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I have a problem with defining a function and most of time I get confused by Set or = and SetDelayed or :=. I read the help section but I didn't find out what the difference is between defining a function as y[x_] := ... and y[x_] = ...

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3  
The documentation contains a tutorial dedicated to explaining the difference. –  Simon Woods Jul 29 '12 at 13:28
1  
I discuss that in great detail here. –  Ted Ersek Jul 29 '12 at 13:45
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1 Answer

References and intro

First, let me point out that = is shorthand for Set and := for SetDelayed; this facilitates searching the docs. Also, as Simon Woods points out in a comment to the question, there is a tutorial on this.

Explanation

The basic distinction is this: y[x_]=expr means evaluate expr, then whenever you see y[something] evaluate evaluate what resulted. On the other hand, y[x_]:=expr means "whenever you see y[something], evaluate expr anew".

Here's how to see it:

a = 5;
y[x_] = a*x

y[3]
a = 10
y[3]
(*
15
10
15
*)

That is, when you define y, it evaluates the right hand side to 5*x and assigns that; if you change a later, it never sees it. On the other hand,

a = 5;
f[x_] := a*x

f[3]
a = 10
f[3]
(*
15
10
30
*)

Compare also:

?? y

Mathematica graphics

So, the value of a at the time of definition has been "baked in", while with SetDelayed, we get

??f

Mathematica graphics

that is, the value of a at execution time is what will be used.

Pitfalls

Here is an example where using SetDelayed results in a calculation being unnecessarily performed multiple times:

fsd[x_] := Integrate[z, {z, 0, x}]
gs[x_] = Integrate[z, {z, 0, x}];

If I try with a number, they give the same answer. But look at the DownValues:

??fsd

Mathematica graphics

??gs

Mathematica graphics

So, in gs, the integration has already been done, while in fsd it is performed anew every time fsd is evaluted. Observe:

t1 = Table[fsd[x], {x, 0, 1, .05}]; // AbsoluteTiming
t2 = Table[gs[x], {x, 0, 1, .05}]; // AbsoluteTiming
(*
{0.061729, Null}
{0.000061, Null}
*)

and t1 == t2 evaluates to True. The reason for the timing differences is precisely that the symbolic integration is done every time for one, only once for the other.

Another possible pitfall is using an already-defined symbold for the right hand side. For instance, consider the difference between these:

ClearAll[f, g];
x = 5;
f[x_] := Sin[x];
g[x_] = Sin[x];

f[1]
g[1]
(*
Sin[1]
Sin[5]
*)

A simple way to avoid this is to simply use a formal symbol:

h[\[FormalX]_] = Sin[\[FormalX]]

which looks like this in the FrontEnd:

Mathematica graphics

Memoization

As a final note, one may combine Set and SetDelayed to implement memoization. Here is how to calculate a Fibonacci number recursively, with

ClearAll[fib];
fib[1] = 1;
fib[2] = 1;
fib[n_Integer] := fib[n] = fib[n - 1] + fib[n - 2]

and without

ClearAll[fibnaive];
fibnaive[1] = 1;
fibnaive[2] = 1;
fibnaive[n_Integer] := fibnaive[n - 1] + fibnaive[n - 2]

memoization. The idea behind this is explained, for instance, here or here. You can also find some elaborations here.

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for being clear y[x_]= : it is like one time function y[x_]:= it is like normal funtion ?? –  DSaad Jul 29 '12 at 13:17
2  
+1 but please include the names Set and SetDelayed in your answer, for reference and easy searching. –  Mr.Wizard Jul 29 '12 at 13:19
    
Is there a difference then between y[x_]= and y[x_]:=y[x]= for caching values ? –  Faysal Aberkane Jul 29 '12 at 13:53
    
@DSaad, no, take what acl said more literally. y[x_]=rhs means "evaluate the rhs to get the expression of the FUNCTION", while y[x_]:=rhs means that rhs is literally the expression of the function –  Rojo Jul 29 '12 at 14:37
    
@FaysalAberkane definately. y[x_]= doesn't cache any values at all –  Rojo Jul 29 '12 at 14:39
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