Something wrong with histrograms...?

ROOT
1

Hi!
I’m writing a script that read the x and weight values from an input file and make a histrogram with those values.
This is the script, really simple:

void plotter(string input)
{
	TCanvas *c = new TCanvas("name", "canvas", 800, 500);
	TH1D *h1 = new TH1D("histo", "title", 161, 1.0000E-12, 1.0000E+04);
	gPad->SetLogx();
	double x;
	double w;
	string line;
	
	ifstream file_input (input.c_str());
	while ( getline(file_input, line) )
	{
		stringstream(line) >> x >> w;
		h1->Fill(x, w);
	}
	h1->Draw("HIST");
}

And this is the input file:

1.00E-012	1
1.26E-012	2
1.58E-012	3
2.00E-012	4
2.51E-012	5
3.16E-012	6
3.98E-012	7
5.01E-012	8
6.31E-012	9
7.94E-012	10
1.00E-011	11
1.26E-011	12
1.58E-011	13
2.00E-011	14
2.51E-011	15
3.16E-011	16
3.98E-011	17
5.01E-011	18
6.31E-011	19
7.94E-011	20
1.00E-010	21
1.26E-010	22
1.58E-010	23
2.00E-010	24
2.51E-010	25
3.16E-010	26
3.98E-010	27
5.01E-010	28
6.31E-010	29
7.94E-010	30
1.00E-009	31
1.26E-009	32
1.58E-009	33
2.00E-009	34
2.51E-009	35
3.16E-009	36
3.98E-009	37
5.01E-009	38
6.31E-009	39
7.94E-009	40
1.00E-008	41
1.26E-008	42
1.58E-008	43
2.00E-008	44
2.51E-008	45
3.16E-008	46
3.98E-008	47
5.01E-008	48
6.31E-008	49
7.94E-008	50
1.00E-007	51
1.26E-007	52
1.58E-007	53
2.00E-007	54
2.51E-007	55
3.16E-007	56
3.98E-007	57
5.01E-007	58
6.31E-007	59
7.94E-007	60
1.00E-006	61
1.26E-006	62
1.58E-006	63
2.00E-006	64
2.51E-006	65
3.16E-006	66
3.98E-006	67
5.01E-006	68
6.31E-006	69
7.94E-006	70
1.00E-005	71
1.26E-005	72
1.58E-005	73
2.00E-005	74
2.51E-005	75
3.16E-005	76
3.98E-005	77
5.01E-005	78
6.31E-005	79
7.94E-005	80
1.00E-004	81
1.26E-004	82
1.58E-004	83
2.00E-004	84
2.51E-004	85
3.16E-004	86
3.98E-004	87
5.01E-004	88
6.31E-004	89
7.94E-004	90
1.00E-003	91
1.26E-003	92
1.58E-003	93
2.00E-003	94
2.51E-003	95
3.16E-003	96
3.98E-003	97
5.01E-003	98
6.31E-003	99
7.94E-003	100
1.00E-002	101
1.26E-002	102
1.58E-002	103
2.00E-002	104
2.51E-002	105
3.16E-002	106
3.98E-002	107
5.01E-002	108
6.31E-002	109
7.94E-002	110
1.00E-001	111
1.26E-001	112
1.58E-001	113
2.00E-001	114
2.51E-001	115
3.16E-001	116
3.98E-001	117
5.01E-001	118
6.31E-001	119
7.94E-001	120
1.00E+000	121
1.26E+000	122
1.58E+000	123
2.00E+000	124
2.51E+000	125
3.16E+000	126
3.98E+000	127
5.01E+000	128
6.31E+000	129
7.94E+000	130
1.00E+001	131
1.26E+001	132
1.58E+001	133
2.00E+001	134
2.51E+001	135
3.16E+001	136
3.98E+001	137
5.01E+001	138
6.31E+001	139
7.94E+001	140
1.00E+002	141
1.26E+002	142
1.58E+002	143
2.00E+002	144
2.51E+002	145
3.16E+002	146
3.98E+002	147
5.01E+002	148
6.31E+002	149
7.94E+002	150
1.00E+003	151
1.26E+003	152
1.58E+003	153
2.00E+003	154
2.51E+003	155
3.16E+003	156
3.98E+003	157
5.01E+003	158
6.31E+003	159
7.94E+003	160
1.00E+004	161

The result is completely different from what I expected. As you can see in the attachments, I made a comparison between the histrogram made by root and a graph with the same input file made by excel.

