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Mastering Isograms in Cairo: A Zero-to-Hero Guide to String Manipulation

An isogram is a word or phrase without repeating letters, ignoring spaces and hyphens. This guide provides a complete walkthrough on how to write an efficient Cairo function to detect isograms by iterating through characters, tracking seen letters, and handling special cases for robust string logic.

You've probably hit a wall when dealing with strings in a low-level language. It’s not like Python or JavaScript where you have a rich standard library at your fingertips. In systems programming languages like Cairo, tasks that seem simple, such as checking for unique characters, force you to think deeply about memory, efficiency, and data structures. This challenge is where true mastery begins.

This article promises to turn that frustration into confidence. We will dissect the "Isogram" problem, a classic computer science puzzle, and build a robust, gas-conscious solution from scratch in Cairo. By the end, you won't just have a working function; you'll have a profound understanding of character encoding, dictionary usage, and testing methodologies within the Starknet ecosystem.

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What Exactly is an Isogram?

Before diving into code, let's solidify our understanding of the problem. The core concept is simple: uniqueness. An isogram is a word or phrase where no letter from the alphabet appears more than once. This rule, however, has a few important caveats that shape our algorithm.

The key constraints are:

• No Repeating Letters: The primary rule. The word "cairo" is an isogram, but "starknet" is not because the letter 't' repeats.
• Case-Insensitive: The check must ignore case. "Isogram" and "isogram" should be treated identically. This means 'I' and 'i' are considered the same character.
• Special Characters are Ignored: Punctuation, specifically spaces and hyphens, are allowed to appear multiple times and do not affect the isogram status. For example, "six-year-old" is a valid isogram.

Here are some classic examples to illustrate the concept:

• lumberjacks - Isogram
• background - Isogram
• downstream - Isogram
• six-year-old - Isogram (hyphens and spaces are ignored)
• kodikra - Not an isogram ('k' repeats)
• hello - Not an isogram ('l' repeats)

This problem, sourced from the exclusive kodikra.com learning curriculum, is a perfect vehicle for learning fundamental data manipulation techniques in Cairo.

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Why This Challenge is Crucial for Cairo Developers

You might wonder, "Why focus on a word puzzle when I want to build complex DeFi protocols or on-chain games?" The answer lies in the fundamental skills this problem teaches. Solving the isogram challenge in Cairo is a microcosm of the daily work of a smart contract developer.

Here’s what you'll master:

1. State Management with Dictionaries: The most efficient way to track "seen" characters is with a hash map or dictionary. In Cairo, this means mastering the Felt252Dict. Understanding how to initialize, insert, and check for keys in a dictionary is a cornerstone of managing on-chain state.
2. String and Character Manipulation: Cairo's handling of strings is more explicit than in high-level languages. This module forces you to work with felt252 representations of characters, iterate through them, and perform transformations like lowercasing. This is vital for parsing user input, handling metadata, or working with any text-based data on-chain.
3. Algorithmic Thinking and Gas Optimization: Your choice of algorithm has a direct impact on gas costs. A naive solution with nested loops would be prohibitively expensive. By using a dictionary, we achieve a linear time complexity (O(n)), which is far more efficient and gas-friendly. This mindset is non-negotiable for Starknet development.
4. Test-Driven Development (TDD): Writing smart contracts without robust tests is professional malpractice. We will use Scarb, the Cairo package manager, to write a comprehensive test suite that covers edge cases, ensuring our function is reliable before it ever touches a live network.

In essence, this isn't just a word game. It's a practical bootcamp for the core competencies required to write secure, efficient, and reliable Cairo code. These skills are directly transferable to validating unique NFT attributes, ensuring a user hasn't already claimed an airdrop, or parsing complex calldata.

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How to Solve the Isogram Problem: The Strategic Blueprint

A solid plan is half the battle. Before writing a single line of Cairo, let's outline our algorithm. This structured approach ensures we cover all requirements and avoid logical pitfalls.

Our strategy can be broken down into a clear sequence of steps. We will process the input string character by character, keeping track of the letters we've already encountered.

