Bit field using a for loop

Isogram
Isogram in Rust 
const A_LCASE: u8 = 97;
const Z_LCASE: u8 = 122;
const A_UCASE: u8 = 65;
const Z_UCASE: u8 = 90;

pub fn check(candidate: &str) -> bool {
    let mut letter_flags: u32 = 0;

    for letter in candidate.bytes() {
        if letter >= A_LCASE && letter <= Z_LCASE {
            if letter_flags & (1 << (letter - A_LCASE)) != 0 {
                return false;
            } else {
                letter_flags |= 1 << (letter - A_LCASE);
            }
        } else if letter >= A_UCASE && letter <= Z_UCASE {
            if letter_flags & (1 << (letter - A_UCASE)) != 0 {
                return false;
            } else {
                letter_flags |= 1 << (letter - A_UCASE);
            }
        }
    }
    return true;
}

This solution uses the ASCII value of the letter to set the corresponding bit position.

First, some const values are set. These values will be used for readability in the body of the check function.

An unsigned 32-bit integer (u32) will be used to hold the twenty-six bits needed to keep track of the letters in the English alphabet.

The for loop loops through the bytes of candidate. Each letter is a u8 which is tested for being a through z or A through Z. The ASCII values defined as the const values are used for that. The ASCII value for a is 97, and for z is 122. The ASCII value for A is 65, and for Z is 90.

  • If the lower-cased letter is subtracted by a, then a will result in 0, because 97 minus 97 equals 0. z would result in 25, because 122 minus 97 equals 25. So a would have 1 shifted left 0 places (so not shifted at all) and z would have 1 shifted left 25 places.
  • If the upper-cased letter is subtracted by A, then A will result in 0, because 65 minus 65 equals 0. Z would result in 25, because 90 minus 65 equals 25. So A would have 1 shifted left 0 places (so not shifted at all) and Z would have 1 shifted left 25 places.

In that way, both a lower-cased z and an upper-cased Z can share the same position in the bit field.

So, for an unsigned thirty-two bit integer, if the values for a and Z were both set, the bits would look like

      zyxwvutsrqponmlkjihgfedcba
00000010000000000000000000000001

You can use the bitwise AND operator to check if a bit has already been set. If it has been set, you know the letter is duplicated and you can immediately return false. If it has not been set, you can use the bitwise OR operator to set the bit. If the loop completes without finding a duplicate letter (and returning false), the function returns true.

6th Nov 2024 · Found it useful?

Other Approaches to Isogram in Rust

Other ways our community solved this exercise
pub fn check(candidate: &str) -> bool {
    let mut hs = std::collections::HashSet::new();
    candidate
        .bytes()
        .filter(|c| c.is_ascii_alphabetic())
        .map(|c| c.to_ascii_lowercase())
        .all(|c| hs.insert(c))
}
Filter with All on a HashSet

Use Filter with All on a HashSet to return the answer. 

candidate.bytes()
    .filter_map(|c| {
        c.is_ascii_alphabetic()
            .then(|| 1u32 << (c.to_ascii_lowercase() - A_LCASE))
    })
    .try_fold(0u32, |ltr_flags, ltr| {
        (ltr_flags & ltr == 0).then(|| ltr_flags | ltr)
    }).is_some()
Bit field used functionally

Use a bit field functionally to keep track of used letters.