1. Reverse a given string.
2. Count the number of vowels in a string.
3. Check if a string is a palindrome.
4. Find the length of a string without using built-in functions.
5. Convert a string to uppercase.
6. Convert a string to lowercase.
7. Count the number of words in a string.
8. Replace all spaces in a string with hyphens.
9. Find the frequency of a character in a string.
10. Remove all vowels from a string.
11. Find the first non-repeating character in a string.
12. Check if two strings are anagrams.
13. Remove all spaces from a string.
14. Replace all occurrences of a character in a string with another character.
15. Reverse the order of words in a string.
16. Find all substrings of a string.
17. Check if a string contains only digits.
18. Check if a string contains only alphabets.
19. Convert the first letter of each word in a string to uppercase.
20. Remove duplicate characters from a string.
21. Find the most frequent character in a string.
22. Concatenate two strings without using built-in functions.
23. Find the second most frequent character in a string.
24. Remove the first occurrence of a character in a string.
25. Remove all occurrences of a character in a string.
26. Check if a string starts with a given substring.
27. Check if a string ends with a given substring.
28. Check if a string contains a given substring.
29. Find the longest word in a string.
30. Convert a string to title case.
31. Find the number of uppercase and lowercase characters in a string.
32. Remove leading and trailing spaces from a string.
33. Convert a string to ASCII values.
34. Convert ASCII values back to a string.
35. Split a string into individual characters.
36. Find the shortest word in a string.
37. Count the number of punctuation marks in a string.
38. Reverse only the vowels in a string.
39. Swap the case of each character in a string.
40. Check if a string contains only uppercase characters.