This article represents different ways in which one or more columns in a data frame could be converted to factor when working with R programming language. Please feel free to comment/suggest if I missed mentioning one or more important points. Also, sorry for the typos.

Following are the key points described later in this article:

• Convert single column to factor
• Convert multiple columns to factor

Following data frame, df, is used in the code sample below:

  param_a param_b param_c diagnosis param_d
1      23    0.61   10452  positive       y
2      18    0.85    9876  positive       n
3      22    0.32    6534  negative       y
4      37    0.56    8743  positive       y
5      15    0.44    9876  negative       n
6      25    0.13    4321  negative       n
7      55    0.51    7685  positive       y


In above data frame, both diagnosis and param_d are character vectors. One could quickly check classes of all columns using the following command:

sapply(df, class)


Convert Single Column to Factor

Following is demonstrated the code samples along with help text. Pay attention that one could use lapply method to change the single column to factor. However, it does throw warning message.

# Invoke as.factor method on dataframe$columnName df$param_d <- as.factor(df\$param_d)

# Invoke as.factor method on columns represented array notation
df[, 'param_d'] <- as.factor( df[, 'param_d'] )

# Use lapply method; Both of below makes param_d column as factor
df[, 'param_d'] <- lapply(df[, 'param_d'], factor)
df[, c("param_d")] <- lapply(df[, c("param_d")], factor)


Convert Multiple Columns to Factor

Use lapply method to change columns to factor.

df[, c("param_d", "diagnosis")] &lt;- lapply(df[, c("param_d", "diagnosis")], factor)