index() and count()

What you'll learn

Topic	Focus
index()	Find the first position
ValueError	What happens when text is missing
count()	Count repeated characters or words
Difference	index() vs count()

Visual roadmap

Think of these methods as answering two different questions.

1. index() → Find the first position

index() returns the starting position of the first match.

s = "python"

print(s.index("p"))   # 0
print(s.index("o"))   # 4
print(s.index("n"))   # 5

Memory: index() answers "Where is it?"

2. Finding a word (substring)

index() also works with multiple characters.

s = "python is an oop lang"

print(s.index("is"))    # 7
print(s.index("an"))    # 10
print(s.index("thon"))  # 2

How Python calculates:

	p	y	t	h	o	n	(space)	i	s
Index	0	1	2	3	4	5	6	7	8

3. When the value is missing

If the substring doesn't exist, index() raises a ValueError.

s = "python"

s.index("K")
# ValueError: substring not found

Warning: index() does not return -1; it throws an exception.

4. Finding every occurrence

index() finds only the first match. Use enumerate() to locate all matches.

s = "MISSISSIPPI"

for index, ch in enumerate(s):
    if ch == "I":
        print(index, "--->", ch)

Output:

1 ---> I
4 ---> I
7 ---> I
10 ---> I

Memory: enumerate() gives (index, value) together.

5. count() → Count occurrences

count() tells you how many times something appears.

s = "MISSISSIPPI"

print(s.count("M"))   # 1
print(s.count("S"))   # 4
print(s.count("P"))   # 2
print(s.count("I"))   # 4

Memory: count() answers "How many?"

6. Counting in numbers

Numbers must first become a string.

n = 123321456234123412

s = str(n)

print(s.count("1"))   # 4
print(s.count("2"))   # 5

Why str()?

s = "12345"

print(s.count("1"))   # Correct

# print(s.count(1))
# TypeError

count() expects a string argument, not an integer.

7. index() vs count()

These methods solve different problems.

Feature	index()	count()
Purpose	Find position	Count occurrences
Returns	Integer index	Integer count
Not found	ValueError	0

Playground example

s = "MISSISSIPPI"

print("index of I:", s.index("I"))

print("count M:", s.count("M"))
print("count S:", s.count("S"))
print("count I:", s.count("I"))

word = "python is an oop lang"

print("index 'is':", word.index("is"))
print("index 'an':", word.index("an"))

Expected output:

index of I: 1
count M: 1
count S: 4
count I: 4
index 'is': 7
index 'an': 10

Use the lesson playground at the bottom of the page to run this code live.

Quick knowledge check

1. What does "python".index("o") return?
   Answer: 4

2. What happens if the substring is missing?
   Answer: index() raises ValueError, while count() returns 0.

3. How many "S" characters are in "MISSISSIPPI"?
   Answer: 4

30-second revision sheet

Method	Easy memory
index()	Find the first position
count()	Count occurrences
enumerate()	Find every position

Interview shortcut: Where? → index() · How many? → count() · Missing? → Error vs 0

Next: Strip Methods.