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Reference Guide
double32.C File Reference

Detailed Description

View in nbviewer Open in SWAN Tutorial illustrating use and precision of the Double32_t data type You should run this tutorial with ACLIC: a dictionary will be automatically created.

root > .x double32.C+

The following cases are supported for streaming a Double32_t type depending on the range declaration in the comment field of the data member:

Case Declaration
A Double32_t fNormal;
B Double32_t fTemperature; //[0,100]
C Double32_t fCharge; //[-1,1,2]
D Double32_t fVertex[3]; //[-30,30,10]
E Double32_t fChi2; //[0,0,6]
F Int_t fNsp;
Double32_t* fPointValue; //[fNsp][0,3]
  • Case A fNormal is converted from a Double_t to a Float_t
  • Case B fTemperature is converted to a 32 bit unsigned integer
  • Case C fCharge is converted to a 2 bits unsigned integer
  • Case D the array elements of fVertex are converted to an unsigned 10 bits integer
  • Case E fChi2 is converted to a Float_t with truncated precision at 6 bits
  • Case F the fNsp elements of array fPointvalue are converted to an unsigned 32 bit integer. Note that the range specifier must follow the dimension specifier.

Case B has more precision than case A: 9 to 10 significative digits and 6 to 7 digits respectively. The range specifier has the general format: [xmin,xmax] or [xmin,xmax,nbits]. Examples

  • [0,1]
  • [-10,100];
  • [-pi,pi], [-pi/2,pi/4],[-2pi,2*pi]
  • [-10,100,16]
  • [0,0,8] Note that:
  • If nbits is not specified, or nbits <2 or nbits>32 it is set to 32
  • If (xmin==0 and xmax==0 and nbits <=14) the double word will be converted to a float and its mantissa truncated to nbits significative bits.


Lets assume an original variable double x. When using the format 0,0,8 you get the best relative precision when storing and reading back the truncated x, say xt. The variance of (x-xt)/x will be better than when specifying a range for the same number of bits. However the precision relative to the range (x-xt)/(xmax-xmin) will be worse, and vice-versa. The format [0,0,8] is also interesting when the range of x is infinite or unknown.

