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Re: How to plot functions
Posted:
Mar 11, 2013 1:41 AM


On 3/10/13 at 12:48 AM, radressss@gmail.com (radres) wrote:
>Let's say I've defined a function f[c]=c*x^2
>when I try to plot this directly,
>Plot[ f[3], {x, 10, 10} ]
>it doesn't plot anything.
Right. Define f as follows:
f[c_]:= c*x^2
There are two key points. First, I've used SetDelayed (:=) rather than Set (=). This results in the function f being evaluated when it is used by Plot, not before. Using Set causes f to be evaluated when it is defined. That is usually not the behavior you want for a function.
Second, and more important, I've used c_ not c as the argument for f. The pattern c_ matches anything and gives it a local name of c. So, call f with a numeric value, Mathematica replaces the c on the right hand side with the values supplied.
Without the blank(_) portion and with Set, you have defined f for the symbol c, not anything else. That is f[3] is undefined resulting in no plot being displayed. Notice, the difference between
In[7]:= f[c] = c*x^2; f[3]
Out[7]= f[3]
and
In[8]:= g[c_] = c*x^2; g[3]
Out[8]= 3*x^2



