# Problem when plotting a graph after for and if else if

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Le Duc Long on 9 Jun 2020
Commented: Le Duc Long on 9 Jun 2020
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C = 100;
g = 20;
s = 0.5;
T = 1;
t = 0.68;
x = 0:0.1:1
if x <t
d = 32/(1.-x*0.2)
else d = C*(1-g/C)^2./(2*(1.-x*g/C))+900*T*((x-1.)+sqrt((x-1).^2+12*C*(x-t)./(T*s*g*3600)))
end
plot(x,d)
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Anyone help me. I dont understant why d(x=0)=21.64.
while: d(x=0<0.68)=32*(1-0)=32
Thanks so much!

#### 1 Comment

Le Duc Long on 9 Jun 2020
Thank you so much.

Image Analyst on 9 Jun 2020
Edited: Image Analyst on 9 Jun 2020
It's because of the way you set up your if statement. You're comparing the whole x to t so you're getting a logical vector and then you're basically saying if vector. You should either vectorize the indexes, or use a for look like this:
C = 100;
g = 20;
s = 0.5;
T = 1;
t = 0.68;
x = 0:0.1:1
for k = 1 : length(x)
if x(k) < t
d(k) = 32 ./ (1 - x(k) * 0.2);
fprintf('For k = %d, x(k) = %.1f, and d(k) = %.1f. x is less than t.\n', k, x(k), d(k));
else
d(k) = C*(1-g/C)^2./(2*(1.-x(k)*g/C))+900*T*((x(k)-1.)+sqrt((x(k)-1).^2+12*C*(x(k)-t)./(T*s*g*3600)));
fprintf('For k = %d, x(k) = %.1f, and d(k) = %.1f. x is greater than t.\n', k, x(k), d(k));
end
end
plot(x, d, 'b.-', 'MarkerSize', 20)
grid on;
d
Here is the vectorized version:
C = 100;
g = 20;
s = 0.5;
T = 1;
t = 0.68;
x = 0:0.1:1
% Initialize all elements to this equation.
d = C*(1-g/C)^2./(2*(1.-x*g/C))+900*T*((x-1.)+sqrt((x-1).^2+12*C*(x-t)./(T*s*g*3600)));
% Now handle case there x is less than t.
indexes = x < t
d(indexes) = 32 ./ (1 - x(indexes) * 0.2);
plot(x, d, 'b.-', 'MarkerSize', 20)
grid on;
d