Why doesn't get a numerical value

i have assigned all the Variable that i have to assign,but why not get a numerical value?

codes are as this

%此处如果不行可以单独拿出来 
clc;
clear all;
close all;

syms n a d

%sn=n*pi/2*diff(besseli(0,n*pi*a/d),a)/(besseli(0,n*pi*a/d))

sn=(pi^2*n^2*besseli(1, (pi*a*n)/d))/(2*d*besseli(0, (pi*a*n)/d));

h=2;%水深
D1=0.8060;%圆柱吃水深度

a=2.2497;%圆柱半径

%d=h-D1;%圆柱底部到水底的距离
d=1.1940;


s0=vpa(subs(sn,n,0),5)

s1=vpa(subs(sn,n,1),5)
 
s2=vpa(subs(sn,n,2),5)


%s0=0.;
%s1=3.76552;
%s2=15.8176;

and the result are as this

 
s0 =
 
0.0
 
 
s1 =
 
(4.9348*besseli(1.0, (3.1416*a)/d))/(d*besseli(0.0, (3.1416*a)/d))
 
 
s2 =
 
(19.739*besseli(1.0, (6.2832*a)/d))/(d*besseli(0.0, (6.2832*a)/d))
 
>> 

why

Answer 1

[madhan ravi]

Use should use subs() all at once (instead your variables are still symbolic) but below is another work around :

%此处如果不行可以单独拿出来 
clc;
clear all;
close all;
syms n 
h=2;%水深
D1=0.8060;%圆柱吃水深度
a=2.2497;%圆柱半径
%d=h-D1;%圆柱底部到水底的距离
d=1.1940;
%sn=n*pi/2*diff(besseli(0,n*pi*a/d),a)/(besseli(0,n*pi*a/d))
sn=(pi^2*n^2*besseli(1, (pi*a*n)/d))/(2*d*besseli(0, (pi*a*n)/d));
s0=vpa(subs(sn,n,0),5)
s1=vpa(subs(sn,n,1),5) 
s2=vpa(subs(sn,n,2),5)

Gives:

 
s0 =
 
0.0
 
s1 =
 
3.7655
 
s2 =
 
15.818