Joint, Marginal, and Conditional Distributions

/**
 * Fintech Math Bootcamp · Lesson 059 of 120
 * Joint, Marginal, and Conditional Distributions
 * Module 06: Probability and Random Variables
 *
 * Scenario: Seeing default risk inside customer segments
 * Rule:     marginal = sum joint cells; conditional = joint / group mass
 *
 * Try it:   What is the denominator for default conditional on segment B?
 *
 * Lesson article: https://thefintechbuilder.com/financial-mathematics-statistics-and-data-foundations/probability-and-random-variables/joint-marginal-and-conditional-distributions/
 * Free course:    https://courses.thefintechbuilder.com
 * Synthetic teaching example, not financial advice or a production library.
 */

export function lesson059() {
  const joint=[[.63,.07],[.24,.06]]; // segment rows; repay/default columns
  if (Math.abs(joint.flat().reduce((s,v)=>s+v,0) - 1) > 1e-12)
    throw new Error("Joint probabilities must sum to 1");
  const defaults=joint[0][1]+joint[1][1];
  const segmentB=joint[1][0]+joint[1][1];
  const result={defaults,segmentB,defaultGivenB:joint[1][1]/segmentB};
  return result;
}

export const checkedResult = {"defaults":0.13,"segmentB":0.3,"defaultGivenB":0.2};

// Run this file directly: npx tsx lessons/06-probability-and-random-variables/059-joint-marginal-and-conditional-distributions.ts
if (process.argv[1] && import.meta.url.endsWith(process.argv[1].replace(/\\/g, "/").split("/").pop()!)) {
  console.log(JSON.stringify(lesson059(), null, 2));
}

# Fintech Math Bootcamp · Lesson 059 of 120
# Joint, Marginal, and Conditional Distributions
# Module 06: Probability and Random Variables
#
# Scenario: Seeing default risk inside customer segments
# Rule:     marginal = sum joint cells; conditional = joint / group mass
#
# Try it:   What is the denominator for default conditional on segment B?
#
# Lesson article: https://thefintechbuilder.com/financial-mathematics-statistics-and-data-foundations/probability-and-random-variables/joint-marginal-and-conditional-distributions/
# Free course:    https://courses.thefintechbuilder.com
# Synthetic teaching example, not financial advice or a production library.

import json


def lesson059() -> dict:
    joint = [[0.63, 0.07], [0.24, 0.06]]  # segment rows; repay/default columns
    if abs(sum(v for row in joint for v in row) - 1) > 1e-12:
        raise ValueError("Joint probabilities must sum to 1")
    defaults = joint[0][1] + joint[1][1]
    segment_b = joint[1][0] + joint[1][1]
    return {"defaults": defaults, "segmentB": segment_b, "defaultGivenB": joint[1][1] / segment_b}


if __name__ == "__main__":
    print(json.dumps(lesson059(), indent=2))

/**
 * Fintech Math Bootcamp · Lesson 059 of 120
 * Joint, Marginal, and Conditional Distributions
 * Module 06: Probability and Random Variables
 *
 * Scenario: Seeing default risk inside customer segments
 * Rule:     marginal = sum joint cells; conditional = joint / group mass
 *
 * Try it:   What is the denominator for default conditional on segment B?
 *
 * Lesson article: https://thefintechbuilder.com/financial-mathematics-statistics-and-data-foundations/probability-and-random-variables/joint-marginal-and-conditional-distributions/
 * Free course:    https://courses.thefintechbuilder.com
 * Synthetic teaching example, not financial advice or a production library.
 */

import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

public class Main {
    static Map<String, Object> lesson059() {
        double[][] joint = {{0.63, 0.07}, {0.24, 0.06}}; // segment rows; repay/default columns
        double total = 0;
        for (double[] row : joint) for (double v : row) total += v;
        if (Math.abs(total - 1) > 1e-12) {
            throw new IllegalArgumentException("Joint probabilities must sum to 1");
        }
        double defaults = joint[0][1] + joint[1][1];
        double segmentB = joint[1][0] + joint[1][1];
        Map<String, Object> result = new LinkedHashMap<String, Object>();
        result.put("defaults", defaults);
        result.put("segmentB", segmentB);
        result.put("defaultGivenB", joint[1][1] / segmentB);
        return result;
    }

    public static void main(String[] args) {
        System.out.println(toJson(lesson059(), ""));
    }

