Write a Python program to create a dictionary from a string ‘w3resource’ such that each individual character m

Programming Problems · Q9

Q. Write a Python program to create a dictionary from a string ‘w3resource’ such that each individual character mates a key and its index value for fist occurrence males the corresponding value in dictionary. Expected output : {'3': 1, 's': 4, 'r': 2, 'u': 6, 'w': 0, 'c': 8, 'e': 3, 'o': 5}

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Concept understanding — Dictionary Key Access

Dictionary Key Access

Imagine you have a physical address book. Each entry has a name (like "Ravi") and a phone number (like "[PII:phone]"). To find Ravi's number, you don't flip through every page — you look up "Ravi" directly. That name is the key, and the number is the value.

A dictionary in programming is exactly that: a collection of key-value pairs. The key is what you know; the value is what you want to retrieve. Key access is the operation of giving the dictionary a key and getting back the value tied to it.

The Intuition

Think of a real dictionary (the book). You look up a word (the key) to find its definition (the value). You never look up a definition to find the word — that would be backwards. Similarly, in a Python dictionary, you always access values by their keys, never the other way around.

Note

Keys must be unique and immutable (strings, numbers, tuples). Values can be anything — numbers, strings, lists, even other dictionaries.

The Precise Statement

In Python, dictionary key access is done using square brackets [] with the key inside, or the .get() method.

Syntax:

dictionary_name[key]

dictionary_name.get(key, default_value)

Example:

student = {"name": "Anjali", "age": 17, "grade": "A"}
print(student["name"])   # Output: Anjali
print(student.get("age")) # Output: 17

What Happens Under the Hood

When you write student["name"], Python:

Takes the key "name".
Computes its hash (a fixed-size number derived from the key's content).
Uses that hash to jump directly to the memory location where the value "Anjali" is stored.

This is why dictionary access is fast — it doesn't scan through all entries. It's a direct lookup, like using an index in a book's index rather than reading every page.

Watch out

If you use a key that does not exist with square brackets, Python raises a KeyError and your program crashes. The .get() method is safer — it returns None (or a default you provide) instead of crashing.

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Guard the assignment so a repeated character keeps its first index (r stays 2, e stays 3).

d = {}
for i, ch in enumerate('w3resource'):
    if ch not in d:
        d[ch] = i
print(d)

{'w': 0, '3': 1, 'r': 2, 'e': 3, 's': 4, 'o': 5, 'u': 6, 'c': 8} …

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Loop over the string with enumerate() and add char: index to the dictionary only if the character isn't already a key — that "only first time" guard is what keeps the first

occurrence's index. (The stem's odd wording — "mates a key", "males the value" — is a typo for

makes: each character becomes a key, its first index the value.)

The idea

'w3resource' has a repeated letter (r at indices 2 and 7, e at 3 and 9). If we wrote

d[ch] = i unconditionally, a later occurrence would overwrite the earlier index. The guard

if ch not in d blocks that overwrite, preserving the first occurrence.

Program

# Dictionary of character -> index of FIRST occurrence
text = 'w3resource'
d = {}
for i, ch in enumerate(text):   # enumerate yields (index, character) pairs
    if ch not in d:             # keep only the FIRST occurrence's index
        d[ch] = i
print(d)

Dry run

i	ch	Already a key?	Action
0	w	no	`d['w'] = 0`
1	3	no	`d['3'] = 1`
2	r	no	`d['r'] = 2`
3	e	no	`d['e'] = 3`
4	s	no	`d['s'] = 4`
5	o	no	`d['o'] = 5`
6	u	no	`d['u'] = 6`
7	r	yes	skipped — index 2 kept
8	c	no	`d['c'] = 8`
9	e	yes	skipped — index 3 kept

Output

{'w': 0, '3': 1, 'r': 2, 'e': 3, 's': 4, 'o': 5, 'u': 6, 'c': 8}

This contains exactly the same 8 key-value pairs as the book's expected output

{'3': 1, 's': 4, 'r': 2, 'u': 6, 'w': 0, 'c': 8, 'e': 3, 'o': 5} — only the printing order …

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Q.Write a Python program to create a dictionary from a string ‘w3resource’ such that each individual character mates a key and its index value for fist occurrence males the corresponding value in dictionary. Expected output : {'3': 1, 's': 4, 'r': 2, 'u': 6, 'w': 0, 'c': 8, 'e': 3, 'o': 5}

The idea

Program

Dry run

Output

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Q.Write a Python program to create a dictionary from a string ‘w3resource’ such that each individual character mates a key and its index value for fist occurrence males the corresponding value in dictionary.
Expected output : {'3': 1, 's': 4, 'r': 2, 'u': 6, 'w': 0, 'c': 8, 'e': 3, 'o': 5}

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