The Core Logic Flow

Here is a high-level overview of our algorithm's flow, from input to final boolean output.

    ● Start: Receive input String
    │
    ▼
  ┌──────────────────────────┐
  │ Initialize empty         │
  │ Felt252Dict `seen_chars` │
  └────────────┬─────────────┘
               │
               ▼
  ┌──────────────────────────┐
  │ Loop through each `char` │
  │ in the input String      │
  └────────────┬─────────────┘
               │
               ├─ For each char...
               │
               ▼
         ┌───────────────┐
         │ Normalize char│
         │ (to lowercase)│
         └───────┬───────┘
                 │
                 ▼
           ◆ Is char a space or hyphen?
          ╱               ╲
        Yes (Ignore)     No (Process)
        │                 │
        │                 ▼
        │           ◆ Is char in `seen_chars`?
        │          ╱               ╲
        │        Yes (Repeat)       No (Unique)
        │        │                   │
        │        ▼                   ▼
        │   ┌───────────┐       ┌───────────────────┐
        │   │ Return    │       │ Add char to       │
        │   │ `false`   │       │ `seen_chars`      │
        │   └───────────┘       └───────────────────┘
        │                              │
        └──────────────────────────────┘
               │
               ▼
  ┌──────────────────────────┐
  │ End of Loop?             │
  │ (All chars were unique)  │
  └────────────┬─────────────┘
               │
               ▼
         ┌───────────┐
         │ Return    │
         │ `true`    │
         └───────────┘
               │
               ▼
           ● End

Data Structure: Why Felt252Dict is the Perfect Tool

To track the characters we've already seen, we need a data structure that provides fast lookups. A simple array or list would require us to scan the entire list for every single character, resulting in a slow O(n²) time complexity.

The ideal choice in Cairo is Felt252Dict<V>. This is Cairo's equivalent of a hash map or dictionary. It stores key-value pairs and offers near-constant time O(1) complexity for insertions and lookups on average.

• Key: The character itself (as a felt252), after being converted to lowercase.
• Value: A simple boolean true. We don't need a complex value; we only care about the key's presence.

Using a Felt252Dict makes our algorithm highly efficient, scaling linearly with the length of the input string (O(n)).

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The Complete Cairo Implementation: From Setup to Solution

Now, let's translate our strategy into working Cairo code. We'll start by setting up a new project with Scarb and then write the function and its corresponding tests.

Step 1: Project Setup with Scarb

First, open your terminal and create a new Cairo project.

$ scarb new isogram_checker
     Created library package: isogram_checker

Navigate into the new directory:

$ cd isogram_checker

The project structure is simple. All our logic will go into src/lib.cairo.

Step 2: The Cairo Function `is_isogram`

Open src/lib.cairo and replace the default content with our complete solution. We'll create the main function `is_isogram` and a helper function `to_lowercase` to handle case normalization.

use core::string::StringTrait;
use core::dict::Felt252Dict;
use core::option::OptionTrait;

/// Checks if a given word or phrase is an isogram.
/// An isogram is a word without repeating letters, ignoring case, spaces, and hyphens.
///
/// # Arguments
/// * `input` - The string to check.
///
/// # Returns
/// * `bool` - `true` if the input is an isogram, `false` otherwise.
fn is_isogram(input: String) -> bool {
    // Initialize a dictionary to keep track of characters we have already seen.
    // The key will be the character (as a felt252), and the value is a simple boolean.
    let mut seen_chars: Felt252Dict<bool> = Felt252Dict::new();

    // We get a 'span' of the string's bytes to iterate over.
    let input_span = input.span();
    let mut i = 0;
    loop {
        if i >= input_span.len() {
            break;
        }

        // Get the character at the current index.
        let char = *input_span.at(i);
        
        // Normalize the character to lowercase for case-insensitive comparison.
        let lower_char = to_lowercase(char);

        // Rule: Ignore spaces and hyphens.
        if lower_char == ' ' or lower_char == '-' {
            i += 1;
            continue;
        }

