dt =1/fs;
- N0=fs/50; %number of samples/cycle
- m=50; %no. of cycles
- t = dt*(0:m*N0); %data window
- fi=50; %Frequency test
- ww=wgn(201,1,-40);
- size(transpose(ww))
- t =dt*(0:200);
- y=sin(2*pi*fi*t + 0.3);
- v=sin(2*pi*fi*t + 0.3)+transpose(ww)
- tmax=1;
- n=N0-1:-1:0;
- f0=50;
- f=50.88;
- Hc=2/N0*cos(2*pi*n/N0+pi/N0);
- Hs=-2/N0*sin(2*pi*n/N0+pi/N0);
- t_est=[];
- f_est=[];
- j_max=tmax*fs;
- for j=1:j_max+1
- x=v(((j-1):(j+ N0-2))*(dt));
- c(j)=x*Hc';
- s(j)=x*Hs';
- if(j>N0)
- Ac(j-N0)=sqrt(sum(c(end-N0+1:end).^2)/N0);
- As(j-N0)=sqrt(sum(s(end-N0+1:end).^2)/N0);
- cc(j-N0)=c(end-N0+1:end)*Hc';
- ss(j-N0)=c(end-N0+1:end)*Hs';
- if(j>2*N0)
- Acc(j-2*N0)=sqrt(sum(cc(end-N0+1:end).^2)/N0);
- Ass(j-2*N0)=sqrt(sum(ss(end-N0+1:end).^2)/N0);
- ccc(j-2*N0)=cc(end-N0+1:end)*Hc';
- ccs(j-2*N0)=cc(end-N0+1:end)*Hs';
- ssc(j-2*N0)=ss(end-N0+1:end)*Hc';
- sss(j-2*N0)=ss(end-N0+1:end)*Hs';
- ff=f0*N0/pi*atan(tan(pi/N0)*((ccc(j-2*N0).^2+ccs(j-2*N0).^2)./(ssc(j-2*N0).^2+sss(j-2*N0).^2)).^.25);
- t_est=[t_est;(j-1)*dt];
- f_est=[f_est;ff];
- end
- end
- end
- t_est;
- f_est
- plot(t_est, f_est,'red')
- hold on
- RMSE = sqrt(mean((f_est-fi).^2))
- xlabel('time')
- ylabel('frequency')
- title('3LDFT WHITE NOISE ')
- plot (t,fi)
- hold off

christina fayez
on 12 Jun 2020

I have another error

clear all; close all; clc

Attempted to access v(1.005); index must be a positive integer or logical.

Error in code (line 41)

v_new=v(i.*dt);

>>

- %frequency and amplitude
- fs=200; %sampling freq.
- dt =1/fs;
- N=fs/200%number of samples/cycle
- m=6 %no. of cycles
- t =dt*(0:400); %dt*(0:m*N); %data window
- fi=50; %Frequency test
- ww=wgn(201,1,-40);
- size(transpose(ww))
- t =dt*(0:200);
- y=sin(2*pi*fi*t + 0.3);
- v=sin(2*pi*fi*t + 0.3)+transpose(ww);
- tmax=1;
- % v : as function of time
- % fs : sampling frequency (Hz)
- % tmax : time of final estimation
- % to test: [t,f]=ZC(@(t)(220*sin(2*pi*50.1*t+pi/2)),50*512,1)
- % to test: [t,f]=ZC(@(t)(220*sin(2*pi*50.1*t+pi/2)+randn(1)*.1),50*512,1)
- dt=1/fs;
- v_old=v(1);
- i_max=tmax*fs;
- if(v_old==0)
- old_cross=0;
- else
- old_cross=-1;
- end
- new_cross_detected=0;
- t_est=[];
- f_est=[];
- for i=200:1:400
- v_new=v(i.*dt);
- if(v_old.*v_new<0)
- new_cross=(i-1+v_old/(v_old-v_new))*dt;
- new_cross_detected=1;
- elseif(v_new==0)
- new_cross=i*dt;
- new_cross_detected=1;
- end
- if(new_cross_detected==1)
- if(old_cross~=-1)
- t_est=[t_est;i*dt];
- f_est=[f_est;1/(2*(new_cross-old_cross))];
- end
- old_cross=new_cross;
- new_cross_detected=0;
- end
- v_old=v_new;
- end
- t_est
- f_est
- plot(t_est,f_est,'red')
- o=rms(fi)
- c=rms(f_est)
- RMSE = sqrt(mean(c - o).^2)
- hold on
- xlabel('time')
- ylabel('frequency')
- title('ZC white noise')
- plot (t,fi)
- hold off

Steven Lord
on 13 Jun 2020

There's no such thing as element 1.005 of an array in MATLAB. If as line 19 states you want v to be a function of t rather than a vector created using a vector of values t you should probably define it as an anonymous function.

v = @(t) sin(2*pi*fi*t + 0.3)+transpose(ww);

If you do this, v(1.005) will not be an attempt to index into a vector but will be an attempt to evaluate the anonymous function when t is 1.005.

As a simpler example that shows the technique:

x = 1:10;

y1 = @(x) x.^2;

y1(5.5) % works, returns (5.5)^2 = 30.25

y2 = x.^2;

y2(5.5) % will not work

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