Program 10-7: Write a program to count the number of times a character appears in a given string.

Question

Accepted Answer

> [!TLDR]
> One pass over the string with an if/else on `ch in dic` counts every character: existing key -> +1, new character -> new key with count 1.

**The idea.** Counting occurrences is the classic dictionary pattern: the character is the key, its running count the value. The membership test `ch in dic` splits the two cases — first sighting (create the key) versus repeat (increment). One linear pass, O(n), no nested searching.

**Program 10-7:**

```python
st = input("Enter a string: ")
dic = {}
for ch in st:
    if ch in dic:                  # seen before -> increment its count
        dic[ch] = dic[ch] + 1
    else:                          # first time -> create the key with count 1
        dic[ch] = 1
print("Number of times each character appears in the string:")
for key in dic:
    print(key, ":", dic[key])
```

**Sample run:**

```
Enter a string: HelloWorld
Number of times each character appears in the string:
H : 1
e : 1
l : 3
o : 2
W : 1
r : 1
d : 1
```

**Trace for 'Hello' (first five characters):**

| ch | in dic? | action        | dic after                          |
|----|---------|---------------|------------------------------------|
| H  | no      | create, 1     | {'H': 1}                           |
| e  | no      | create, 1     | {'H': 1, 'e': 1}                   |
| l  | no      | create, 1     | {'H': 1, 'e': 1, 'l': 1}           | …

Q.Program 10-7: Write a program to count the number of times a character appears in a given string.

Concept understanding — Character Counting

ch	in dic?	action	dic after
H	no	create, 1	{'H': 1}
e	no	create, 1	{'H': 1, 'e': 1}
l	no	create, 1	{'H': 1, 'e': 1, 'l': 1}

Q.Program 10-7: Write a program to count the number of times a character appears in a given string.

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