    // --- Minimal JSON printer: maps keep insertion order, 2-space indent. ---

    static String toJson(Object value, String indent) {
        if (value == null) return "null";
        if (value instanceof String) return "\"" + value + "\"";
        if (value instanceof Double) return formatNumber((Double) value);
        if (value instanceof Number) return value.toString();
        if (value instanceof double[]) {
            List<Object> items = new ArrayList<Object>();
            for (double v : (double[]) value) items.add(v);
            return toJson(items, indent);
        }
        String inner = indent + "  ";
        StringBuilder sb = new StringBuilder();
        if (value instanceof Map) {
            Map<?, ?> map = (Map<?, ?>) value;
            if (map.isEmpty()) return "{}";
            sb.append("{\n");
            int i = 0;
            for (Map.Entry<?, ?> entry : map.entrySet()) {
                sb.append(inner).append('"').append(entry.getKey()).append("\": ")
                  .append(toJson(entry.getValue(), inner))
                  .append(++i < map.size() ? ",\n" : "\n");
            }
            return sb.append(indent).append('}').toString();
        }
        List<?> list = (List<?>) value;
        if (list.isEmpty()) return "[]";
        sb.append("[\n");
        for (int i = 0; i < list.size(); i++) {
            sb.append(inner).append(toJson(list.get(i), inner))
              .append(i + 1 < list.size() ? ",\n" : "\n");
        }
        return sb.append(indent).append(']').toString();
    }

    static String formatNumber(double v) {
        if (Double.isNaN(v) || Double.isInfinite(v)) return "null";
        if (v == Math.rint(v) && Math.abs(v) < 1e15) return Long.toString((long) v);
        return Double.toString(v);
    }
}

// Fintech Math Bootcamp · Lesson 059 of 120
// Joint, Marginal, and Conditional Distributions
// Module 06: Probability and Random Variables
//
// Scenario: Seeing default risk inside customer segments
// Rule:     marginal = sum joint cells; conditional = joint / group mass
//
// Try it:   What is the denominator for default conditional on segment B?
//
// Lesson article: https://thefintechbuilder.com/financial-mathematics-statistics-and-data-foundations/probability-and-random-variables/joint-marginal-and-conditional-distributions/
// Free course:    https://courses.thefintechbuilder.com
// Synthetic teaching example, not financial advice or a production library.

package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"math"
	"os"
)

// Distributions holds a marginal (defaults), a group mass (segment B) and a conditional.
type Distributions struct {
	Defaults      float64 `json:"defaults"`
	SegmentB      float64 `json:"segmentB"`
	DefaultGivenB float64 `json:"defaultGivenB"`
}

func lesson059() (Distributions, error) {
	joint := [][]float64{{0.63, 0.07}, {0.24, 0.06}} // segment rows; repay/default columns
	total := 0.0
	for _, row := range joint {
		for _, v := range row {
			total += v
		}
	}
	if math.Abs(total-1) > 1e-12 {
		return Distributions{}, errors.New("Joint probabilities must sum to 1")
	}
	defaults := joint[0][1] + joint[1][1]
	segmentB := joint[1][0] + joint[1][1]
	return Distributions{Defaults: defaults, SegmentB: segmentB, DefaultGivenB: joint[1][1] / segmentB}, nil
}

func main() {
	result, err := lesson059()
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(1)
	}
	out, _ := json.MarshalIndent(result, "", "  ")
	fmt.Println(string(out))
}

/**
 * Fintech Math Bootcamp · Lesson 059 of 120
 * Joint, Marginal, and Conditional Distributions
 * Module 06: Probability and Random Variables
 *
 * Scenario: Seeing default risk inside customer segments
 * Rule:     marginal = sum joint cells; conditional = joint / group mass
 *
 * Try it:   What is the denominator for default conditional on segment B?
 *
 * Lesson article: https://thefintechbuilder.com/financial-mathematics-statistics-and-data-foundations/probability-and-random-variables/joint-marginal-and-conditional-distributions/
 * Free course:    https://courses.thefintechbuilder.com
 * Synthetic teaching example, not financial advice or a production library.
 */

