I have a dataset with a missing column header for first column. How to name it so that I can find it using 

    a$
![screenshot][1]  
[1]: https://image.ibb.co/nNv7Xn/column_name.jpg 

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I used following command but got error

    a$miRNAs<-row.names(a)

    Warning message:
    In a$miRNAs <- row.names(a) : Coercing LHS to a list

How to find the solution?
It is rownames not a column. Since you have mentioned dplyr, we can convert it to columns using packages from tidyverse, here is an example:

    # example dataset with rownames
    a <- mtcars[1:3, 1:3]
    a
    #                mpg cyl disp
    # Mazda RX4     21.0   6  160
    # Mazda RX4 Wag 21.0   6  160
    # Datsun 710    22.8   4  108
    
    library(dplyr)
    library(tibble)
    
    a <- a %>% 
      rownames_to_column(var = "myName")
    
    a
    #          myName  mpg cyl disp
    # 1     Mazda RX4 21.0   6  160
    # 2 Mazda RX4 Wag 21.0   6  160
    # 3    Datsun 710 22.8   4  108

