Handling CSV files

Let's learn how to input and output data using .csv files

We'll cover the following

What are CSV files?

A CSV file is a Comma Separated Values file. It’s just like any other ordinary text file, however, it has the .csv extension and uses special characters as separators between data.

Suppose we have a table of students’ names, their ages, and their scores on a particular test. The table will look something like this.

Name Age Marks
Andrew 28 55.2
Mathew 23 99.5
Dany 29 71
Philip 29 21.9
John 28 44
Brian 23 11.5
Monica 29 45

In a .csv file format, we can store the above data as:

Name,   Age, Marks
Andrew, 28,  55.2
Mathew, 23,  99.5
Dany,   29,  71
Philip, 29,  21.9
John,   28,  44
Brian,  23,  11.5
Monica, 29,  45

CSV files can be used for exchanging data between different applications that is input and output data. CSV files mostly use the comma character to separate (or delimit) data, but sometimes use other characters, like semicolons ;.

Taking Input from CSV file

Taking input from a .csv file is easy. We use the function read.csv() to fetch all the data. Simply pass the file path to this function and you will get all the data.

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main.r
data.csv
csvData = read.csv("data.csv")
print(csvData)

Data Type of Input from CSV

The data that we receive from a .csv file can be best described as a table; therefore, this data is stored as a data frame by the compiler.

We can use the class() function to confirm this.

Remember, the class() function returns the high level data type of any object.

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main.r
data.csv
csvData = read.csv("data.csv")
class(csvData)

In the code snippet above, we use the class() function on the variable csvData that stores the entire data from the data.csv file.

Now that we know that the data from a .csv file is returned as a data frame we can simply perform all data manipulation techniques we learned for data frames on this data as well.

For example, we can print the maximum marks present in the data:

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main.r
data.csv
csvData = read.csv("data.csv")
maximumMarks <- max(csvData$Marks)
print(maximumMarks)

In Line number 3 we first reach the entire Marks column by using the syntax:

csvData$Marks

Since the variable csvData is a data frame we were able to access the columns using this method. Then for the data of this entire column, we use the max() built-in R function to find the maximum value.

Output Data Frame to CSV file

We noticed that when we fetch data from a .csv file it is in the format of a data frame. Similarly, we can output a data frame to a .csv file as well.

Let’s take an example:

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myDataframe <- data.frame(foo = c(1, 2, 3, 4, 5), bar = c(T, F, T, F, T)) 
print(myDataframe)

Now, let’s modify the above code, so that it writes a csv file with the same data.

For that we will need the function write.csv(). It takes the data to be written and the file path as arguments.

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myDataframe <- data.frame(foo = c(1, 2, 3, 4, 5), bar = c(T, F, T, F, T)) 
write.csv(myDataframe, "output/outMyDataframe.csv")