|AHELP for CIAO 4.2||
Create a one-dimensional histogram of a data array
simple_hist(array; nbins=10, min=NULL, max=NULL, step=NULL);
The simple_hist() routine calculates a one-dimensional histogram of the input data (the array argument). For more control over the calculation use the histogram module.
The routine can be loaded into Sherpa by saying:
|array||The array of data to histogram.|
|nbins||10||The number of bins to use to create the histogram (only used when step is NULL).|
|min||NULL||The value to use for the lower edge of the first bin; if NULL then the minimum data value will be used.|
|max||NULL||The value to use for the upper edge of the last bin; if NULL then the minimum data value will be used. It can be over-ridden if step is not NULL.|
|step||NULL||The bin width to use (this overrides the nbins value).|
The return value is an object with the following fields:
|xlow||The lower edges of each bin.|
|xhigh||The upper edges of each bin.|
|y||The number of elements of array that fall within each bin.|
The histogram is calculated using evenly-spaced bins; for all but the last bin the range is half-open, so that if it has edges xlow_i and xhigh_i then a value falls in this bin if it has a value in the range
xlow_i <= x < xhigh_i
The last bin is considered closed, so a value falls in this bin if
xlow_last <= x <= xhigh_last
If step is not given (i.e. it is NULL), then the bin width is calculated as
(max - min) / nbins
If step is given then it is used as the bin width and the upper edge will be adjusted to ensure there are an integer number of bins: so if the input values are min=0, max=1, and step=0.4 the histogram will be calculated for the bins 0 to 0.4, 0.4 to 0.8, and 0.8 to 1.2.
chips> x = [0, 1, 2, 3, 2, 3, 4, 2, 1]; chips> h = simple_hist(x); chips> add_histogram(h.xlow, h.xhigh, h.y);
The return value of h is an object with the fields "xlow", "xhigh", and "y", which contain the lower and upper bin edges and the histogram values respectively. These fields are used to plot up the histogram using the ChIPS add_histogram() routine.
chips> print (h); xlow = Float64 xhigh = Float64 y = Int32 chips> print (h.y); 1 0 2 0 0 3 0 2 0 1
chips> x = [0, 1, 2, 3, 2, 3, 4, 2, 1]; chips> h = simple_hist(x; step=1);
The step size (width) of each bin is set to 1, which results in:
chips> print (h); xlow = Float64 xhigh = Float64 y = Int32 chips> print (h.y); 1 2 3 3
chips> img = read_image("emap.fits"); chips> ivals = get_piximgvals(img); chips> ix = where(isinf(ivals) | isnan(ivals) == 0); chips> h = simple_hist(ivals[ix]; nbins=20);
Here we use the Crates read_image() and get_piximgvals() to read in the contents of the image "emap.fits". Since it may contain "invalid" numbers, such as NaN and Infinity, we filter out these values before calculating the histogram (using 20 bins).
The version scheme has changed to a date-based system. There should be no changes to the behavior of this routine.