String operations — concatenation, repetition

1.2.10 · Coding › Introduction to Programming (Python)

1. Concatenation with `+`

YEH kya karta hai. "foot" + "ball" → "football". Koi space automatically insert nahi hoti — tumhe bas wohi characters milte hain jo tum provide karte ho.

YEH kaise kaam karta hai (first principles se). String ko slots ki list ki tarah socho:

$"abc" \equiv [a_{0}, b_{1}, c_{2}]$

Jab tum s + t karte ho, Python ek brand-new buffer banata hai jiska length len(s) + len(t) hota hai, pehle s ke saare characters copy karta hai, phir uske baad t ke saare characters. Originals unchanged rehte hain — strings immutable hoti hain, isliye koi "operation" in-place edit nahi karta; yeh ek fresh string return karta hai.

Worked example Ek greeting banana

name = "Ada"
msg = "Hello, " + name + "!"
print(msg)          # Hello, Ada!

"Hello, " + name → "Hello, Ada". Yeh step kyun? + pehle literal copy karta hai phir name ke 3 chars; humne jo space type ki thi woh preserve hoti hai.
... + "!" → "Hello, Ada!". Kyun? + left-associative hai, isliye yeh left→right evaluate karta hai, ek-ek piece jodhta hota hai.
len(msg) = $7 + 3 + 1 = 11$ . Kyun? lengths add hoti hain.

Worked example Type matters

age = 19
print("Age: " + str(age))   # Age: 19

str(age) kyun? str aur int ke beech + undefined hai — Python andaaza lagaane se mana karta hai. Humein pehle number ko text mein convert karna hoga.

2. Repetition with `*`

YEH kyun exist karta hai. 40-character ki line ke liye "-" + "-" + "-" + ... likhna bahut takleef deh hai. "-" * 40 ise ek baar mein keh deta hai.

YEH kaise kaam karta hai. s * n ko define kiya gaya hai repeated concatenation ke roop mein:

$s * n = n copies s + s + \dots + s$

Toh yeh concatenation ke saare rules inherit karta hai.

Worked example Ek divider line

print("=" * 10)     # ==========
print("ab" * 3)     # ababab

"=" * 10 → 10 equals signs. Kyun? ek char das baar repeat hota hai → length $10 \times 1 = 10$ .
"ab" * 3 → "ababab". Kyun? poora 2-char unit repeat hota hai, length $3 \times 2 = 6$ .

Worked example Edge cases jo beginners ko surprise karte hain

print("hi" * 0)     # ''  (empty string)
print("hi" * -2)    # ''  (empty string)

* 0 → empty. Kyun? kisi bhi cheez ki zero copies kuch bhi nahi hoti — $0 \times len = 0$ ke saath consistent hai.
* -2 → empty. Kyun? Python kisi bhi negative count ko zero copies treat karta hai (tumhare paas negative repeats nahi ho sakte). Forecast-then-verify: bahut log guess karte hain ki yeh error dega — nahi deta, bas "" return karta hai.

Figure — String operations — concatenation, repetition

3. Dono ko combine karna

Worked example Ek word ko frame karo

word = "GO"
print("*" * 6)
print("*" + " " + word + " " + "*")
print("*" * 6)

Output:

******
* GO *
******

Dono kyun mix karein? * top/bottom borders fast banata hai; + middle row ko piece-by-piece assemble karta hai. Har tool apne kaam ke liye (80/20: yeh do string-building ke vast majority ko cover karte hain jo tum likhoge).

Common mistakes

Flashcards

Python mein do strings ko concatenate karne wala operator kaunsa hai?

+

Kaunsa operator ek string ko n baar repeat karta hai?

* (integer ke saath)

len(s + t) ko len(s) aur len(t) ke terms mein kya hoga?

len(s) + len(t)

len(s * n) kya hai?

n * len(s) jab n ≥ 0

"hi" * 0 kya return karta hai?

'' (empty string)

"hi" * -3 kya return karta hai?

'' (negative counts empty string dete hain, koi error nahi)

"3" + 4 kyun fail karta hai?

Tum str ko int ke saath concatenate nahi kar sakte; pehle str(4) se convert karo.

"ab" * "2" kyun fail karta hai?

Repetition ko integer count chahiye, string nahi; int("2") use karo.

Python strings mutable hain ya immutable?

Immutable — +/* brand-new strings return karte hain.

Kya "a"+"b" mein space insert hoti hai?

Nahi — sirf exactly wohi characters milte hain jo tum provide karte ho.

Recall Feynman: 12-saal ke bacche ko explain karo

Socho letter beads ek dhaage pe hain. Concatenation (+) matlab ek bead-dhaage ko doosre ke end se baandh dena taaki ek lamba dhaaga bane — "cat" + "dog" ban jaata hai "catdog", kuch extra nahi ghusa. Repetition (*) ek copy machine hai: "ha" * 3 print karta hai "hahaha". Tum kisi cheez ko "minus do baar" repeat nahi kar sakte, isliye Python tumhe ek empty bead-dhaaga de deta hai. Aur tum kabhi letter-beads ko number-beads ke saath nahi jod sakte bina pehle number ko letters mein convert kiye — isliye str(4) exist karta hai.

Connections

Strings — definition and indexing — string kya hoti hai operate karne se pehle.
Immutability of strings — kyun +/* naye objects return karte hain.
Type conversion — str(), int(), float() — str + int errors ka fix.
Operator overloading — same symbol, alag matlab operand type se.
f-strings and .format() — jab concatenation lamba ho jaaye to cleaner alternatives.
len() and built-in functions — characters ginne ke liye.

1. Concatenation with +

2. Repetition with *

3. Dono ko combine karna

Common mistakes

Flashcards

Connections

Concept Map

1. Concatenation with `+`

2. Repetition with `*`