This article describes several commands related to importing and exporting data taking care of specific file structures. Various manipulations of the data are also discussed.

Lets begin with clearing all variable definitions of the current Mathematica session.

Clear["Global`*"]

Our first goal is to import data from a file which contains, lets say, 3 columns; first column is the X-data whereas 2nd and 3rd columns contain Y-data. There is also some Header and Footer text which we need to skip while importing. An example file "DataFile.txt" is shown below.

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Test data

Xdata Ydata1   Ydata2

1         1                1.000

2         4           1.414

3         9           1.732

4         16           4.000

Created on Nov 23, 2016

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HeaderLineNum = 2;

FooterLineNum = 1;

TotalColNum = 3;

file = OpenRead["C:\\Users\\TANAY\\Desktop\\DataFile.txt"];

Skip[file, String, HeaderLineNum];

data = ReadList[file, Table[Number, {i, 1, TotalColNum}]][[;; -(FooterLineNum + 1)]];

Close[file];

Output: {{1, 1, 1.}, {2, 4, 1.414}, {3, 9, 1.732}, {4, 16, 4}}

To select individual columns use the following commands:

xdata = data[[All, 1]];

ydata1 = data[[All, 2]];

ydata2 = data[[All, 3]];

To select individual rows use the following commands:

row1 = data[[1, All]];

row2 = data[[2, All]];

To select two columns from the imported data use the following commands:

data12 = data[[All, {1, 2}]]; (* Column 1 and 2 *)

data13 = data[[All, {1, 3}]]; (* Column 1 and 3 *)

To select all rows of columns 1 through 3 use the following commands:

data1to3 = data[[All, 1 ;; 3]] (* Column 1 to 3 *)

One can use these reconstructed arrays to make a plot:

ListLinePlot[{data12, data13}]; (* Output: two line; not shown *)

Lets first generate a 2D array and store it into variable "arr2d":

arr2d = {};

For[i = 0, i <= 3, arr2d = Join[arr2d, {{i, i^2, N[Sqrt[i]]}} ]; i = i + 1;]

Print[arr2d]

Output: {{0,0,0.},{1,1,1.},{2,4,1.41421},{3,9,1.73205}}

To save the data into a .xlsx (or .xls)  format use the following command:

Export["C:\\Users\\TANAY\\Desktop\\OutputFile.xlsx", arr2d];

OutputFile:

To know the current working directory use:

Directory[]

Output: C:\Users\TANAY\Documents

To set current working directory to a different location use:

SetDirectory["C:\\Users\\TANAY\\Desktop"]

Output: C:\Users\TANAY\Desktop

To set current working directory to the folder where the Mathematica notebook resides:

SetDirectory[NotebookDirectory[]];

In order to find all files (including directories) in the working directory use:

FileNames[]

In order to find all txt files that start with letters "C" or "D" in the working directory use:

files = FileNames[{"C*.txt", "D*.txt"}];

In order to find all directories that contain the letter "C" in the working directory use:

folders = Select[FileNames["*C*"], DirectoryQ];

To create a constant array of length 11 with all elements being 0 use:

ConstantArray[0, 11]

To rotate an array by 2 units use:

RotateRight[array,2]

RotateLeft[array,2]

To select all or first instance

list1 = Table[k, {k, 1, 10}]

Select[list1, # > 5 &]

SelectFirst[list1, # > 5 &]

To select all or first instance positions

Position[list1, _?(# > 5 &)]

Position[list1, x_ /; x > 5]

FirstPosition[list1, _?(# > 5 &)]

FirstPosition[list1, x_ /; x > 5]

To know the kernel time used use:

TimeUsed[]

Operation; // Timing