R-粘贴文本的功能-其中的NULL字答案

【问题标题】：R- function to paste text - NULL word inside itR-粘贴文本的功能-其中的NULL字
【发布时间】：2016-07-12 19:15:01
【问题描述】：

我需要创建一个函数来接收变量名和操作并创建代码以在 sql 中运行。我写了一个函数：

texto <- function(var,opera) {
    paste0(
        opera, '(b.', var, ') as ', var, '_', opera, '_12,', '\n',
        opera, '(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M6 then b.', var, ' else 0 end) as ', var,'_', opera,'_6,', '\n',
        opera, '(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M3 then b.', var, ' else 0 end) as ', var,'_', opera,'_3,', '\n',
        opera, '(case when(b.cd_Anio*100+b.cd_Mes)=a.cd_periodo_observacion then b.', var, ' else 0 end) as ', var,'_', opera,'_1,'
    )
}

带有变量和操作列表：

vars <- list('vl_Saldo_Prom_Deu_Aco', 'vl_Saldo_Prom_Deu_Exc')
ops <- list('sum', 'max')

我运行一个循环：

for (i in vars) {
    for (j in ops) {
        print(cat(texto(i,j)))
    }
}

我得到：

sum(b.vl_Saldo_Prom_Deu_Aco) as vl_Saldo_Prom_Deu_Aco_sum_12,
sum(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M6 then b.vl_Saldo_Prom_Deu_Aco else 0 end) as vl_Saldo_Prom_Deu_Aco_sum_6,
sum(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M3 then b.vl_Saldo_Prom_Deu_Aco else 0 end) as vl_Saldo_Prom_Deu_Aco_sum_3,
sum(case when(b.cd_Anio*100+b.cd_Mes)=a.cd_periodo_observacion then b.vl_Saldo_Prom_Deu_Aco else 0 end) as vl_Saldo_Prom_Deu_Aco_sum_1,NULL
max(b.vl_Saldo_Prom_Deu_Aco) as vl_Saldo_Prom_Deu_Aco_max_12,
max(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M6 then b.vl_Saldo_Prom_Deu_Aco else 0 end) as vl_Saldo_Prom_Deu_Aco_max_6,
max(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M3 then b.vl_Saldo_Prom_Deu_Aco else 0 end) as vl_Saldo_Prom_Deu_Aco_max_3,
max(case when(b.cd_Anio*100+b.cd_Mes)=a.cd_periodo_observacion then b.vl_Saldo_Prom_Deu_Aco else 0 end) as vl_Saldo_Prom_Deu_Aco_max_1,NULL
sum(b.vl_Saldo_Prom_Deu_Exc) as vl_Saldo_Prom_Deu_Exc_sum_12,
sum(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M6 then b.vl_Saldo_Prom_Deu_Exc else 0 end) as vl_Saldo_Prom_Deu_Exc_sum_6,
sum(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M3 then b.vl_Saldo_Prom_Deu_Exc else 0 end) as vl_Saldo_Prom_Deu_Exc_sum_3,
sum(case when(b.cd_Anio*100+b.cd_Mes)=a.cd_periodo_observacion then b.vl_Saldo_Prom_Deu_Exc else 0 end) as vl_Saldo_Prom_Deu_Exc_sum_1,NULL
max(b.vl_Saldo_Prom_Deu_Exc) as vl_Saldo_Prom_Deu_Exc_max_12,
max(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M6 then b.vl_Saldo_Prom_Deu_Exc else 0 end) as vl_Saldo_Prom_Deu_Exc_max_6,
max(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M3 then b.vl_Saldo_Prom_Deu_Exc else 0 end) as vl_Saldo_Prom_Deu_Exc_max_3,
max(case when(b.cd_Anio*100+b.cd_Mes)=a.cd_periodo_observacion then b.vl_Saldo_Prom_Deu_Exc else 0 end) as vl_Saldo_Prom_Deu_Exc_max_1,NULL

