Filling a histogram

ROOT
1

Hey, not sure if this is the place for this post. I am learning the basics, trying to fill a histogram of 10 bins with specific values with Fill().

Bin Value
1 2
2 4
3 0
4 5
5 1
6 7
7 8
8 6
9 4
10 5

How can I do this? I tried to create an array and fill it with a loop like this:

root [14] TH1F hist3(“hist3”,“practice”,10,1,10);
root [15] int y [10] = { 2, 4, 0, 5, 1, 7, 8, 6, 4, 5 };
root [16] for (int i=0;i<5;i++) hist3.Fill(y[i]);
root [17] hist3.Draw()
root [18] hist3.Draw();

but this is what was drawn.

Shouldn’t I have 10 events with the frequency of the value’s occurrence shown by the vertical axis?

Thanks for any help.

hist3.gif696×472 5.66 KB

2

Hi lpf,

with “10,1,10” you end up having 10 bins, each 0.9 units wide:

1 <= x <1.9 1.9 <= x < 2.8 2.8 <= x < 3.7 3.7 <= x < 4.6 4.6 <= x < 5.5 ...
In your code you loop from index 0 to index 4, so you are trying to fill the histo with values 2, 4, 0, 5, 1.

2 goes in the second bin: 4 in the fourth; 0 goes in the “underflow” bin (outside, to the left); 5 in the fifth bin, and 1 in the first. The plot you have is correct: 4 bins with 1 event each.

EDIT: I have a couple of ideas of what you were expecting, maybe you can try these two different sets of commands and see if they are useful… :wink:

root [0] TH1F hist3("hist3","practice",10,0,10); // changed 1 to 0 root [1] int y [10] = { 2, 4, 0, 5, 1, 7, 8, 6, 4, 5 }; root [2] for (int i=0;i<10;i++) hist3.Fill(y[i]); // changed <5 to <10 root [3] hist3.Draw();

root [0] TH1F hist3("hist3","practice",10,1,10); root [1] int y [10] = { 2, 4, 0, 5, 1, 7, 8, 6, 4, 5 }; root [2] for (int i=0;i<10;i++) hist3.SetBinContent(i+1,y[i]); // replaced Fill with SetBinContent root [3] hist3.Draw();