class22 - PHYS 5900 Class 22 11am-12:15pm Zi-Wei Lin 3.3...

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PHYS 5900 Class 22 (10/19/2009 11am-12:15pm) Zi-Wei Lin 3.3 Functions Pure Functions Named functions and variables are global. They need to be cleared after they are no longer needed in order to avoid problems. So it is better to minimize the number of named functions and variables. To minimize the number of variables, we prefer replacements (transformation rules) over assignments. To minimize the number of named functions, we prefer pure functions. They are anonymous functions (i.e. without names). In[1]:= ?Function Function @ body D or body & is a pure function. The formal parameters are H or 1 L , 2, etc. Function @ x , body D is a pure function with a single formal parameter x . Function @8 x 1 , x 2 , < , body D is a pure function with a list of formal parameters. Forms of a pure function include: Function[x, body] Function[x, body][arg] Function[{x1, x2, ...}, body] Function[{x1, x2, ...}, body][arg] Instead of defining a named function as: In[2]:= f @ x_ D : = 3 * x In[3]:= f @ y D Out[3]= 3y
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we can use a pure function instead: In[4]:= Function @ x, 3 * x [email protected] y D Out[4]= 3y In[5]:= Clear @ f D Short forms of a pure function include: Function[body] or body& Function[body][arg] or body&[arg] where # or (#1, #2 ...) refers to the formal parameter x or (x1, x2 ...) , and arg will replace # . In[6]:= ?# represents the first argument supplied to a pure function. n represents the n th argument. In[7]:= Function @ 3 * [email protected] y D Out[7]= 3y In[8]:= 3 * & @ y D Out[8]= 3y Instead of used a named function: In[9]:= test @ x_ D : = x > 4; In[10]:= Select @8 1, a, x^2, 3, 5., 1 + x, 7 < , test D Out[10]= 8 5., 7 < We can use a pure function: In[11]:= Select @8 1, a, x^2, 3, 5., 1 + x, 7 < , > 4 & D Out[11]= 8 5., 7 < In[12]:= Clear @ test D Example: Earlier we used a test function to constrain a pattern: In[13]:= myExpr = 1 + x + 2 * x^2 + 3 * x^3 + Sin @ x D ; In[14]:= test @ expr_ D : = PolynomialQ @ expr, x D 2 class22.nb
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In[15]:= Select @ myExpr, test D Out[15]= 1 + x + 2x 2 + 3x 3 Now we can use a pure function instead (without defining a test function): In[16]:= Select @ myExpr, Function @ v, PolynomialQ @ v, x DDD Out[16]= 1 + x + 2x 2 + 3x 3 Note: in the above we ask for a polynomial in x , so we have to use another variable name ( v ) for the formal parameter in the pure function. or use the short form of a pure function: either In[17]:= Select @ myExpr, Function @ PolynomialQ @ , x DDD Out[17]= 1 + x + 2x 2 + 3x 3 or In[18]:= Select @ myExpr, PolynomialQ @ , x D & D Out[18]= 1 + x + 2x 2 + 3x 3 In[19]:= Clear @ test, myExpr D Example: Instead of defining a named function as: In[20]:= q @ x_ D : = 1 H 1 + x L In[21]:= Nest @ q, x, 4 D Out[21]= 1 1 + 1 1 + 1 1 + 1 1 + x we can use a pure function instead: In[22]:= Nest @ Function @ v, 1 H 1 + v LD , x, 4 D Out[22]= 1 1 + 1 1 + 1 1 + 1 1 + x class22.nb 3
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or use the short form: either In[23]:= Nest @ Function @ 1 H 1 + LD , x, 4 D Out[23]= 1 1 + 1 1 + 1 1 + 1 1 + x or In[24]:= Nest @H 1 H 1 + LL &, x, 4 D Out[24]= 1 1 + 1 1 + 1 1 + 1 1 + x In[25]:= Clear @ q D Assign attributes to a pure function e.g. by using Function[x,body,attr] This option is given inside ?Function (follow >> ) In[26]:= ?Function Function @ body D or body & is a pure function. The formal parameters are H or 1 L , 2, etc.
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