从函数输出中提取值：在 R 中答案

【问题标题】：Extracting values from a function output: In R从函数输出中提取值：在 R 中
【发布时间】：2016-05-11 13:50:10
【问题描述】：

对于一个示例数据框：

df <- structure(list(region = structure(c(1L, 1L, 1L, 1L, 1L, 2L, 2L, 
2L, 2L, 2L, 2L), .Label = c("a", "b", "c", "d"), class = "factor"), 
    result = c(0L, 1L, 1L, 0L, 0L, 1L, 0L, 0L, 0L, 1L, 0L), weight = c(0.126, 
    0.5, 0.8, 1.5, 5.3, 2.2, 3.2, 1.1, 0.1, 1.3, 2.5)), .Names = c("region", 
"result", "weight"), row.names = c(NA, 11L), class = "data.frame")

我正在使用函数计算relative risk：

#Relative risk function
calcRelativeRisk <- function(mymatrix,alpha=0.05,referencerow=2)
{
  numrow <- nrow(mymatrix)
  myrownames <- rownames(mymatrix)
  for (i in 1:numrow)
  {
    rowname <- myrownames[i]
    DiseaseUnexposed <- mymatrix[referencerow,1]
    ControlUnexposed <- mymatrix[referencerow,2]
    if (i != referencerow)
    {
      DiseaseExposed <- mymatrix[i,1]
      ControlExposed <- mymatrix[i,2]
      totExposed <- DiseaseExposed + ControlExposed
      totUnexposed <- DiseaseUnexposed + ControlUnexposed
      probDiseaseGivenExposed <- DiseaseExposed/totExposed
      probDiseaseGivenUnexposed <- DiseaseUnexposed/totUnexposed

      # calculate the relative risk
      relativeRisk <- probDiseaseGivenExposed/probDiseaseGivenUnexposed
      print(paste("category =", rowname, ", relative risk = ",relativeRisk))

      # calculate a confidence interval
      confidenceLevel <- (1 - alpha)*100
      sigma <- sqrt((1/DiseaseExposed) - (1/totExposed) +
                      (1/DiseaseUnexposed) - (1/totUnexposed))
      # sigma is the standard error of estimate of log of relative risk
      z <- qnorm(1-(alpha/2))
      lowervalue <- relativeRisk * exp(-z * sigma)
      uppervalue <- relativeRisk * exp( z * sigma)
      print(paste("category =", rowname, ", ", confidenceLevel,
                  "% confidence interval = [",lowervalue,",",uppervalue,"]"))
    }
  }
}

首先创建 xtab：

df$region <- factor(df$region)
result <- xtabs(weight ~ region + result, data=df)
result

然后使用函数计算相对风险：

calcRelativeRisk(result,alpha=0.05)
[1] "category = a , relative risk =  1.26904794624327"
[1] "category = a ,  95 % confidence interval = [ 0.751148304223936 , 2.14402759189898 ]"

我想标记相对风险（“RR”）和置信区间“RR_upper”和“RR_lower”。这是因为我正在创建一个大表，此代码在多个数据帧上运行。如何从 R 输出中提取这些值？（然后围绕它们等）。我想我可以更改函数打印选项，但由于我没有创建函数，我想知道是否还有其他方法？

【问题讨论】：

标签： r

【解决方案1】：

最好的办法是将打印语句更改为输出数据的内容。如果将calcRelativeRisk(result,alpha=0.05) 包装在str 或typeof 中，则会得到NULL。 print 语句不保存值并返回一个 NULL 对象。

在 print 语句之后，在函数的最后一行，添加一个命名向量：

c("relative_risk"=relativeRisk,  "lowervalue"=lowervalue, "uppervalue"=uppervalue)

这将返回一个长度为 3 的数字向量，然后您可以使用它来构建表。

所以你的函数的底部看起来像这样：

    lowervalue <- relativeRisk * exp(-z * sigma)
      uppervalue <- relativeRisk * exp( z * sigma)
      # print values of interest
      print(paste("category =", rowname, ", ", confidenceLevel,
                  "% confidence interval = [",lowervalue,",",uppervalue,"]"))


    }
  }
  # return values of interest
  c("relative_risk"=relativeRisk,  "lowervalue"=lowervalue, "uppervalue"=uppervalue)
}

要检索这些值，只需使用以下命令：

myValues <- calcRelativeRisk(result,alpha=0.05)

【讨论】：