1 <chapter name="Histograms">
5 The <code>Hist</code> class gives a simple implementation of
6 one-dimensional histograms, useful for quick-and-dirty testing,
7 without the need to link to more sophisticated packages.
8 For this reson it is used in many of the
9 <aloc href="SampleMainPrograms">sample main programs</aloc>
10 found in the <code>examples</code> subdirectory.
12 <h3>Basic principles</h3>
14 We here provide a simple overview of what is involved.
15 As a first step you need to declare a histogram, with name,
16 title, number of bins and <ei>x</ei> range (from, to).
18 Hist ZpT( "Z0 pT spectrum", 100, 0., 100.);
20 Alternatively you can first declare it and later define it:
23 ZpT.book( "Z0 pT spectrum", 100, 0., 100.);
26 Once declared, its contents can be added by repeated calls to
31 where the first argument is the <ei>x</ei> value and the second the
32 weight. Since the weight defaults to 1 the last argument could have
33 been omitted in this case.
36 A set of overloaded operators have been defined, so that histograms
37 can be added, subtracted, divided or multiplied by each other. Then the
38 contents are modified accordingly bin by bin. Thus the relative
39 deviation between two histograms <code>data</code> and
40 <code>theory</code> can be found as
42 diff = (data - theory) / (data + theory);
44 assuming that <code>diff</code>, <code>data</code> and <code>theory</code>
45 have been booked with the same number of bins and <ei>x</ei> range. That
46 responsibility rests on the user; some checks are made for compatibility,
47 but not enough to catch all possible mistakes.
50 Also overloaded operations with double real numbers are available.
51 Again these four operations are defined bin by bin, i.e. the
52 corresponding amount is added to, subtracted from, multiplied by or
53 divided by each bin. The double number can come before or after the
54 histograms, with obvious results. Thus the inverse of a histogram
55 <code>result</code> is given by <code>1. / result</code>.
56 The two kind of operations can be combined, e.g.
58 allpT = ZpT + 2. * WpT
60 Finally, also the <code>+=, -+, *=, /=</code> are overloaded, with
61 the right-hand side being either a histogram or a real number.
63 <h3>Output format</h3>
66 A histogram can be printed by making use of the overloaded <<
71 The printout format is inspired by the old HBOOK one. To understand
72 how to read this format, consider the simplified example
92 The key feature is that the <code>Contents</code> and
93 <code>Low edge</code> have to be read vertically. For instance,
94 the first bin has the contents
95 <code>3 * 10^2 + 4 * 10^1 + 7 * 10^0 = 347</code>. Correspondingly,
96 the other bins have contents 179, 123, 207 and 283. The first bin
97 stretches from <code>-(1 * 10^1 + 0 * 10^0) = -10</code> to the
98 beginning of the second bin, at <code>-(0 * 10^1 + 5 * 10^0) = -5</code>.
101 The visual representation above the contents give a simple impression
102 of the shape. An <code>X</code> means that the contents are filled up
103 to this level, a digit in the topmost row the fraction to which the
104 last level is filled. So the 9 of the first column indicates this bin
105 is filled 9/10 of the way from <code>3.00*10^2 = 300</code> to
106 <code>3.50*10^2 = 350</code>, i.e. somewhere close to 345,
107 or more precisely in the range 342.5 to 347.5.
110 The printout also provides some other information, such as the
111 number of entries, i.e. how many times the histogram has been filled,
112 the total weight inside the histogram, the total weight in underflow
113 and overflow, and the mean value and root-mean-square width (disregarding
114 underflow and overflow). The mean and width assumes that all the
115 contents is in the middle of the respective bin. This is especially
116 relevant when you plot a integer quantity, such as a multiplicity.
117 Then it makes sense to book with limits that are half-integers, e.g.
119 Hist multMPI( "number of multiparton interactions", 20, -0.5, 19.5);
121 so that the bins are centered at 0, 1, 2, ..., respectively. This also
122 avoids ambiguities which bin gets to be filled if entries are
123 exactly at the border between two bins. Also note that the
124 <code>fill( xValue)</code> method automatically performs a cast
125 to double precision where necessary, i.e. <code>xValue</code>
130 We here collect a more complete and formal overview of the methods.
132 <method name="Hist::Hist()">
133 declare a histogram, but does not define it.
136 <method name="Hist::Hist(string title, int numberOfBins, double xMin,
138 declare and define a histogram, where
139 <argument name="title">
140 is a string with the title of the histogram at output,
142 <argument name="numberOfBins">
143 is the number of bin the <ei>x</ei> range will be subdivided into,
144 limited to be at most 1000,
146 <argument name="xMin">
147 is the lower edge of the histogram,
149 <argument name="xMax">
150 is the upper edge of the histogram.
154 <method name="Hist::Hist(const Hist& h)">
155 creates an identical copy of the histogram in the argument,
156 including bin contents.
159 <method name="Hist::Hist(string title, const Hist& h)">
160 creates an identical copy of the histogram in the argument,
161 including bin contents, except that a new title is provided
165 <method name="Hist& Hist::operator=(const Hist& h)">
166 copies all properties of the histogram in the argument,
167 except that the original histogram title is retained.
