how can I write a multiplication of variables to graph?

23 次查看（过去 30 天）
Erwin Avendaño
Erwin Avendaño 2018-3-11
评论： John D'Errico 2018-3-11
I want to graph x. ^ 2-4 * xy +5 but matlab does not let me say that it is badly written or that it declares variable "xy" but it is that "xy" is not a variable, it is two variables multiplying, what should I do? :(

请先登录，再回答此问题。

回答（2 个）

John D'Errico
John D'Errico 2018-3-11
编辑：John D'Errico 2018-3-11
MATLAB does not know that you have decided that xy is the product of two variables. Mind reading is not a strong point for computers. Should it know that xy is not just a variable, named xy? You knew that you have to multiply by 4. So why not an operator between x and y?
Multiplication between variables x and y in MATLAB is accomplished by x*y or x.*y, depending on what you are doing, and the shapes of x and y.
Therefore, when you write xy, MATLAB looks for a variable named xy. Failing that, it looks for a function named xy. Failing that, it throws up its virtual hands, and gives up, throwing an error. I suppose, they might have decided to see if variables named x and y were defined separately. But suppose you wrote an expression with something like abcdefgh? Should MATLAB try to look if ANY combination of existing variables and functions could be used here, inserting an implicit * where it wanted? That is surely a fast path to buggy code.
  5 个评论
显示 3更早的评论
Erwin Avendaño
Erwin Avendaño 2018-3-11
What happens is that X, and if they have values because I put [x, y] = (2: .2: 4,1: .2: 6) but even so it tells me an error that declares the variable "xy"
John D'Errico
John D'Errico 2018-3-11
If you try to define x and y as
[x, y] = (2: .2: 4,1: .2: 6)
This is not in itself legal MATLAB syntax. So x and y are never defined in the first place as you want to write it.
You could never write reliable code that understands how to combine variables automatically, if parts of the name are written like that. Since it is completely valid to define a variable with the name xy, then all MATLAB can do is see if you have already defined a variable by that name. Otherwise, MATLAB would essentially be limited to only single character variable names, thus the 52 possible variables {a,b,c,...,z,A,B,...,Z}. Surely it should be possible to define more than 52 variables in a programming language?

请先登录，再进行评论。

Steven Lord
Steven Lord 2018-3-11
MATLAB does not implicitly multiply variables whose names are next to one another. If you want to use the product of x and y in your expression, you must use either the * operator (for matrix multiplication) or the .* operator (for element-wise multiplication) between them.
x = 1:10
y = x+1
productOfxAndy = x.*y

类别

在 Help Center 和 File Exchange 中查找有关 Entering Commands 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by