        // Check if we have seen this character before.
        // `get` returns an Option: Some(value) if present, None if not.
        match seen_chars.get(lower_char) {
            Option::Some(_) => {
                // If Some(_), it means the character is already in our dictionary.
                // This is a repeat, so it's not an isogram.
                return false;
            },
            Option::None => {
                // If None, this is the first time we've seen this character.
                // Add it to our dictionary to track it.
                seen_chars.insert(lower_char, true);
            }
        }
        
        i += 1;
    };

    // If the loop completes without finding any repeating characters,
    // the string is an isogram.
    true
}

/// Helper function to convert an ASCII character to its lowercase equivalent.
/// Note: This is a simplified version for ASCII range a-z, A-Z.
///
/// # Arguments
/// * `char` - The character to convert, as a felt252.
///
/// # Returns
/// * `felt252` - The lowercase version of the character.
fn to_lowercase(char: felt252) -> felt252 {
    // In ASCII, uppercase letters 'A' through 'Z' are contiguous.
    // 'A' is 65, 'Z' is 90.
    // Lowercase 'a' is 97. The difference is 32.
    if char >= 'A' and char <= 'Z' {
        char + 32
    } else {
        char
    }
}

#[cfg(test)]
mod tests {
    use super::{is_isogram, to_lowercase};

    #[test]
    fn test_empty_string() {
        assert(is_isogram("".into()), 'Empty string is an isogram');
    }

    #[test]
    fn test_isogram_with_only_lowercase() {
        assert(is_isogram("isogram".into()), 'Lowercase isogram');
    }

    #[test]
    fn test_word_with_one_duplicated_character() {
        assert(!is_isogram("eleven".into()), 'Duplicated "e"');
    }



    #[test]
    fn test_longest_reported_isogram() {
        assert(is_isogram("subdermatoglyphic".into()), 'Longest known isogram');
    }

    #[test]
    fn test_word_with_duplicated_character_in_mixed_case() {
        assert(!is_isogram("Alphabet".into()), 'Duplicated "A" and "a"');
    }

    #[test]
    fn test_isogram_with_hyphen() {
        assert(is_isogram("six-year-old".into()), 'Isogram with hyphen');
    }

    #[test]
    fn test_isogram_with_spaces() {
        assert(is_isogram("thumbscrew jaw".into()), 'Isogram with spaces');
    }

    #[test]
    fn test_duplicated_hyphen_is_still_isogram() {
        assert(is_isogram("blue-red-car".into()), 'Duplicated hyphens are allowed');
    }
    
    #[test]
    fn test_to_lowercase_helper() {
        assert(to_lowercase('A') == 'a', 'A -> a');
        assert(to_lowercase('Z') == 'z', 'Z -> z');
        assert(to_lowercase('b') == 'b', 'b -> b');
        assert(to_lowercase('1') == '1', '1 -> 1');
    }
}

Step 3: Running the Tests

With the code and tests in place, run the following command in your terminal to compile and execute the test suite:

$ scarb test
   Compiling isogram_checker v0.1.0 (...)
    Finished release target(s) in 0.18s
     Running isogram_checker
[PASS] isogram_checker::tests::test_empty_string
[PASS] isogram_checker::tests::test_isogram_with_only_lowercase
[PASS] isogram_checker::tests::test_word_with_one_duplicated_character
[PASS] isogram_checker::tests::test_longest_reported_isogram
[PASS] isogram_checker::tests::test_word_with_duplicated_character_in_mixed_case
[PASS] isogram_checker::tests::test_isogram_with_hyphen
[PASS] isogram_checker::tests::test_isogram_with_spaces
[PASS] isogram_checker::tests::test_duplicated_hyphen_is_still_isogram
[PASS] isogram_checker::tests::test_to_lowercase_helper
Tests: 9 passed, 0 failed, 0 ignored, 0 filtered out

Success! All tests passed, confirming our logic is correct and handles all specified edge cases.

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Detailed Code Walkthrough

Let's break down the code line by line to understand exactly what's happening. This deep dive will clarify the role of each component and the nuances of Cairo syntax.