170 <method name="void Hist::book(string title, int numberOfBins,
171 double xMin, double xMax)">
172 define a histogram that previously was only declared;
173 see above for the meaning of the arguments.
176 <method name="void Hist::name(string title)">
177 change the title of a histogram, but keep other properties unchanged.
180 <method name="void Hist::null()">
181 reset bin contents, but keep other histogram properties unchanged.
184 <method name="void Hist::fill(double xValue, double weight)">
185 fill the histogram, where
186 <argument name="xValue">
187 is the <ei>x</ei> position where the filling should occur, and
189 <argument name="weight" default="1.">
190 is the amount of weight to be added at this <ei>x</ei> value.
194 <method name="friend ostream& operator<<(ostream& os, const Hist& h)">
195 appends a simple histogram printout (see above for format) to the
196 <code>ostream</code>, while leaving the histogram object itself
197 unchanged. At most 100 columns are allowed to be displayed.
198 If the number of bins is larger than 100 then the contents of
199 adjacent bins are added to give the value in each column. (Two by two
200 up to 200 bins, three by three up to 300, and so on, with the very
201 last column possibly summing fewer rows than the others.)
204 <method name="void Hist::table(ostream& os = cout)">
206 <methodmore name="void Hist::table(string fileName)">
207 print a two-column table, where the first column gives the center of
208 each bin and the second one the corresponding bin contents. The desired
209 output stream or file name can be provided as argument. The former
210 is more flexible (e.g., it allows easy append to an existing file),
211 whereas the latter is simpler for the case that each histogram should
212 be a file of its own. The table may be useful for plotting e.g. with
216 <method name="friend void table(const Hist& h1, const Hist& h2,
217 ostream& os = cout)">
219 <methodmore name="friend void table(const Hist& h1, const Hist& h2,
221 print a three-column table, where the first column gives the center of
222 each bin and the second and third ones the corresponding bin contents
223 of the two histograms. Only works if the two histograms have the same
224 x axis (within a tiny tolarance), else nothing will be done.
227 <method name="double Hist::getBinContent(int iBin)">
228 return the value in bin <code>iBin</code>, ranging from 1 through
229 <code>numberOfBins</code>, with <code>0</code> for underflow and
230 <code>numberOfBins + 1</code> for overflow.
233 <method name="int Hist::getEntries()">
234 return the number of entries, i.e. the number of time that
235 <code>fill(...)</code> has been called.
238 <method name="bool Hist::sameSize(const Hist& h)">
239 checks that the number of bins and upper and lower limits are the
240 same as in the histogram in the argument.
243 <method name="void Hist::takeLog(bool tenLog = true)">
244 by default take 10-logarithm of current contents bin by bin. With
245 optional argument <code>false</code> instead take <ei>e</ei>-logarithm
246 of contents bin by bin. If to be used, then right before the
250 <method name="void Hist::takeSqrt()">
251 take square root of current contents bin by bin, with negative contents
255 <method name="Hist& Hist::operator+=(const Hist& h)">
257 <methodmore name="Hist& Hist::operator-=(const Hist& h)">
258 adds or subtracts the current histogram by the contents of the
259 histogram in the argument if <code>sameSize(...)</code> is true,
263 <method name="Hist& Hist::operator*=(const Hist& h)">
265 <methodmore name="Hist& Hist::operator/=(const Hist& h)">
266 multiplies or divides the current histogram by the contents of the
267 histogram in the argument if <code>sameSize(...)</code> is true,
271 <method name="Hist& Hist::operator+=(double f)">
273 <methodmore name="Hist& Hist::operator-=(double f)">
274 adds or subtracts each bin content by the common offset <ei>f</ei>.
277 <method name="Hist& Hist::operator*=(double f)">
279 <methodmore name="Hist& Hist::operator*=(double f)">
280 multiplies or divides each bin content by the common factor <ei>f</ei>.
283 <method name="friend Hist operator+(double f, const Hist& h1)">
285 <methodmore name="friend Hist operator+(const Hist& h1, double f)">
287 <methodmore name="friend Hist operator+(const Hist& h1, const Hist h2)">
288 add a constant to a histogram or two histograms to each other, bin by bin.
291 <method name="friend Hist operator-(double f, const Hist& h1)">
293 <methodmore name="friend Hist operator-(const Hist& h1, double f)">
295 <methodmore name="friend Hist operator-(const Hist& h1, const Hist h2)">
296 subtract a histogram from a constant, a constant from a histogram,
297 or two histograms from each other, bin by bin.
300 <method name="friend Hist operator*(double f, const Hist& h1)">
302 <methodmore name="friend Hist operator*(const Hist& h1, double f)">
304 <methodmore name="friend Hist operator*(const Hist& h1, const Hist h2)">
305 multiply a constant by a histogram or two histograms by each other,
309 <method name="friend Hist operator/(double f, const Hist& h1)">
311 <methodmore name="friend Hist operator/(const Hist& h1, double f)">
313 <methodmore name="friend Hist operator/(const Hist& h1, const Hist h2)">
314 divide a constant by a histogram, a histogram by a constant,
315 or two histograms by each other, bin by bin.
320 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->