List methods — append, insert, remove, pop, sort, reverse, count, index

1.2.22 · Coding › Introduction to Programming (Python)

YE methods exist kyun karte hain?

Ek list data ko ek sequence mein store karti hai jisme positions hoti hain jinhe indices kehte hain (jo 0 se shuru hoti hain). Jab data aa jaye, tab aapko zaroorat padti hai:

Badhaane ki → append, insert
Ghataane ki → remove, pop
Reorder karne ki → sort, reverse
Query karne ki → count, index

Inke bina aapko har baar poora list haath se rebuild karna padta — jo slow aur error-prone hota. Ye methods wo bookkeeping aapke liye kar dete hain.

HAR method kya karta hai (contract)

Definition Aath methods

lst.append(x) → x ko end mein add karta hai. ==None== return karta hai (in place mutate karta hai).
lst.insert(i, x) → x ko index i pe daalte hain, i se sab cheez daayein shift ho jaati hai. None return karta hai.
lst.remove(x) → value x ki pehli occurrence delete karta hai. Agar absent ho toh ValueError raise karta hai. None return karta hai.
lst.pop(i=-1) → index i pe jo item hai use remove karke return karta hai (agar i na ho toh last item).
lst.sort() → in place reorder karta hai, by default ascending. None return karta hai.
lst.reverse() → order ko in place ulta karta hai. None return karta hai.
lst.count(x) → return karta hai ki x kitni baar aaya.
lst.index(x) → pehle x ka index return karta hai. Agar absent ho toh ValueError raise karta hai.

YE under the hood kaise kaam karta hai (behaviour derive karo)

Ek list $0, 1, 2, \dots, n - 1$ positions mein laid out hoti hai jahan $n =$ len(lst).

append — iski index derive karo. Length $n$ wali list ka end agle free slot pe hota hai: $new index of appended item = n = len(lst)$ Append karne ke baad, length $n + 1$ ho jaati hai.

insert(i, x) — $j \geq i$ pe har cheez ko $j + 1$ pe move karna padta hai jagah banane ke liye: $old item at j ⟶ new position j + 1 (j \geq i)$ Isliye front ke paas insert "slow" hai (bahut shifts) jabki append "fast" hai (koi shift nahi).

pop(i) — lst[i] return karta hai, phir $j > i$ pe har cheez $j - 1$ pe move ho jaati hai gap close karne ke liye. Length $n - 1$ ho jaati hai.

Figure — List methods — append, insert, remove, pop, sort, reverse, count, index

Worked examples

Worked example Step by step list banana aur edit karna

nums = [3, 1, 2]
nums.append(5)        # [3, 1, 2, 5]
nums.insert(1, 9)     # [3, 9, 1, 2, 5]
nums.remove(1)        # [3, 9, 2, 5]
last = nums.pop()     # last = 5 ; nums = [3, 9, 2]
first = nums.pop(0)   # first = 3 ; nums = [9, 2]

insert(1, 9) kyun? Index 1 pe 1 tha; 1 shift hokar index 2 pe chala gaya jagah banane ke liye, toh 9 index 1 pe land kiya. Phir remove(1) kyun? Ye value 1 (pehla wala) ke hisaab se search karta hai aur use delete karta hai — index ke hisaab se nahi. pop() se 5 kyun milta hai lekin pop(0) se 3 kyun? Koi argument nahi = last item; 0 = front item, aur ye remove kiye huye ko return karta hai (remove ki tarah nahi).

Worked example Sorting, reversing, querying

data = [4, 2, 4, 1]
data.sort()                # [1, 2, 4, 4]
data.reverse()             # [4, 4, 2, 1]
c = data.count(4)          # c = 2
i = data.index(2)          # i = 2

sort() pehle aur reverse() baad mein kyun? reverse sort nahi karta — ye sirf current order ko flip karta hai. Descending paane ke liye pehle ascending sort karo, phir reverse karo (ya data.sort(reverse=True)). index(2) 2 kyun hai? Reverse karne ke baad list [4,4,2,1] hai; 2 position 2 pe baith jaata hai.

Worked example Forecast-then-Verify (run karne se pehle predict karo)

s = ['b', 'a', 'c', 'a']
s.sort()                   # forecast: ['a','a','b','c'] ?
print(s.count('a'))        # forecast: 2 ?
print(s.index('b'))        # forecast: 2 ?

Chalao. Strings alphabetically sort hoti hain, toh ['a','a','b','c'] ✓, count('a')=2 ✓, index('b')=2 ✓.

Recall Feynman: 12 saal ke bacche ko samjhao

Socho numbered lockers ki ek row hai. append ek bag ko end mein agle khaali locker mein daal deta hai. insert ek bag ko kisi chosen locker mein squeeze karta hai aur uske baad walon ko ek neeche dhakelta hai. pop ek bag nikaal ke aapko de deta hai. remove ek specific bag dhundh ke phenk deta hai (aapko kuch wapas nahi deta). sort saare bags ko chhote se bade order mein lagata hai, reverse poori row ko ulta kar deta hai. count batata hai "kitne red bags hain?" aur index batata hai "pehla red bag kis locker mein hai?" Waapas-karo-ya-nahi ka ye difference hi poora trick hai: pop aapko kuch deta hai, baaki nahi dete.

Flashcards

list.append(x) kya return karta hai?

None — ye list ko in place mutate karta hai, x ko end mein add karta hai.

remove aur pop mein kya difference hai?

remove(value) pehli matching value delete karta hai aur None return karta hai; pop(index) index ke hisaab se remove karta hai aur item return karta hai.

pop() bina argument ke kya karta hai?

LAST element ko remove karke return karta hai.

sort() kya return karta hai, aur uski jagah naya sorted list kaise paayein?

None return karta hai (in place sort karta hai); naye list ke liye sorted(lst) use karo.

Kya reverse() list ko sort karta hai?

Nahi — ye sirf current order ko flip karta hai; descending chahiye toh pehle sort karo.

lst.remove(x) ke saath kya hoga agar x present na ho?

ValueError raise karta hai.

count(x) kya return karta hai?

List mein x kitni baar aaya, woh number.

Missing element ke liye index(x) kya return karta hai?

ValueError raise karta hai; warna pehli occurrence ka index.

[3,1,2].insert(1,9) ke baad list kya hai?

[3, 9, 1, 2] — index ≥1 pe items right shift ho jaate hain.

Ek call mein descending sort kaise karo?

lst.sort(reverse=True).

Connections

Lists — creation and indexing
Mutability vs Immutability in Python
sorted() vs list.sort()
Tuples — why no append/sort
Time complexity — append O(1) vs insert O(n)
Strings — index and count methods
Exceptions — ValueError and IndexError