http://www.reddit.com/r/askscience/comments/35uljq/if_i_wanted_to_randomly_find_someone_in_an/cr89r0o


#options:


#initialize:




simulate=function(move2=F,verbose=F){
    die=function(x=1){floor(runif(x,1,11))}
    coords=function(who){pos=which(park==who);y=ceiling(pos/10);x=pos%%10;if(x==0)x=10;return(c(x,y))}

    park= matrix(data=".",nrow=10,ncol=10,byrow=T)

    #place A
    posA=die(2)
    park[posA[1],posA[2]]="A"

    #place B
    overlap=T
    while(overlap){
        posB=die(2)
        if(! all(posB==posA)){overlap=F}
        }
    park[posB[1],posB[2]]="B"


    #plotpark=function(){for(i in 1:10){for(j in 1:10){cat(park[park$x==i & park$y==j,"who"], " ") };print("")}}#print()
    plotpark=function(){print(park)}
    if(verbose)plotpark()

    found=FALSE
    counter=0
    if(all(posB==posA)){
        warning("+++ REDO FROM START +++               >> collision")
        found=T
        }

    while(!found){
        counter=counter+1

        # ####  A
        #get pos:
        pos=coords("A")
        x=pos[1]
        y=pos[2]
        if(verbose==2)cat("A: ",x,y,"\n")
        park[x,y]=counter

        #valid moves:
        dirs=c("U","UR","R","DR","D","DL","L","UL")
        if(y==1)dirs=grep("[^L]$",dirs,value=T)
        if(y==10)dirs=grep("[^R]$",dirs,value=T)
        if(x==1)dirs=grep("^[^U]",dirs,value=T)
        if(x==10)dirs=grep("^[^D]",dirs,value=T)

        if(verbose==2)print(dirs)

        dirs=sample(dirs,1)

        if(verbose==2)print(dirs)

        if(length(grep("D",dirs))==1) x=x+1
        if(length(grep("U",dirs))==1) x=x-1
        if(length(grep("R",dirs))==1) y=y+1
        if(length(grep("L",dirs))==1) y=y-1

        if(verbose==2)cat(x,y,"\n")

        newA=c(x,y)

        # ####  B
        #get pos:
        pos=coords("B")
        x=pos[1]
        y=pos[2]
        if(verbose==2)cat("B: ",x,y,"\n")
        park[x,y]=counter

        #valid moves:
        dirs=c("U","UR","R","DR","D","DL","L","UL")
        if(y==1)dirs=grep("[^L]$",dirs,value=T)
        if(y==10)dirs=grep("[^R]$",dirs,value=T)
        if(x==1)dirs=grep("^[^U]",dirs,value=T)
        if(x==10)dirs=grep("^[^D]",dirs,value=T)

        if(verbose==2)print(dirs)

        dirs=sample(dirs,1)

        if(verbose==2)print(dirs)

        if(length(grep("D",dirs))==1) x=x+1
        if(length(grep("U",dirs))==1) x=x-1
        if(length(grep("R",dirs))==1) y=y+1
        if(length(grep("L",dirs))==1) y=y-1

        if(verbose==2)cat(x,y,"\n")

        newB=c(x,y)


        if(all(newA==newB))
            {
            found=T
            park[x,y]="X"
            }
            else{
            park[newA[1],newA[2]]="A"
            park[newB[1],newB[2]]="B"
            }

        #Search nieghbouring squares

        if(verbose==2)plotpark()
    }
    if(verbose)plotpark()
    if(verbose)cat("\n\nFound in: ",counter)
return(counter)
}

simulate()

nruns=10000

dataA=rep(0,nruns)
dataAB=rep(0,nruns)
for(i in 1:nruns)dataA[i]=simulate()
for(i in 1:nruns)dataAB[i]=simulate(T)


summary(data)
summary(dataAB)