#include "ROOT/TSeq.hxx"
#include "TCanvas.h"
#include "TFile.h"
#include "TGraph.h"
#include "TH1.h"
#include "TLegend.h"
#include "TMath.h"
#include "TRandom3.h"
#include "TTree.h"
class DemoDouble32 {
Double_t fD64; // reference member with full double precision
Double32_t fF32; // saved as a 32 bit Float_t
Double32_t fI32; //[-pi,pi] saved as a 32 bit unsigned int
Double32_t fI30; //[-pi,pi,30] saved as a 30 bit unsigned int
Double32_t fI28; //[-pi,pi,28] saved as a 28 bit unsigned int
Double32_t fI26; //[-pi,pi,26] saved as a 26 bit unsigned int
Double32_t fI24; //[-pi,pi,24] saved as a 24 bit unsigned int
Double32_t fI22; //[-pi,pi,22] saved as a 22 bit unsigned int
Double32_t fI20; //[-pi,pi,20] saved as a 20 bit unsigned int
Double32_t fI18; //[-pi,pi,18] saved as a 18 bit unsigned int
Double32_t fI16; //[-pi,pi,16] saved as a 16 bit unsigned int
Double32_t fI14; //[-pi,pi,14] saved as a 14 bit unsigned int
Double32_t fI12; //[-pi,pi,12] saved as a 12 bit unsigned int
Double32_t fI10; //[-pi,pi,10] saved as a 10 bit unsigned int
Double32_t fI8; //[-pi,pi, 8] saved as a 8 bit unsigned int
Double32_t fI6; //[-pi,pi, 6] saved as a 6 bit unsigned int
Double32_t fI4; //[-pi,pi, 4] saved as a 4 bit unsigned int
Double32_t fI2; //[-pi,pi, 2] saved as a 2 bit unsigned int
Double32_t fR14; //[0, 0, 14] saved as a 32 bit float with a 14 bits mantissa
Double32_t fR12; //[0, 0, 12] saved as a 32 bit float with a 12 bits mantissa
Double32_t fR10; //[0, 0, 10] saved as a 32 bit float with a 10 bits mantissa
Double32_t fR8; //[0, 0, 8] saved as a 32 bit float with a 8 bits mantissa
Double32_t fR6; //[0, 0, 6] saved as a 32 bit float with a 6 bits mantissa
Double32_t fR4; //[0, 0, 4] saved as a 32 bit float with a 4 bits mantissa
Double32_t fR2; //[0, 0, 2] saved as a 32 bit float with a 2 bits mantissa
DemoDouble32() = default;
void Set(Double_t ref)
fD64 = fF32 = fI32 = fI30 = fI28 = fI26 = fI24 = fI22 = fI20 = fI18 = fI16 = fI14 = fI12 = fI10 = fI8 = fI6 =
fI4 = fI2 = fR14 = fR12 = fR10 = fR8 = fR6 = fR4 = fR2 = ref;
void double32()
const auto nEntries = 40000;
const auto xmax = TMath::Pi();
const auto xmin = -xmax;
// create a Tree with nEntries objects DemoDouble32
TFile::Open("DemoDouble32.root", "recreate");
TTree tree("tree", "DemoDouble32");
DemoDouble32 demoInstance;
auto demoInstanceBranch = tree.Branch("d", "DemoDouble32", &demoInstance, 4000);
for (auto i : ROOT::TSeqI(nEntries)) {
demoInstance.Set(r.Uniform(xmin, xmax));
// Now we can proceed with the analysis of the sizes on disk of all branches
// Create the frame histogram and the graphs
auto branches = demoInstanceBranch->GetListOfBranches();
const auto nb = branches->GetEntries();
auto br = static_cast<TBranch *>(branches->At(0));
const Long64_t zip64 = br->GetZipBytes();
auto h = new TH1F("h", "Double32_t compression and precision", nb, 0, nb);
auto gcx = new TGraph();
auto gdrange = new TGraph();
auto gdval = new TGraph();
// loop on branches to get the precision and compression factors
for (auto i : ROOT::TSeqI(nb)) {
auto br = static_cast<TBranch *>(branches->At(i));
const auto brName = br->GetName();
h->GetXaxis()->SetBinLabel(i + 1, brName);
const auto cx = double(zip64) / br->GetZipBytes();
gcx->SetPoint(i, i + 0.5, cx);
if (i == 0 ) continue;
tree.Draw(Form("(fD64-%s)/(%g)", brName, xmax - xmin), "", "goff");
const auto rmsDrange = TMath::RMS(nEntries, tree.GetV1());
const auto drange = TMath::Max(0., -TMath::Log10(rmsDrange));
gdrange->SetPoint(i-1, i + 0.5, drange);
tree.Draw(Form("(fD64-%s)/fD64", brName), "", "goff");
const auto rmsDVal = TMath::RMS(nEntries, tree.GetV1());
const auto dval = TMath::Max(0., -TMath::Log10(rmsDVal));
gdval->SetPoint(i-1, i + 0.5, dval);
tree.Draw(Form("(fD64-%s) >> hdvalabs_%s", brName, brName), "", "goff");
auto hdval = gDirectory->Get<TH1F>(Form("hdvalabs_%s", brName));
hdval->GetXaxis()->SetTitle("Difference wrt reference value");
auto c = new TCanvas(brName, brName, 800, 600);
auto c1 = new TCanvas("c1", "c1", 800, 600);
// Finally build a legend
auto legend = new TLegend(0.3, 0.6, 0.9, 0.9);
legend->SetHeader(Form("%d entries within the [-#pi, #pi] range", nEntries));
legend->AddEntry(gcx, "Compression factor", "lp");
legend->AddEntry(gdrange, "Log of precision wrt range: p = -Log_{10}( RMS( #frac{Ref - x}{range} ) ) ", "lp");
legend->AddEntry(gdval, "Log of precision wrt value: p = -Log_{10}( RMS( #frac{Ref - x}{Ref} ) ) ", "lp");
Definition: Converters.cxx:921
ROOT::R::TRInterface & r
Definition: Object.C:4
#define c(i)
Definition: RSha256.hxx:101
#define h(i)
Definition: RSha256.hxx:106
double Double32_t
Definition: RtypesCore.h:58
double Double_t
Definition: RtypesCore.h:57
long long Long64_t
Definition: RtypesCore.h:71
@ kRed
Definition: Rtypes.h:64
@ kBlack
Definition: Rtypes.h:63
@ kBlue
Definition: Rtypes.h:64
@ kFullSquare
Definition: TAttMarker.h:51
@ kFullTriangleUp
Definition: TAttMarker.h:51
@ kFullCircle
Definition: TAttMarker.h:51
#define gDirectory
Definition: TDirectory.h:229
float xmin
Definition: THbookFile.cxx:93
float xmax
Definition: THbookFile.cxx:93
char * Form(const char *fmt,...)
A pseudo container class which is a generator of indices.
Definition: TSeq.hxx:66
A TTree is a list of TBranches.
Definition: TBranch.h:91
Long64_t GetZipBytes(Option_t *option="") const
Return total number of zip bytes in the branch if option ="*" includes all sub-branches of this branc...
Definition: TBranch.cxx:2102
The Canvas class.
Definition: TCanvas.h:27
static TFile * Open(const char *name, Option_t *option="", const char *ftitle="", Int_t compress=ROOT::RCompressionSetting::EDefaults::kUseCompiledDefault, Int_t netopt=0)
Create / open a file.
Definition: TFile.cxx:3942
A TGraph is an object made of two arrays X and Y with npoints each.
Definition: TGraph.h:41
1-D histogram with a float per channel (see TH1 documentation)}
Definition: TH1.h:571
TAxis * GetXaxis()
Get the behaviour adopted by the object about the statoverflows. See EStatOverflows for more informat...
Definition: TH1.h:316
This class displays a legend box (TPaveText) containing several legend entries.
Definition: TLegend.h:23
virtual void SetTitle(const char *title="")
Set the title of the TNamed.
Definition: TNamed.cxx:164
virtual const char * GetName() const
Returns name of object.
Definition: TNamed.h:47
Random number generator class based on M.
Definition: TRandom3.h:27
A TTree represents a columnar dataset.
Definition: TTree.h:78
return c1
Definition: legend1.C:41
Short_t Max(Short_t a, Short_t b)
Definition: TMathBase.h:212
constexpr Double_t Pi()
Definition: TMath.h:38
Double_t RMS(Long64_t n, const T *a, const Double_t *w=0)
Return the Standard Deviation of an array a with length n.
Definition: TMath.h:1157
Double_t Log10(Double_t x)
Definition: TMath.h:754
Definition: tree.py:1
Rene Brun

Definition in file double32.C.