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Count

Usage

Count[list, pattern]给出了list中匹配pattern的元素个数。
Count[expr, pattern, levelspec] 给出了匹配pattern的子表达式的总个数,这些子表达式出现在expr中由levelspec指定的级别上。


Notes

• 级别规定描述在A.3.6节.
• 参见Mathematica全书: 1.8.5节 and 2.3.2节.
•同时参见: FreeQ, MemberQ, Cases, Select, Position.
Further Examples

(See the Further Examples for FullSimplify for a demonstration of how Count can be used to affect the outcome of a simplification.)

This counts the number of occurrences of Real numbers in the list. (An Integer is not considered to be Real, any more than a Real is considered to be Complex.)

In[1]:=  

Out[1]=

This command returns the number of summands that are powers. (Note that the first argument to Count need not be a list.)

In[2]:=  

Out[2]=

This returns the number of expressions that occur in the sum at any level and are powers. This count is higher; the actual matches can be seen by replacing Count with Cases.

In[3]:=  

Out[3]=