# how to use for loop in equation

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shiv gaur on 1 Apr 2022
Tstart = 0.0;
Tend = 1000.0;
Nt = 20000;
dT = (Tend-Tstart)/Nt;
X0 = 0.0;
Y0 = 1.0;
Z0 = 0.0;
N = 20;
for A=0.1:1
B=0.2;
C=5.7;
T = zeros(Nt+1,1);
X = zeros(Nt+1,1);
Y = zeros(Nt+1,1);
Z = zeros(Nt+1,1);
a = zeros(N+1,1);
b = zeros(N+1,1);
c = zeros(N+1,1);
T(1) = 0.0;
X(1) = X0;
Y(1) = Y0;
Z(1) = Z0;
for j = 2:Nt+1
a(1) = X(j-1);
b(1) = Y(j-1);
c(1) = Z(j-1);
for k = 1:N
SC = 0.0;
for i= 0:k-1
SC = SC+ a(i+1)*c(k-i);
end
a(k+1) = (-b(k) - c(k))/k;
b(k+1) = (a(k)+A*b(k))/k ;
c(k+1) = (SC+B-C*(c(k)))/k ;
end
x = a(1);
y = b(1);
z = c(1);
for k = 2:N+1
x = x + a(k)*dT^(k-1);
y = y + b(k)*dT^(k-1);
z = z + c(k)*dT^(k-1);
end
end
T(j) = T(j-1) + dT;
X(j) = x;
Y(j) = y;
Z(j) = z;
end
plot(A,X)
how to plot the A vs X
shiv gaur on 1 Apr 2022
what is the problem in looping I am not able to plot

Riccardo Scorretti on 1 Apr 2022
Hi Shiv,
up to my understanding you want to execute a kind of parametric study, basing on a parameter A (and perhaps in future on B and C). If so, the point is that:
1. indeed, A iterates only one time with value A = 0.1,
2. most importantly, you was trying to plot X (= a vector) versus A (= a scalar). If the purpose is to plot the result of computation for several values of A, you should plot X as a function of T inside the outer loop, and hold on the graphic.
Perhaps, you meant to do this?
Tstart = 0.0;
Tend = 1000.0;
Nt = 20000;
dT = (Tend-Tstart)/Nt;
X0 = 0.0;
Y0 = 1.0;
Z0 = 0.0;
N = 20;
figure; % ***
for A=0.1:1
B=0.2;
C=5.7;
T = zeros(Nt+1,1);
X = zeros(Nt+1,1);
Y = zeros(Nt+1,1);
Z = zeros(Nt+1,1);
a = zeros(N+1,1);
b = zeros(N+1,1);
c = zeros(N+1,1);
T(1) = 0.0;
X(1) = X0;
Y(1) = Y0;
Z(1) = Z0;
for j = 2:Nt+1
a(1) = X(j-1);
b(1) = Y(j-1);
c(1) = Z(j-1);
for k = 1:N
SC = 0.0;
for i= 0:k-1
SC = SC+ a(i+1)*c(k-i);
end
a(k+1) = (-b(k) - c(k))/k;
b(k+1) = (a(k)+A*b(k))/k ;
c(k+1) = (SC+B-C*(c(k)))/k ;
end
x = a(1);
y = b(1);
z = c(1);
for k = 2:N+1
x = x + a(k)*dT^(k-1);
y = y + b(k)*dT^(k-1);
z = z + c(k)*dT^(k-1);
end
end
T(j) = T(j-1) + dT;
X(j) = x;
Y(j) = y;
Z(j) = z;
plot(T,X); % ***
hold on; % ***
end % plot(A,X)

R2021b

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