这几乎没问题，除了 NULL 字。我想知道为什么要打印它以及如何避免它。

【问题讨论】：

标签： r function paste cat

【解决方案1】：

cat() 函数将文本直接打印到标准输出流。它不返回任何东西。或者更准确地说，它总是不可见地返回一个 NULL 值，正如Value 部分下的文档所述。因此，您的 print() 调用正在打印 NULL，这会导致将未经修饰的 NULL 打印到标准输出流。

因此，NULL 文本不是您在texto() 中生成的 SQL 的一部分。 NULL 在您的控制台上的出现是您错误的print() 调用的产物，仅此而已。

您可以通过完全删除 print() 调用来解决问题。没有必要，因为cat() 已经将文本直接打印到标准输出。

但是，由于在生成的 SQL 的末尾没有包含换行符，我建议您通过将其作为第二个参数传递给 cat() 获取的可变参数列表，在 cat() 调用中添加一个换行符。因此我们有：

for (i in vars) for (j in ops) cat(texto(i,j),'\n');
## sum(b.vl_Saldo_Prom_Deu_Aco) as vl_Saldo_Prom_Deu_Aco_sum_12,
## sum(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M6 then b.vl_Saldo_Prom_Deu_Aco else 0 end) as vl_Saldo_Prom_Deu_Aco_sum_6,
## sum(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M3 then b.vl_Saldo_Prom_Deu_Aco else 0 end) as vl_Saldo_Prom_Deu_Aco_sum_3,
## sum(case when(b.cd_Anio*100+b.cd_Mes)=a.cd_periodo_observacion then b.vl_Saldo_Prom_Deu_Aco else 0 end) as vl_Saldo_Prom_Deu_Aco_sum_1,
## max(b.vl_Saldo_Prom_Deu_Aco) as vl_Saldo_Prom_Deu_Aco_max_12,
## max(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M6 then b.vl_Saldo_Prom_Deu_Aco else 0 end) as vl_Saldo_Prom_Deu_Aco_max_6,
## max(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M3 then b.vl_Saldo_Prom_Deu_Aco else 0 end) as vl_Saldo_Prom_Deu_Aco_max_3,
## max(case when(b.cd_Anio*100+b.cd_Mes)=a.cd_periodo_observacion then b.vl_Saldo_Prom_Deu_Aco else 0 end) as vl_Saldo_Prom_Deu_Aco_max_1,
## sum(b.vl_Saldo_Prom_Deu_Exc) as vl_Saldo_Prom_Deu_Exc_sum_12,
## sum(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M6 then b.vl_Saldo_Prom_Deu_Exc else 0 end) as vl_Saldo_Prom_Deu_Exc_sum_6,
## sum(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M3 then b.vl_Saldo_Prom_Deu_Exc else 0 end) as vl_Saldo_Prom_Deu_Exc_sum_3,
## sum(case when(b.cd_Anio*100+b.cd_Mes)=a.cd_periodo_observacion then b.vl_Saldo_Prom_Deu_Exc else 0 end) as vl_Saldo_Prom_Deu_Exc_sum_1,
## max(b.vl_Saldo_Prom_Deu_Exc) as vl_Saldo_Prom_Deu_Exc_max_12,
## max(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M6 then b.vl_Saldo_Prom_Deu_Exc else 0 end) as vl_Saldo_Prom_Deu_Exc_max_6,
## max(case when(b.cd_Anio*100+b.cd_Mes)>=a.cd_periodo_observacion_M3 then b.vl_Saldo_Prom_Deu_Exc else 0 end) as vl_Saldo_Prom_Deu_Exc_max_3,
## max(case when(b.cd_Anio*100+b.cd_Mes)=a.cd_periodo_observacion then b.vl_Saldo_Prom_Deu_Exc else 0 end) as vl_Saldo_Prom_Deu_Exc_max_1,

有关 R 返回值可见性的更多信息，我建议阅读http://rfunction.com/archives/799，以及invisible() 和withVisible() 这两个函数。

【讨论】：