Elo score calculator functions

elo.R

### Functions for calculating Elo probabilities and scores ###

# Uses the weighted Elo formula created by 538, with a K of 250, a shape of 0.4 and an offset of 5
# https://fivethirtyeight.com/features/how-were-forecasting-the-2016-us-open/



### Define a function that calculates probabilities on the basis of Elo score inputs

#p1elo is p1's current Elo score
#p2elo is p2's current Elo score

Elo.prob <- function(p1elo,p2elo){
  p1exp <- (1+(10^((p2elo-p1elo)/400)))^(-1)
  return(p1exp) # returns the probability of p1 winning the match
}



### Define a function that calculates new Elo scores on the basis of previous scores, probabilities and match result

# p1elo - previous Elo score, p1 
# score - zero (lost match) or one (won match)
# p1exp - expected match outcome (value between zero and one, determined  by previous formula)
# numGames_ID1 - number of matches played by p1. Elo scores are affected less by any given match when a player has played many matches

Elo.newscore <- function(p1elo,score,p1exp,numGames_ID1){
  newscore <- p1elo + 250/((5+numGames_ID1)^0.4)*(score-p1exp)
  return(newscore)
}