Mathematica 教程

# Special Forms of Assignment

Particularly when you write procedural programs in Mathematica, you will often need to modify the value of a particular variable repeatedly. You can always do this by constructing the new value and explicitly performing an assignment such as x=value. Mathematica, however, provides special notations for incrementing the values of variables, and for some other common cases.
 i++ increment the value of i by 1 i-- decrement i ++i pre-increment i --i pre-decrement i i+=di add di to the value of i i-=di subtract di from i x*=c multiply x by c x/=c divide x by c

Modifying values of variables.

This assigns the value 7x to the variable t.
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This increments the value of t by 18x.
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The value of t has been modified.
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This sets t to 8, multiplies its value by 7, then gives the final value of t.
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 The value of i++ is the value of i before the increment is done.
 The value of ++i is the value of i after the increment.
 x=y=value assign the same value to both x and y {x,y}={value1,value2} assign different values to x and y {x,y}={y,x} interchange the values of x and y

Assigning values to several variables at a time.

This assigns the value 5 to x and 8 to y.
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This interchanges the values of x and y.
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Now x has value 8.
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And y has value 5.
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You can use assignments to lists to permute values of variables in any way.
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When you write programs in Mathematica, you will sometimes find it convenient to take a list, and successively add elements to it. You can do this using the functions PrependTo and AppendTo.
 PrependTo[v,elem] prepend elem to the value of v AppendTo[v,elem] append elem v={v,elem} make a nested list containing elem

Assignments for modifying lists.

This assigns the value of v to be the list {5, 7, 9}.
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This appends the element 11 to the value of v.
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Now the value of v has been modified.
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Although AppendTo[v, elem] is always equivalent to v=Append[v, elem], it is often a convenient notation. However, you should realize that because of the way Mathematica stores lists, it is usually less efficient to add a sequence of elements to a particular list than to create a nested structure that consists, for example, of lists of length 2 at each level. When you have built up such a structure, you can always reduce it to a single list using Flatten.
This sets up a nested list structure for w.
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You can use Flatten to unravel the structure.
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