Pythonの文字列結合と繰り返しの基本(+と*の使い方)

# 基本的な結合
a = "Hello"
b = "World"
msg = a + " " + b  # スペースも明示的に入れる
print(msg)

# 変数を含む結合
first = "Python"
second = "入門"
title = first + " " + second
print(title)

# スペースと改行の挿入
name = "Taro"
greeting = "Hello, " + name + "!\n" + "Welcome to Python."
print(greeting)

# 連続する長い文を読みやすく書く方法(丸括弧で改行)
long_msg = (
    "これは長いメッセージです。" +
    "途中で改行しても丸括弧の中なら1つの式として続けられます。" +
    "最後に改行を入れます。\n"
)
print(long_msg)

# これはエラーになります
age = 20
# print("年齢: " + age)  # コメントを外すとTypeError

try:
    result = "年齢: " + age  # 故意にエラーを発生させる
except TypeError as e:
    print("TypeErrorが発生:", e)

age = 20
msg = "年齢: " + str(age) + "歳"
print(msg)

# 複数の数値を扱うときも同様
year = 2025
month = 9
text = "今日は " + str(year) + "/" + str(month)
print(text)

# 多くの部分をつなぐときは join を使う
parts = ["Hello", "Python", "World"]
msg_plus = parts[0] + " " + parts[1] + " " + parts[2]  # 小規模ならOK
msg_join = " ".join(parts)  # 規模が大きくなるほどこちらが有利
print("msg_plus:", msg_plus)
print("msg_join:", msg_join)

# 数値が混ざる場合は str にしてから join
numbers = [2025, 9, 10]
date_text = "-".join(map(str, numbers))  # map(str, ...)で一括変換
print("date_text:", date_text)

# 繰り返しの基本
laugh = "ha"
print(laugh * 3)

# 指定幅の区切り線
line = "-" * 20
print(line)

print("ha" * 0)   # 空文字列
print("ha" * -3)  # 空文字列
print(len("ha" * -1))  # 長さで確認

# 区切り付きの繰り返し
token = "abc"
n = 5
with_sep = ",".join([token] * n)  # "abc,abc,abc,abc,abc"
print(with_sep)

# パターン自体を繰り返すなら * でOK
pattern = "abc,"
raw = pattern * n  # 末尾のカンマが余る
fixed = raw.rstrip(",")  # 末尾の余分な区切りを削除
print(raw)
print(fixed)

# 任意の幅の飾り枠を作る
title = " MENU "
width = 30
# 上下の線は "=" を幅に合わせて繰り返す
border = "=" * width
# 行内は左右に余白を確保して整形
padding = (width - len(title)) // 2
line = "-" * padding + title + "-" * (width - len(title) - padding)

print(border)
print(line)
print(border)

name = "Hanako"
city = "Tokyo"
age = 23

# + で丁寧に結合し、数値は str に変換
msg = "Name: " + name + "\n" + "City: " + city + "\n" + "Age: " + str(age)
print(msg)

# 複数のパーツを join でまとめる
parts = ["User", name, "from", city]
sentence = " ".join(parts)  # スペース区切りで結合
print(sentence)

# 簡易テーブルの作成
headers = ["ID", "Name", "Score"]
rows = [
    ["001", "Alice", "89"],
    ["002", "Bob",   "95"],
    ["003", "Cara",  "78"],
]

# 列幅を決める(今回は固定幅)
w_id, w_name, w_score = 5, 10, 6

# 罫線を * で構築
hline = "+" + "-" * w_id + "+" + "-" * w_name + "+" + "-" * w_score + "+"

print(hline)
# ヘッダ行
header_line = "|" + headers[0].ljust(w_id) + "|" + headers[1].ljust(w_name) + "|" + headers[2].rjust(w_score) + "|"
print(header_line)
print(hline)

# データ行
for r in rows:
    line = "|" + r[0].ljust(w_id) + "|" + r[1].ljust(w_name) + "|" + r[2].rjust(w_score) + "|"
    print(line)

print(hline)

value = None

# これはエラー
try:
    print("値: " + value)
except TypeError as e:
    print("エラー:", e)

# 安全な対処1: 空文字に置き換える(表示しない)
safe = value or ""  # None や空は "" になる
print("値: " + safe)

# 安全な対処2: 明示的に "None" と出したいなら str を使う
print("値: " + str(value))

# 遅くなりがちなやり方
words = ["py"] * 5_000  # 例として同じ単語を多数
s = ""
for w in words:
    s += w  # 都度新しい文字列が作られる

# 推奨のやり方
t = "".join(words)

print(len(s), len(t), s == t)

# 意図せぬ余分なスペース
a = "Hello "
b = " World"
merged = a + b  # "Hello  World" ← スペースが2つ
print(repr(merged))  # reprで可視化

# 対策1: 余計な空白を事前に削る
clean = a.rstrip() + " " + b.lstrip()
print(repr(clean))

# 対策2: 区切りは join に任せる
parts = [a.strip(), b.strip()]
joined = " ".join(parts)
print(repr(joined))

# 改行の混入にも注意
line1 = "first line\n"
line2 = "second line"
text = line1 + line2  # 間に改行が入る
print(text)