functional-test.R

# define mapper function
map <- function(list, fn) {
    i <- 0;
    len <- length(list);
    result <- vector(length = len);

    while (i < len) {
        i <- i + 1;
        result[i] <- fn(list[i]);
    }

    return(result);
};

# define filter function
filter <- function(list, fn) {
    i <- 0;
    len <- length(list);
    result <- vector();

    while (i < len) {
        i <- i + 1;
        item <- list[i];
        if (fn(item)) {
            result <- append(result, item);
        }
    }

    return(result);
}

# define reduce function
reduce <- function(list, reducer, initial) {
    i <- 0;
    len <- length(list);

    current <- initial;

    while (i < len) {
        i <- i + 1;
        current <- reducer(current, list[i]);
    }

    return(current);
};

# define dataset
data <- c(1, 2, 3);

# filter out even numbers: c(1, 3)
data <- filter(data, function(n) {
    return(n %% 2 != 0);
})

# map numbers to the power of 2: c(1, 9)
data <- map(data, function(n) {
    return(n ** 2);
});

# sum list of numbers.  equivelant to sum(data): 10
data <- reduce(data, function(c, n) {
    return(c + n) + 1;
}, 0);

# print the number 10
print(data);

This example shows how to write a map, filter, and reduce function using the R language.