MATLAB: “Index exceeds the number of array elements (1).”

Question

load('air.txt')
Air.temp = air(:,1);
Air.press = air(:,2);
Air_R = 0.287;
load('argon.txt')
Argon.press = argon(:,2);
Argon.temp = argon(:,1);
Argon_R = 0.208;
load('helium.txt')
Helium.press = helium(:,2);
Helium.temp = helium(:,1);
Helium_R = 2.080;
for k = 1:length(air)
    Air(k).vol = (Air_R*Air(k).temp)./Air(k).press;
end
for k = 1:length(argon)
    Argon(k).vol = (Argon_R*Argon(k).temp)./Argon(k).press;
end
for k = 1:length(helium)
    Helium(k).vol = (Helium_R*Helium(k).temp)./Helium(k).press;
end 

Using the ideal gas law to caululate the velocity of argon, helium and air, but I keep getting and Error.

"Index exceeds the number of array elements (1)."

When I run the code through I only get the volume of air butnot the volumes of argon or helium? I tried to also create a auser defined function, but I wasn't able to figure out what was wrong with that either.

Answer 1

The way you define your struct in the first few lines results in Air being a 1x1 struct with two fields (temp and press), each of which is 38x1. So calling Air(2).temp throws an error because Air is only 1x1. To use the indices correctly, your first for loop should read

for k = 1:length(air)
    Air.vol(k) = (Air_R*Air.temp(k))./Air.press(k);
end