Matlab showing 0 instead of small value

In the following code delta_rho_m, delta_p_m (=(mass_density_CGS - mean(mass_density_CGS))) are very small, and delta_p_m is showing as 0 for all the data, at the same time (mass_density_CGS - mean(mass_density_CGS)) is giving values. I request for help to solve this issue

clc;
clear; % all;

data = load('NH_VOY2_Data.txt');
% data = load('UxUyUz.txt');

mp = 1.67*10^(-24); %proton mass gm
mp_SI = 1.67*10^(-27); %proton mass kg

cc_m3 = 10^(-6); % m^3

k = 1.3807 * 10^(-16); % Boltzmann constant in CGS cm2 g s-2 K-1
% k = 1.380649*10^(-23); % boltzmann constant SI J/K

gamma = 5.00/3.00; % 

time = data(:, 1);
density_CGS = data(:, 2);
speed_km = data(:, 3); % 
temperature = data(:, 4); 
distance = data(:, 5); 
latitude = data(:, 6); 
longitude = data(:, 7); 

pressure_CGS = zeros(size(density_CGS)); %pressure_SI = zeros(size(density));

speed_R = zeros(size(speed_km));
speed_T = zeros(size(speed_km));
speed_N = zeros(size(speed_km));
speed = zeros(size(speed_km));
%distance_sun = zeros(size(speed));

%for i = 1:length(density_CGS)   
    mass_density_CGS = density_CGS * mp; %\rho
    delta_rho_m = mass_density_CGS - mean(mass_density_CGS) % \delta \rho_m i.e. fluctuation in mass density  

delta_rho_m = 24x1

1.0e-25 * -0.0844 -0.0372 -0.0310 -0.0436 -0.0454 0.0288 -0.0247 -0.0228 -0.0316 -0.0064

    
    delta_p_m = pressure_CGS - mean (pressure_CGS) % \delta p_m i.e. fluctuation in pressure

delta_p_m = 24x1

0 0 0 0 0 0 0 0 0 0

    %speed_SI(i) = speed_km(i) * 1000.00; % SI unit from km to m
%end

for i = 1:length(speed_km)    
    speed_R(i) = speed_km(i) * cosd(latitude(i)) * cosd(longitude(i));
    speed_T(i) = speed_R(i) * tand(longitude(i));
    speed_N(i) = speed_km(i) * sind(latitude(i));
    speed(i) = sqrt(speed_R(i)^2 + speed_T(i)^2 + speed_N(i)^2);
    pressure_CGS(i) = density_CGS(i) * k * temperature(i); % P = nkT but here it is \rho kT; need to check
end

mean_mass_density = mean(mass_density_CGS); %\rho_0 as r_0
mean_pressure_CGS = mean(pressure_CGS); % p_0 
rho_0 = mean_mass_density;
P_0 = mean_pressure_CGS;

a_0 = sqrt((gamma*P_0)/rho_0); % sound speed
% a_02 = a_0^2;% (a_0)^2

pressure_CGS - mean (pressure_CGS)

ans = 24x1

1.0e-14 * -0.0927 -0.0092 -0.1341 0.0340 -0.1948 0.2477 0.2828 0.2249 0.1063 0.0416

Answer 1

[Sam Chak]

Hi @Ismita

When examining the flow of execution, you will notice that the 'pressure_CGS' vector is initialized with twenty-four zeros. Consequently, calculating the average of the elements in the vector will yield zero as the result. Therefore, subtracting zero from zero will also result in zero. This occurs prior to the computation within the for-loop.

pressure_CGS = zeros(size(density_CGS));
...
delta_p_m = pressure_CGS - mean(pressure_CGS) % \delta p_m i.e. fluctuation in pressure

[Ismita]

thank you

[Sam Chak]

You are welcome, @Ismita. If you find the explanation helpful, please consider clicking 'Accept' ✔ on the answer to close the issue.