# fplot in Matlab error

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Aseel Alamri on 9 Nov 2020
Commented: Star Strider on 9 Nov 2020
% I am trying to plot these three eqyations but evrytime I hit run this error shows "Error using fplot (line 149)
Invalid parameter ' ...'."
clear all;
syms V(t) a b C
time = [3.46
4.58
5.67
6.64
7.63
8.41
9.32
10.27
11.19
12.39
13.42
15.19
16.24
17.23
18.18
19.29
21.23
21.99
24.33
25.58
26.43
27.44
28.43
30.49
31.34
32.34
33
35.2
36.34
37.29
38.5
39.67
41.37
42.58
45.39
46.38
48.29
49.24
50.19
51.14
52.1
54
56.33
57.33
59.38];
Voltage = [0.0158
0.0264
0.0326
0.0445
0.0646
0.0933
0.1454
0.2183
0.2842
0.4977
0.6033
0.8441
1.2163
1.447
2.3298
2.5342
3.0064
3.4044
3.2046
4.5241
4.3459
5.1374
5.5376
4.8946
5.066
6.1494
6.8548
5.9668
6.6945
6.6395
6.8971
7.2966
7.2268
6.8815
8.0993
7.2112
7.0694
7.4971
6.9974
6.7219
7.0523
7.1095
7.0694
8.0562
7.2268];
a = 0.3389;
b = 0.0489;
sol1 = (a*tanh((a*(C+t))/2)+1)/(2*b)
a = 0.4340;
b = 0.2158;
sol2 = ((a./b).*C.*exp(a.*t))./((a./b)+C.*exp(a.*(t-1)))
syms V(t) a b C
cond = [V(0) == C]
a = 0.2375;
b = 0.1179;
eq = diff(V) == a*V-b*V*log(V);
sol3 = dsolve(eq, cond)
fplot(sol1,time,'ro')
hold on
plot(time,sol2,'B')
plot(time, sol3,'G')
hold off
legend('Data', 'Gompertz', 'Bertalanffy', 'logistic');
title(' three models')
xlabel('Time (days)');
ylabel('Voltage');

Star Strider on 9 Nov 2020
Note that ‘sol1’ is a function of both ‘C’ and ‘t’. Since ‘C’ is undefined, fplot throws the error. One option is to use another function, and provide a range of values for ‘C’:
fsurf(sol1,[min(Voltage) max(Voltage) min(time) max(time)])
The arguments are taken in lexical order, so here ‘C’ is assigned ‘Voltage’ and ‘t’ is assigned ‘time’.
##### 2 CommentsShowHide 1 older comment
Star Strider on 9 Nov 2020
Not necessarily.
I have no idea what your equations are modeling, so I cannot suggest an approach. I would keep them as they are (at least ‘sol1’ as a function only of time), and define ‘C’ as some numeric scalar constant before you calculate ‘sol1’. The fplot function requires that the functions it is given are functions of only one variable.
I leave that to you to decide how you want to approach that.
Also, note that:
sol1 = @(t) ((a*tanh((a*(Voltage+time))/2)+1)/(2*b))
is not actually a function of ‘t’. I would just leave it the way it is in the code you posted, and define ‘C’ as a numeric scalar.