#include <charconv>
#include <cmath>
#include <iostream>
#include <stdexcept>
#include <string>
#include <utility>
#include <vector>

// --- Minimal JSON value and printer: objects keep insertion order, 2-space indent. ---

struct Json {
    enum class Kind { Null, Number, Text, Array, Object };
    Kind kind = Kind::Null;
    double number = 0;
    std::string text;
    std::vector<std::string> keys;  // object keys, parallel to items
    std::vector<Json> items;        // array elements or object values
};

Json num(double v) { Json j; j.kind = Json::Kind::Number; j.number = v; return j; }
Json str(const std::string& s) { Json j; j.kind = Json::Kind::Text; j.text = s; return j; }
Json arr(const std::vector<Json>& values) { Json j; j.kind = Json::Kind::Array; j.items = values; return j; }

Json arr(const std::vector<double>& values) {
    std::vector<Json> items;
    for (double v : values) items.push_back(num(v));
    return arr(items);
}

Json obj(const std::vector<std::pair<std::string, Json>>& fields) {
    Json j;
    j.kind = Json::Kind::Object;
    for (const auto& [key, value] : fields) { j.keys.push_back(key); j.items.push_back(value); }
    return j;
}

std::string formatNumber(double v) {
    if (!std::isfinite(v)) return "null";
    char buf[64];
    auto end = std::to_chars(buf, buf + sizeof buf, v).ptr;  // shortest round-trip form
    return std::string(buf, end);
}

void writeJson(std::ostream& out, const Json& j, const std::string& indent) {
    switch (j.kind) {
    case Json::Kind::Null: out << "null"; return;
    case Json::Kind::Number: out << formatNumber(j.number); return;
    case Json::Kind::Text: out << '"' << j.text << '"'; return;
    default: break;
    }
    bool isObject = j.kind == Json::Kind::Object;
    if (j.items.empty()) { out << (isObject ? "{}" : "[]"); return; }
    std::string inner = indent + "  ";
    out << (isObject ? "{\n" : "[\n");
    for (size_t i = 0; i < j.items.size(); ++i) {
        out << inner;
        if (isObject) out << '"' << j.keys[i] << "\": ";
        writeJson(out, j.items[i], inner);
        out << (i + 1 < j.items.size() ? ",\n" : "\n");
    }
    out << indent << (isObject ? '}' : ']');
}

// --- Lesson ---

Json lesson059() {
    const double joint[2][2] = {{0.63, 0.07}, {0.24, 0.06}};  // segment rows; repay/default columns
    double total = 0;
    for (const auto& row : joint)
        for (double v : row) total += v;
    if (std::abs(total - 1) > 1e-12) throw std::invalid_argument("Joint probabilities must sum to 1");
    double defaults = joint[0][1] + joint[1][1];
    double segmentB = joint[1][0] + joint[1][1];
    return obj({
        {"defaults", num(defaults)},
        {"segmentB", num(segmentB)},
        {"defaultGivenB", num(joint[1][1] / segmentB)},
    });
}

int main() {
    writeJson(std::cout, lesson059(), "");
    std::cout << '\n';
}

// Fintech Math Bootcamp · Lesson 059 of 120
// Joint, Marginal, and Conditional Distributions
// Module 06: Probability and Random Variables
//
// Scenario: Seeing default risk inside customer segments
// Rule:     marginal = sum joint cells; conditional = joint / group mass
//
// Try it:   What is the denominator for default conditional on segment B?
//
// Lesson article: https://thefintechbuilder.com/financial-mathematics-statistics-and-data-foundations/probability-and-random-variables/joint-marginal-and-conditional-distributions/
// Free course:    https://courses.thefintechbuilder.com
// Synthetic teaching example, not financial advice or a production library.