Why my script completely messes up the histrogram…? o.O
I really don’t understand what’s wrong.

Can you help me, please? :slight_smile:

fileroot.png796×474 3.56 KB



2

void plotter(string input) { TGraph *g = new TGraph(input.c_str()); TCanvas *c = new TCanvas("name", "canvas", 800, 500); gPad->SetLogx(); g->Draw("AP"); }

3

Binned data are not good in your case. do:

{
   TCanvas *c = new TCanvas("name", "canvas", 800, 500);
   gPad->SetLogx();
   double x;
   double w;
   int i = 0;
   string line;
   TGraph *g = new TGraph();

   ifstream file_input ("ziel.dat");
   while ( getline(file_input, line) )
   {
      stringstream(line) >> x >> w;
      g->SetPoint(i,x,w);
      i++;
   }
   g->Draw();
}
4

Actually I’ve been using a TGraphErrors till yesterday, when my colleagues told me that I should use TH1D.
The input file is, indeed an histrogram: in every line the x value is the upper edge of the current bin and the second value is the weight of that bin (i.e. in my input file, 2.00E-012 is the bin’s upper edge of the bin whose weight is 4). You can read the bin’s lower edge from the previous line.
For example, the width of the bin whose weight is 4 is (2.00E-012 - 1.58E-012) since these two values are the upper and the lower value respectively.

1.58E-012 3 2.00E-012 4
It’s a little tricky, but that’s how MCNP writes its output file.

Thanks for you help, guys :slight_smile:

5

I see. You should have done non equidistant bins in that case.
But seems to me a graph is good.

6

[quote=“couet”]I see. You should have done non equidistant bins in that case.
But seems to me a graph is good.[/quote]
What do you mean?

7

root.cern.ch/root/htmldoc/TH1F.html#TH1F:TH1F@3

8

Well, it’s not exactly what you need (“x” values are used as “low-edges” of the histogram instead of “upper-edges”), but suit yourself: void plotter(string input) { TGraph *g = new TGraph(input.c_str()); // note that below, the very first "x" value defines the low-edge of the // very first bin and the very last "x" value defines the "overflow bin" TH1D *h = new TH1D("h", "something screwy", (g->GetN() - 1), g->GetX()); h->FillN(g->GetN(), g->GetX(), g->GetY(), 1); TCanvas *c = new TCanvas("name", "canvas", 800, 500); c->Divide(1, 2); c->cd(1); gPad->SetLogx(); g->Draw("AP"); c->cd(2); gPad->SetLogx(); h->Draw(); c->cd(0); }

9

I finally made it :slight_smile:

I had to change every bin size using an array:

void plotter(string input)
{
	TCanvas *c = new TCanvas("name", "canvas", 800, 500);
	gPad->SetLogx();
	double x;
	double w;
	string line;
	int i = 0;
	
	Double_t x_v[161];
	Double_t y_v[161];
	
	ifstream file_input (input.c_str());
	while ( getline(file_input, line) )
	{
		stringstream(line) >> x >> w;
		x_v[i] = x;
		y_v[i] = w;
		i++;
	}
	
	TH1D *h1 = new TH1D("histo", "title", 160, x_v);
	for (int a = 0; a < 161; a++)
	{
		h1->SetBinContent(h1->FindBin(x_v[a]), y_v[a]);
		h1->SetBinError(a, 0.5);
	}
	cout << h1->GetBinContent(161) << endl;
	h1->Draw("HIST E1");
}

Thanks :slight_smile:

file.png796×474 4.65 KB