如何将单个列名粘贴在R中的列含义前面

我想将月份的名称粘贴到我的数据集的月平均气温前面，但我try 的都不起作用.我不能使用非常复杂的代码，因为它是针对我的课程的，而我们是从基础开始的.对于这个错误，有人有简单的解决方案吗？

以下是我要运行的完整程序:

for(i in raw[2:13]) {
  avg <- round(mean(i), digits = 1)
  monthavg <- cat(colnames(i), ":", avg, "ºC \n")
}

这就是我想要的结果:

January: -9.2 ºC 
February: -7.3 ºC 
March: -3 ºC 
April: 4.2 ºC 
May: 9.6 ºC 
June: 13.4 ºC 
July: 16.4 ºC 
August : 15.4 ºC 
September: 10.7 ºC 
October: 5.4 ºC 
November: -2.3 ºC 
December: -6.8 ºC 

"colname"是需要调试的部分.以下是我try 过的不同变体中仅有的3个结果:

第一次try :

> for(i in raw[2:13]) {
+   avg <- round(mean(i), digits = 1)
+   monthavg <- cat(month.name, ":", avg, "ºC \n")
+ }
January February March April May June July August September October November December : -9.2 ºC 
January February March April May June July August September October November December : -7.3 ºC 
January February March April May June July August September October November December : -3 ºC 
January February March April May June July August September October November December : 4.2 ºC 
January February March April May June July August September October November December : 9.6 ºC 
January February March April May June July August September October November December : 13.4 ºC 
January February March April May June July August September October November December : 16.4 ºC 
January February March April May June July August September October November December : 15.4 ºC 
January February March April May June July August September October November December : 10.7 ºC 
January February March April May June July August September October November December : 5.4 ºC 
January February March April May June July August September October November December : -2.3 ºC 
January February March April May June July August September October November December : -6.8 ºC 

第二次try :

> for(i in raw[2:13]) {
+   avg <- round(mean(i), digits = 1)
+   monthavg <- cat(colnames(i), ":", avg, "ºC \n")
+ }
 : -9.2 ºC 
 : -7.3 ºC 
 : -3 ºC 
 : 4.2 ºC 
 : 9.6 ºC 
 : 13.4 ºC 
 : 16.4 ºC 
 : 15.4 ºC 
 : 10.7 ºC 
 : 5.4 ºC 
 : -2.3 ºC 
 : -6.8 ºC 

第三次try :

> for(i in raw[2:13]) {
+   avg <- round(mean(i), digits = 1)
+   monthavg <- cat(colnames(raw[i]), ":", avg, "ºC \n")
+ }
 : -9.2 ºC 
Error in `[.default`(raw, i) : 
  only 0's may be mixed with negative subscripts