Difference between a vector in maths and programming

Maybe this question is better suited in the math section of the site but I guess stackoverflow is suited too. In mathematics, a vector has a position and a direction, but in programming, a vector is usually defined as:

Vector v开发者_C百科 (3, 1, 5);

Where is the direction and magnitude? For me, this is a point, not a vector... So what gives? Probably I am not getting something so if anybody can explain this to me it would be very appreciated.

---

If we are working in cartesian coordinates, and assume (0,0,0) to be the origin, then a point p=(3,1,5) can be written as

where i, j and k are the unit vectors in the x, y and z directions. For convenience sake, the unit vectors are dropped from programming constructs.

The magnitude of the vector is

and its direction cosines are

respectively, both of which can be done programmatically. You can also take dot products and cross-products, which I'm sure you know about. So the usage is consistent between programming and mathematics. The difference in notations is mostly because of convenience.

However as Tomas pointed out, in programming, it is also common to define a vector of strings or objects, which really have no mathematical meaning. You can consider such vectors to be a one dimensional array or a list of items that can be accessed or manipulated easily by indexing.

---

In mathematics, it is easy to represent a vector by a point - just say that the "base" of the vector is implied to be the origin. Thus, a mathematical point for all practical purposes is also a representation of a mathematical vector, and the vector in your example has the magnitude sqrt(3^2 + 1^2 + 5^2) = 6 and the direction (1/2, 1/6, 5/6) (a normalized vector from the origin).

However, a vector in programming usually has no geometrical use, which means you really aren't interested in things like magnitude or direction. A vector in programming is rather just an ordered list of items. Important here is that the items need not be numbers - it can be anything handled by the language in question! Thus, ("Hello", "little", "world") is also a vector in programming, although it (obviously) has no vector interpretation in the mathematical sense.

---

Practically speaking (!):

A vector in mathematics is only a direction without a position (actually something more general, but to stay in your terminology). In programming you often use vectors for points. You can think of your vector as the vector pointing from the origin (0,0,0) to the point (3,1,5), called the location vector of the point. Consult texts on analytical and affine geometry for more insight.

---

A Vector in computer science is an "one dimensional" data structure (array) (can be thought as direction) with an usually dynamic size (length/magnitude). For that reason it is called as vector. But it's an array at least.

---

A vector also means a set of coordinates. This is how it is used in programming. Just as a set of numbers. You might want to represent position vectors, velocity vectors, momentum vectors, force vectors with a vector object, or you may wish to represent it any way that suits you.

Many times vector quantities may be represented by 4 coordinates instead of 3 (see homogeneous coordinates in computer graphics) so a physical vector is represented by a computer vector with 4 elements. Alternatively you can store direction and magnitude separately, or encode them with 3, 4 or more coordinates.

I guess what I am getting to, is that computer languages are designed to represent physical models, but abstract data containers that the programmer use as tools for his/hers modeling.

---

Vector in math is an element of n-dimensional space over some field(e.g. real/complex number, functions, string). It may have infinite dimension, e.g. functional space L^2. I don't remember infite-dimensional vectors were used in programming (infinite vectors are not vectors with non-limited length, but vector with infite number of elements)

---

The most rigorous statement is that a mathematical vector is a first-order tensor that transforms from one coordinate system to another according to tensor transformation rules. The physical idea to keep in mind is that vectors have both magnitude and direction.

Programming vectors are data structures that need not transform according to any rules and may or may not have a notion of a coordinate system as reference. If you happen to use a vector data structure to hold numbers, they may conform to the mathematical definition. But if you have a vector of objects, it's unlikely that they have anything to do with coordinate transformations.