Histograms in ROOT

ROOT is a popular data analysis tool used in the field of high energy physics. It is a powerful tool that allows users to perform a variety of tasks, including generating histograms. In this article, we will show you how to generate histograms in ROOT.

To create a histogram in ROOT, you first need to have a data set that you want to plot. This data can be stored in a ROOT file or it can be generated directly in your ROOT session. Once you have your data, you can use the TH1 class to create a histogram. This class has several constructors that allow you to specify the number of bins, the range of the data, and other options.

For example, to create a histogram with 10 bins ranging from 0 to 100, you can use the following code:

TH1F h("h", "My Histogram", 10, 0, 100);

The TH1F class is used for histograms with floating point data, while the TH1D class is used for histograms with double precision data. You can also use the TH1I class for histograms with integer data.

Once you have created your histogram, you can fill it with data using the Fill() method. This method takes the value of the data point as an argument and increments the bin corresponding to that value. For example, to fill the histogram with random values between 0 and 100, you can use the following code:

for (int i = 0; i < 1000; i++) {
  h.Fill(gRandom->Uniform(0, 100));
}

The gRandom object is a global random number generator provided by ROOT. The Uniform() method generates a random number between the two specified values (in this case, 0 and 100). Once you have filled your histogram with data, you can draw it on a canvas using the Draw() method. This method takes the name of the histogram and the drawing options as arguments. For example, to draw the histogram with a bar chart style and a blue color, you can use the following code:

h.Draw("BAR", "color=blue");

The BAR option specifies that the histogram should be drawn as a bar chart, while the color=red option specifies that the bars should be red. There are many other drawing options available, such as LINE, SCAT, and LEGO, which can be used to customize the appearance of the histogram.

To draw a histogram as a line chart, you can use the LINE option:

h.Draw("LINE");

In addition to changing the drawing style, you can also customize the colors of the histogram. The Draw() method also takes a string argument that specifies the color. For example, to draw a histogram in red, you can use the following code:

h.Draw("BAR", "color=red");

You can also add labels and titles to the histogram to make it more informative. To add a title to the histogram, you can use the SetTitle() method. This method takes the title as an argument. For example, to add the title “My Histogram” to the histogram, you can use the following code:

h.SetTitle("My Histogram");

To add labels to the x-axis and y-axis, you can use the GetXaxis() and GetYaxis() methods, respectively. These methods return an TAxis object that can be used to customize the labels. For example, to add the labels “X-axis” and “Y-axis” to the x-axis and y-axis, respectively, you can use the following code:

h.GetXaxis()->SetTitle("X-axis");

In addition to these options, the TH1 class provides many other methods and options that can be used to customize histograms. For more information, you can refer to the ROOT documentation.