// --- Minimal JSON value and printer: objects keep insertion order, 2-space indent. ---

#[allow(dead_code)]
enum Json {
    Null,
    Num(f64),
    Str(String),
    Arr(Vec<Json>),
    Obj(Vec<(String, Json)>),
}

#[allow(dead_code)]
fn nums(values: &[f64]) -> Json {
    Json::Arr(values.iter().map(|&v| Json::Num(v)).collect())
}

#[allow(dead_code)]
fn obj(fields: Vec<(&str, Json)>) -> Json {
    Json::Obj(fields.into_iter().map(|(k, v)| (k.to_string(), v)).collect())
}

fn format_number(v: f64) -> String {
    if !v.is_finite() {
        return "null".to_string();
    }
    if v.fract() == 0.0 && v.abs() < 1e15 {
        return format!("{}", v as i64);
    }
    format!("{:?}", v) // shortest round-trip form
}

impl Json {
    fn render(&self, indent: &str) -> String {
        let inner = format!("{}  ", indent);
        match self {
            Json::Null => "null".to_string(),
            Json::Num(v) => format_number(*v),
            Json::Str(s) => format!("\"{}\"", s),
            Json::Arr(items) if items.is_empty() => "[]".to_string(),
            Json::Obj(fields) if fields.is_empty() => "{}".to_string(),
            Json::Arr(items) => {
                let lines: Vec<String> = items.iter().map(|v| format!("{}{}", inner, v.render(&inner))).collect();
                format!("[\n{}\n{}]", lines.join(",\n"), indent)
            }
            Json::Obj(fields) => {
                let lines: Vec<String> = fields
                    .iter()
                    .map(|(k, v)| format!("{}\"{}\": {}", inner, k, v.render(&inner)))
                    .collect();
                format!("{{\n{}\n{}}}", lines.join(",\n"), indent)
            }
        }
    }
}

// --- Lesson ---

fn lesson059() -> Result<Json, String> {
    let joint: [[f64; 2]; 2] = [[0.63, 0.07], [0.24, 0.06]]; // segment rows; repay/default columns
    let total: f64 = joint.iter().flatten().sum();
    if (total - 1.0).abs() > 1e-12 {
        return Err("Joint probabilities must sum to 1".to_string());
    }
    let defaults = joint[0][1] + joint[1][1];
    let segment_b = joint[1][0] + joint[1][1];
    Ok(obj(vec![
        ("defaults", Json::Num(defaults)),
        ("segmentB", Json::Num(segment_b)),
        ("defaultGivenB", Json::Num(joint[1][1] / segment_b)),
    ]))
}

fn main() {
    match lesson059() {
        Ok(result) => println!("{}", result.render("")),
        Err(message) => {
            eprintln!("{}", message);
            std::process::exit(1);
        }
    }
}

/**
 * Fintech Math Bootcamp · Lesson 059 of 120
 * Joint, Marginal, and Conditional Distributions
 * Module 06: Probability and Random Variables
 *
 * Scenario: Seeing default risk inside customer segments
 * Rule:     marginal = sum joint cells; conditional = joint / group mass
 *
 * Try it:   What is the denominator for default conditional on segment B?
 *
 * Lesson article: https://thefintechbuilder.com/financial-mathematics-statistics-and-data-foundations/probability-and-random-variables/joint-marginal-and-conditional-distributions/
 * Free course:    https://courses.thefintechbuilder.com
 * Synthetic teaching example, not financial advice or a production library.
 */

using System;
using System.Linq;
using System.Text.Json;

var options = new JsonSerializerOptions { WriteIndented = true };
Console.WriteLine(JsonSerializer.Serialize(Lesson059(), options));

static object Lesson059()
{
    double[][] joint = { new[] { 0.63, 0.07 }, new[] { 0.24, 0.06 } }; // segment rows; repay/default columns
    if (Math.Abs(joint.SelectMany(row => row).Sum() - 1) > 1e-12)
        throw new ArgumentException("Joint probabilities must sum to 1");
    double defaults = joint[0][1] + joint[1][1];
    double segmentB = joint[1][0] + joint[1][1];
    return new { defaults, segmentB, defaultGivenB = joint[1][1] / segmentB };
}