I am trying to create an array h of size 1001 which can store value of 2 integrals

gl=10;
gr=10;
N=1000;
L=10;
l=10;
h = zeros(1,1000);
syms x;
for i=1:1001
    f =gl*(7/44)*(exp((-1*L*i)/(N*l*cos(x))));
    A=vpa(int(f,-1.57,1.57));
    A
    f =gl*(7/44)*(exp((-1*(L-N)*i)/(N*l*cos(x))));
    B=vpa(int(f,1.57,4.71));
    B
    h(i)=A+B;
end

1- On running instead of value of B I am getting exact expression is printing intead of value of B

2- I want to store all these values in the array h from index 1 to 1001 but its showing error message "Unable to convert expression into double array.".

Plase help and thanks a lot in advance.

[darova]

Try double instead of vpa

[Jan]

@darova: This does not work also. See:

gl=10;
gr=10;
N=1000;
L=10;
l=10;
syms x;
i = 1;
f = gl * (7 / 44) * (exp((-1 * (L-N) * i) / (N * l * cos(x))));
B = vpa(int(f, x, [1.57, 4.71]))

B = 

Answer 1

[Jan]

gl=10;
gr=10;
N=1000;
L=10;
l=10;
syms x;
i = 1;
f = gl * (7 / 44) * (exp((-1 * (L-N) * i) / (N * l * cos(x))));
B = vpaintegral(f, x, [1.58, 4.71])  % Smaller interval, x > pi/2

B = 

3.91702

B = vpaintegral(f, x, [1.57, 4.71])  % Original interval

B = 

For cos(x)==0, or x=pi/2 the exponent gets infinite and the integration fails due to the pole.

[Yongjian Feng]

Ah... cos(x) is in the denominator! You are right.

[Tarun Agarwal]

@Jan Thank you very much It worked

@darova@Yongjian Feng Thanks a lot .

Answer 2

[Yongjian Feng]

It doens't allow you to go outside (-1.57, 1.57). This is essentially (-PI/2, PI/2). Since your f(x) is a function of cos(x), so your f(x) is also a periodic function. Try to keep the integration range within (-PI/2, PI/2), and it should work.