Import tab-delimited file into Mathematica and create a Plot of 2 columns

For my master thesis which is basically about graph theory and community identification in graphs/networks I purchased a copy of Mathematica to help me with creating plots and other visualizations of my data. While I am learning Mathematica step by step every day, I want to share some of my learnings as I have noticed that there is not too much information in blogs although the internal documentation is pretty good. But sometimes I miss the „learn by examples“-way as you can find it in lots of software development blogs. Here we go…

This blog post is about how to import a CSV or tab-delimited file into mathematica and how to work with the data.
You can also  download the related Mathematica notebook of this blogpost.

The file looks like this:

x y
3 9
4 16
5 25
10 100
9 81
8 64
6 36
7 49
1 1
2 4

1. Extracting x and y values from data
also see

1. Import the file into a variable called data

data = Import["~tmp/my-tab-delimited-file.txt", "Table", "HeaderLines" -> 1]
Out: {{3, 9}, {4, 16}, {5, 25}, {10, 100}, {9, 81}, {8, 64}, {6, 36}, {7, 49}, {1, 1}, {2, 4}}
The "HeaderLines" -> 1 is responsible for skipping the first line, if that contains headers for each column. 

2. get xvalues from the first column
xvals = data[[All, 1]]
Out: {3, 4, 5, 10, 9, 8, 6, 7, 1, 2}

3. get yvalues from the 4th column.
yvals = data[[All, 2]]
Out: {9, 16, 25, 100, 81, 64, 36, 49, 1, 4}

Now assume you have a file where the first column contains the x-value and the second column contains the y-value and you want to create a plot of this.

ListPlot[Table[	{data[[n, 1]], data[[n, 2]]},
  		{n, Length[data[[All, 1]]]}]
 		, AxesLabel -> {"my x axis", "my y-axis"}]

This will create a plot where the x-value is taken from the first column and the y-value from the second column.
When I first tried to do that I made the mistake and tried this:

ListPlot[{data[[All, 1]], data[[All, 2]]}
 		, AxesLabel -> {"my x axis", "my y-axis"}]

But this will create two separate plots (in two different colors) and it will assume that data[[All,1]] are the y-values for the first plot (color 1) and data[[All,2]] are the y-values for the second plot (color 2) which is wrong.

Ok, ListPlot created you a nice plot with points. Now we want to have a line-plot where each point is connected by a line.
The easiest way is to just change it from ListPlot to ListLinePlot:

ListLinePlot[Table[	{data[[n, 1]], data[[n, 2]]},
  		{n, Length[data[[All, 1]]]}]
 		, AxesLabel -> {"my x axis", "my y-axis"}]

This creates lines but if the order of your x-values in your file is not sorted (e.g. ordered ascending) then this will just display a chaotic bunch of lines. We need to sort your data by the x-values (first column of your file) to have a proper line plot. So before rendering the ListLinePlot apply the following before:

sorted = Sort[data]
Out: {{1, 1}, {2, 4}, {3, 9}, {4, 16}, {5, 25}, {6, 36}, {7, 49}, {8,
  64}, {9, 81}, {10, 100}}

ListLinePlot[Table[	{sorted[[n, 1]], sorted[[n, 2]]},
  		{n, Length[sorted[[All, 1]]]}]
 		, AxesLabel -> {"my x axis", "my y-axis"}]

This should give you a proper ListLinePlot where the lines are going from low x-values to high x-values.

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