I get "vectors must be the same length". How come?

syms x

x1 = 0:0.1:5

x2 = 5:0.1:35

x = [x1 x2]

C1 = 15

C2 = 15 - 13/30. * (x2-5)

C = [C1 C2]

plot (x,C)

Answer 1

[Torsten]

How do you want to plot a vector of length 302 against a vector of length 352 ? It's not possible.

x1 = 0:0.1:5;
x2 = 5:0.1:35;
x = [x1 x2];
C1 = 15;
C2 = 15 - 13/30. * (x2-5);
C = [C1 C2];
numel(x)

ans = 352

numel(C)

ans = 302

plot (x,C)

Error using plot
Vectors must be the same length.

Answer 2

[Steven Lord]

If you want the value of C1 to be a constant 15 for all the values of x1, you need to explicitly expand C1 to be a vector of that size. MATLAB doesn't "notice that you're going to use x and C in a later call and so automatically expand the size of C1" or anything like that.

x1 = 0:0.1:5;
x2 = 5:0.1:35;
x = [x1 x2];
C1 = repmat(15, size(x1)); % Modified code here
C2 = 15 - 13/30. * (x2-5);
C = [C1 C2];
plot (x,C)

Instead of using repmat you could have multiplied it by an array of ones or added it to an array of zeros of the correct size.

Alternately you could have modified your x1 and your C1 to only include the endpoints.

x3 = [0 5];
C3 = [15 15];
x = [x3, x2];
C = [C3, C2];
figure
plot(x, C)