How do you count how many times characters from String B appear in String A?

Question

How can you determine the total number of times any character from String B appears within String A, taking into account each occurrence of these characters in String A?

Accepted Answer

To determine the total number of times characters from String B appear in String A, you can follow these steps:

### Approach

1. **Initialize a Counter**: Start by initializing a counter to keep track of the total occurrences of characters from String B in String A.

2. **Create a Set for String B**: Convert String B into a set. This helps in quickly checking if a character in String A is one of the characters to count. Using a set is efficient because lookups in sets have an average time complexity of O(1).

3. **Iterate Over String A**: Loop through each character in String A and check if it exists in the set created from String B.

4. **Count Occurrences**: For each character in String A that is present in the set, increment the counter by 1.

5. **Return the Result**: After finishing the iteration over String A, the counter will hold the total number of times characters from String B appear in String A.

### Implementation

Here is an example implementation in Python:

```python
def count_occurrences(string_a, string_b):
    # Convert string_b into a set for O(1) lookup
    char_set = set(string_b)
    
    # Initialize a counter for occurrences
    count = 0
    
    # Iterate over each character in string_a
    for char in string_a:
        # If the character is in the set, increment the counter
        if char in char_set:
            count += 1
            
    return count

# Example usage:
string_a = "hello world"
string_b = "lo"
result = count_occurrences(string_a, string_b)
print(f"Total occurrences: {result}")
```

### Explanation with Example

Consider `string_a = "hello world"` and `string_b = "lo"`. The set created from String B would be `{'l', 'o'}`.

- Iterating through "hello world", you encounter:
  - 'h' (not in set, skip)
  - 'e' (not in set, skip)
  - 'l' (in set, count becomes 1)
  - 'l' (in set, count becomes 2)
  - 'o' (in set, count becomes 3)
  - (and so on for the rest of the string)

The final count after completing the loop will be the total occurrences of characters from String B in String A.

Using this method is efficient and straightforward, ensuring each character from String A is considered exactly once per its appearance, and allows for dynamically sized input strings with linear time complexity relative to the length of String A.

How do you count how many times characters from String B appear in String A?

Question Explain

Answer Example

Related Interview Questions