図：5%成長するファンドが2%配当金を払う場合と払わない場合の基準価額

https://gyazo.com/6b959f6d60450fb9ee7260bc87eeb273

外国税額控除ありのシミュレーション

code:main.py

import matplotlib.pyplot as plt

# 定義された条件

annual_growth_rate = 0.05

dividend_rate = 0.02

initial_investment = 10**7

investment_period = 20

us_tax_rate = 0.1

jp_tax_rate = 0.2

# 資産推移を保持するリスト

# シミュレーション

for year in range(1, investment_period + 1):

nisa_account_final.append(nisa_growth)

# 米国税を差し引く

dividend_non_nisa_after_tax = dividend_non_nisa * (1 - us_tax_rate) * (1 - jp_tax_rate)

# 税額控除のために米国税を計算しておく

us_tax = dividend_non_nisa * us_tax_rate

# 年度ごとにdevidend_non_nisa_after_us_taxを出力

print(f"Year {year}: {us_tax}")

# 米国税を差し引いた後の配当を加算

non_nisa_account_final.append(non_nisa_growth)

# 相対誤差の計算

# グラフの描画

fig, (ax1, ax2) = plt.subplots(2, 1, figsize=(10, 10))

ax1.plot(nisa_account_final, label="NISA Account (Final)")

ax1.plot(regular_growth, label="Regular Growth (5%)")

ax1.set_ylabel("Amount (JPY)")

ax1.set_title("Investment Simulation Over 20 Years (Final Corrected)")

ax1.legend()

ax1.grid(True)

ax2.plot(relative_error, label="Relative Error (NISA + Non-NISA vs Regular Growth)", color="red")

ax2.set_xlabel("Year")

ax2.set_ylabel("Relative Error (%)")

ax2.set_title("Relative Error Between NISA + Non-NISA and Regular Growth")

ax2.legend()

ax2.grid(True)

plt.tight_layout()

plt.show()

https://gyazo.com/523d815b5afcaea19a28a6988b916dfb

code:実行方法

rye init

rye add matplotlib

rye sync

rye run python main.py

code:main.py

import matplotlib.pyplot as plt

# 定義された条件

annual_growth_rate = 0.05

dividend_rate = 0.02

initial_investment = 10**7

investment_period = 20

us_tax_rate = 0.1

jp_tax_rate = 0.2

# 資産推移を保持するリスト

# シミュレーション

for year in range(1, investment_period + 1):

nisa_account_final.append(nisa_growth)

dividend_non_nisa_after_tax = dividend_non_nisa * (1 - us_tax_rate) * (1 - jp_tax_rate)

non_nisa_account_final.append(non_nisa_growth)

# 相対誤差の計算

# グラフの描画

fig, (ax1, ax2) = plt.subplots(2, 1, figsize=(10, 10))

ax1.plot(nisa_account_final, label="NISA Account (Final)")

ax1.plot(regular_growth, label="Regular Growth (5%)")

ax1.set_ylabel("Amount (JPY)")

ax1.set_title("Investment Simulation Over 20 Years (Final Corrected)")

ax1.legend()

ax1.grid(True)

ax2.plot(relative_error, label="Relative Error (NISA + Non-NISA vs Regular Growth)", color="red")

ax2.set_xlabel("Year")

ax2.set_ylabel("Relative Error (%)")

ax2.set_title("Relative Error Between NISA + Non-NISA and Regular Growth")

ax2.legend()

ax2.grid(True)

plt.tight_layout()

plt.show()

--

https://gyazo.com/b6741692d29670b3c6844958a70f4c91

2%の配当を払い、配当の20%の税金が取られ、再投資なし

2%の配当を払い、配当の20%の税金が取られ、配当は手動で全額再投資する

配当が出ず、税金も取られない

前提がおかしいな

実際にはこうだ

配当2%に対して10%の税金がかかる。これが税額控除できない

残りの再投資した配当はNISA枠ではないから、そこからでた配当には外国税が10%、日本の税金が20%かかる

これが考慮されていない

code:py

import matplotlib.pyplot as plt

# Constants

initial_investment = 10000000

annual_addition = 1000000

annual_growth_rate = 1.05

dividend_rate = 0.02

tax_rate = 0.20

years = 20

def calculate_corrected_cases():

for year in range(1, years + 1):

# Case 1: Dividend paid, taxed, not reinvested

taxed_dividend1 = dividend1 * (1 - tax_rate)

