R- 粘贴文本的功能 - 里面有 NULL 字

Question

我需要创建一个接收变量名和操作的函数，并在 sql 中创建运行的代码。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。

Answer 1

cat() 函数将文本直接打印到标准输出流。它没有 return 任何东西。或者更准确地说，它总是无形地 return 是一个 NULL 值，如文档在 Value 部分下所述。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，由于生成的 SQL 末尾没有包含换行符，我建议您将一个换行符作为第二个参数传递给可变参数，从而将其添加到 cat() 调用中cat() 采用的参数列表。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 return 值可见性的更多信息，我建议阅读 http://rfunction.com/archives/799, and the two functions invisible() and withVisible()。

R- 粘贴文本的功能 - 里面有 NULL 字

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