vector1 <- c(1,13,25,30)
vector2 <- c(5,11,22,26,31)

i<-1;
j<-1;
k<-1;

while(i<=length(vector1))
{
   finish <- 0
   while(  finish == 0)
        {
          dif<- vector2[j]-vector1[i];
          a <- dif %% 3

          if (a==0 ){ finish <- 1 }

   j<-j+1;

 }

 i<-i+1;
 k<- k+1;

}

Error in if (a == 0) { : missing value where TRUE/FALSE needed