# 元手から税引き後の配当金を引いているので、間違っていそう

case1.append(growth1 + annual_addition)

# Case 2: Dividend paid, taxed, reinvested

reinvested_dividend2 = dividend2 * (1 - tax_rate)

# (元金-配当金)*成長率

# 配当金を再投資

case2.append(growth2 + reinvested_dividend2 + annual_addition)

# Case 3: No dividend, no tax

case3.append(growth3 + annual_addition)

return case1, case2, case3

# Calculating the corrected cases

case1_corrected, case2_corrected, case3_corrected = calculate_corrected_cases()

# Calculating the relative errors for comparison

# Plotting the main graph and relative errors as subplots

# Main graph

ax1.set_title('Comparison of Investment Strategies Over 20 Years')

ax1.set_xlabel('Year')

ax1.set_ylabel('Amount (yen)')

ax1.legend()

ax1.grid(True)

# Relative errors

ax2.plot(range(years + 1), relative_error_case1_vs_case3, marker='o', label="Relative Error: No Reinvestment vs No Dividend")

ax2.plot(range(years + 1), relative_error_case2_vs_case3, marker='o', label="Relative Error: Reinvestment vs No Dividend")

ax2.set_title('Relative Errors Compared to No Dividend Case Over 20 Years')

ax2.set_xlabel('Year')

ax2.set_ylabel('Relative Error (%)')

ax2.legend()

ax2.grid(True)

plt.tight_layout()

plt.show()

https://gyazo.com/d8a6700665f17df7614895bd7987c545

code:py

import matplotlib.pyplot as plt

# Initial investment amount

initial_investment = 10_000_000 # 1000万円

# Annual growth rate

growth_rate = 0.05 # 5%

# Dividend rate

dividend_rate = 0.02 # 2%

# Number of years

years = 10

# Lists to store yearly investment amounts

# Calculating the yearly amounts for both scenarios

investment_with_dividend_temp = initial_investment

for year in range(years):

profit = investment_with_dividend_temp * growth_rate

dividend = profit * dividend_rate

investment_with_dividend_temp += profit - dividend

investment_with_dividend_list.append(investment_with_dividend_temp)

investment_without_dividend_list.append(initial_investment * (1 + growth_rate) ** (year + 1))

# Calculating the yearly differences between the two scenarios

# Plotting the main graph for investment amount comparison

# Plotting the subplot for yearly differences

plt.show()

同じ条件で毎年100万円積んだ場合

https://gyazo.com/02f65174eaff1a22ea426b7998e2eba6

code:py

import matplotlib.pyplot as plt

# Initial investment amount

initial_investment = 10_000_000 # 1000万円

# Annual growth rate

growth_rate = 0.05 # 5%

# Dividend rate

dividend_rate = 0.02 # 2%

# Number of years

years = 10

# Annual addition

annual_addition = 1_000_000 # 100万円

# Lists to store yearly investment amounts

# Calculating the yearly amounts for all scenarios

investment_with_dividend_temp = initial_investment

investment_without_dividend_temp = initial_investment

investment_with_dividend_and_addition_temp = initial_investment

investment_without_dividend_and_addition_temp = initial_investment

for year in range(years):

# With Dividends

profit_with_dividend = investment_with_dividend_temp * growth_rate

dividend_with_dividend = profit_with_dividend * dividend_rate

investment_with_dividend_temp += profit_with_dividend - dividend_with_dividend

investment_with_dividend_list.append(investment_with_dividend_temp)

# Without Dividends

investment_without_dividend_temp = investment_without_dividend_temp * (1 + growth_rate)

investment_without_dividend_list.append(investment_without_dividend_temp)

# With Dividends and Addition

profit_with_dividend_and_addition = investment_with_dividend_and_addition_temp * growth_rate

dividend_with_dividend_and_addition = profit_with_dividend_and_addition * dividend_rate

investment_with_dividend_and_addition_temp += profit_with_dividend_and_addition - dividend_with_dividend_and_addition + annual_addition

investment_with_dividend_and_addition_list.append(investment_with_dividend_and_addition_temp)

# Without Dividends and Addition

investment_without_dividend_and_addition_temp = (investment_without_dividend_and_addition_temp + annual_addition) * (1 + growth_rate)

investment_without_dividend_and_addition_list.append(investment_without_dividend_and_addition_temp)

# Calculating the yearly differences between the two scenarios

# Plotting the main graph for investment amount comparison

# Plotting the subplot for yearly differences

plt.show()