R/pointstomsec.R

In erpR: Event-Related Potentials (ERP) Analysis, Graphics and Utility Functions

pointstomsec=function(a, lengthsegment, startmsec, endmsec)
{
  if (is.null(startmsec)|is.null(endmsec)){
    stop("startmsec and endmsec must be specified", call.=F)
  }
  totmsec=endmsec-(startmsec) #total duration in msec
  pointsstep=totmsec/(lengthsegment-1) #how many msec is a point.
  # notice the - 1. This is because it doesn't make sense to say how many msec there are in a point.
  # what you actually are counting is how many msec there are between two points.
  # each point step is the space between two points (a small segment between two points). For this reason you have to subtract one.	
  x=((a-1)*pointsstep)+(startmsec)
  # notice that you check a-1. This is because the points in your vector goes from 1 to length of the vector. 
  # when you are at point 1. you are at step 0 (i.e., no "small segment"). The maximum number of segment you can reach is lengthsegment-1.
  
  return(x)
  
}