How to make the for loop the length as the array inside the for loop?

I am making a for loop and I want to subtract each element by each other. I was able to do this but my array is now 1x200 and my original array was 1x201. How do I make it so the for loop array is 1x201?

x1 = [1e6:20000:5e6];
L = 4e6; % in m
D_c = 5000;% in m
D1 = [0:25:5000];
x_o = 1e6; % in m
x_2 = [x_o:20000:5e6];
Tw_o = 300; % in K
LR_w = 0.004; % in K/m
LR_c = -0.007; % in K/m
pz_0 = 1000; % in mb
D_1 = D_c.*(sin((pi/L).*(x1-x_o)));
T_c2 = Tw_o-(((LR_w-LR_c).*D_1)-((LR_c.*D1)));
g = 9.81;
Rd = 287;
p1 = pz_0*(((((T_c2)-(LR_c.*D1))./(T_c2))).^((g)/(Rd*LR_c)));
for i = 1:length(p1)-1
    deltap(i) = p1(i+1)-p1(i);
end

Answer 1

[Star Strider]

You do not need the loop at all. Just use the diff function —

x1 = [1e6:20000:5e6];
L = 4e6; % in m
D_c = 5000;% in m
D1 = [0:25:5000];
x_o = 1e6; % in m
x_2 = [x_o:20000:5e6];
Tw_o = 300; % in K
LR_w = 0.004; % in K/m
LR_c = -0.007; % in K/m
pz_0 = 1000; % in mb
D_1 = D_c.*(sin((pi/L).*(x1-x_o)));
T_c2 = Tw_o-(((LR_w-LR_c).*D_1)-((LR_c.*D1)));
g = 9.81;
Rd = 287;
p1 = pz_0*(((((T_c2)-(LR_c.*D1))./(T_c2))).^((g)/(Rd*LR_c)));

deltap = diff(p1)

deltap = 1×200

-2.8534 -2.8634 -2.8734 -2.8835 -2.8935 -2.9036 -2.9136 -2.9237 -2.9337 -2.9437 -2.9536 -2.9635 -2.9733 -2.9831 -2.9927 -3.0023 -3.0118 -3.0212 -3.0305 -3.0396 -3.0486 -3.0574 -3.0661 -3.0746 -3.0830 -3.0911 -3.0990 -3.1068 -3.1143 -3.1215

for i = 1:length(p1)-1
    deltap(i) = p1(i+1)-p1(i);
end
deltap

deltap = 1×200

-2.8534 -2.8634 -2.8734 -2.8835 -2.8935 -2.9036 -2.9136 -2.9237 -2.9337 -2.9437 -2.9536 -2.9635 -2.9733 -2.9831 -2.9927 -3.0023 -3.0118 -3.0212 -3.0305 -3.0396 -3.0486 -3.0574 -3.0661 -3.0746 -3.0830 -3.0911 -3.0990 -3.1068 -3.1143 -3.1215

If you want to calculate the numerical derivative at eash point instead, use the gradient function —

deltap = gradient(p1)

deltap = 1×201

-2.8534 -2.8584 -2.8684 -2.8784 -2.8885 -2.8985 -2.9086 -2.9186 -2.9287 -2.9387 -2.9486 -2.9585 -2.9684 -2.9782 -2.9879 -2.9975 -3.0071 -3.0165 -3.0258 -3.0350 -3.0441 -3.0530 -3.0618 -3.0704 -3.0788 -3.0870 -3.0951 -3.1029 -3.1105 -3.1179

.

[Matthew]

Is there a way to get these values positive instead of negative? Thanks.

[Star Strider]

One way would be to take the absolute value (the abs function), the other, since they are uniformly negative, would be to multiply them by -1 or just use a unary negative:

deltap = -gradient(p1)

Use whatever works best in your application.

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