处理矩阵或者数据框时，有时会对计算每行或每列中某些数值或某一个或几个字符的个数。当该数字或字符出现一定次数后选择保留或舍弃当前行的数据。
翻了半天Google终于找到两种解决方案。如下：
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> m <- matrix(c("a","b","c","b","b","c","c","c","a","c","c","b"), ncol=3)
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> m
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[,1] [,2] [,3]
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[1,] "a" "b" "a"
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[2,] "b" "c" "c"
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[3,] "c" "c" "c"
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[4,] "b" "c" "b"
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> m[rowSums(m=="b"|m=="a")>1,] # 方法1
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[,1] [,2] [,3]
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[1,] "a" "b" "a"
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[2,] "b" "c" "b"
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> m[apply(m=="b",1,sum)>1,] #方法2
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[1] "b" "c" "b"
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> m[rowSums(m=="b"|m=="a")==2,]
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[1] "b" "c" "b"
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