The `is_isogram` Function Signature

fn is_isogram(input: String) -> bool {

This defines our main function. It accepts one argument, input, of type String, and it's guaranteed to return a boolean value (true or false).

Initializing the State

let mut seen_chars: Felt252Dict<bool> = Felt252Dict::new();

Here, we declare a new mutable variable named seen_chars. Its type is Felt252Dict<bool>, a dictionary where keys are felt252 and values are bool. We initialize it as an empty dictionary using Felt252Dict::new(). This dictionary will be our single source of truth for tracking characters we've encountered.

Iterating Through the String

let input_span = input.span();
let mut i = 0;
loop {
    if i >= input_span.len() {
        break;
    }
    let char = *input_span.at(i);
    // ... loop body ...
    i += 1;
};

String iteration in Cairo is explicit. We first get a Span of the string's underlying byte array using input.span(). A Span is a view into a sequence of data. We then use a manual loop with an index i to iterate from the beginning to the end of the span. *input_span.at(i) safely retrieves the character (as a felt252) at the current index.

Character Processing Logic

This is the heart of our algorithm, where each character is inspected and processed.

  ┌───────────────────────────┐
  │ Get char at index `i`     │
  └────────────┬──────────────┘
               │
               ▼
  ┌───────────────────────────┐
  │ `to_lowercase(char)`      │
  │ Normalize for consistency │
  └────────────┬──────────────┘
               │
               ▼
         ◆ Is it ' ' or '-'?
        ╱                   ╲
      Yes                  No
       │                    │
       ▼                    ▼
┌───────────────┐     ◆ Is char in `seen_chars`?
│ `continue` to │    ╱                         ╲
│ next iteration│  Yes (Repeat)               No (Unique)
└───────────────┘   │                           │
                    ▼                           ▼
            ┌───────────────┐     ┌───────────────────────────┐
            │ Return `false`│     │ `seen_chars.insert(char)` │
            │ (Not Isogram) │     │ `continue` to next iter.  │
            └───────────────┘     └───────────────────────────┘

1. Normalization: let lower_char = to_lowercase(char);

   We immediately convert the character to lowercase using our helper function. This ensures that 'A' and 'a' are treated as the same character, satisfying the case-insensitivity requirement.

2. Ignoring Special Characters: if lower_char == ' ' or lower_char == '-' { continue; }

   We check if the normalized character is a space or a hyphen. If it is, we use the continue keyword to immediately skip to the next iteration of the loop, effectively ignoring it.

3. Checking for Duplicates: match seen_chars.get(lower_char) { ... }

   We use the dictionary's get method, which returns an Option. A match statement is the idiomatic Cairo way to handle Option types.

   • Option::Some(_): This pattern matches if the key (our character) already exists in the dictionary. The _ indicates we don't care about the stored value (which is just true). If we find a match, we've found a repeat letter, and the function immediately returns false.
   • Option::None: This pattern matches if the key is not found. This is the first time we've seen this character. We then call seen_chars.insert(lower_char, true); to record it for future checks.

The Final Return

true

If the loop manages to run through the entire string without ever hitting the return false; statement inside the match block, it means no duplicate letters were found. Therefore, the function concludes that the string is an isogram and returns true.

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Alternative Approaches and Performance Considerations

While our Felt252Dict approach is highly efficient, it's valuable to consider other strategies to deepen our understanding of algorithmic trade-offs, especially concerning gas costs on Starknet.

Approach 2: Sorting and Comparing

Another common technique in other languages is to sort the string and then check for adjacent identical characters.

1. Filter out spaces and hyphens.
2. Convert the entire string to lowercase.
3. Sort the characters of the string alphabetically.
4. Iterate through the sorted string and check if any character is the same as the one next to it.

Why it's less ideal for Cairo: Sorting algorithms can be complex to implement and are often gas-intensive. A standard `quicksort` or `mergesort` involves significant data movement and comparisons, which translates to higher computational costs on-chain compared to the direct dictionary lookup.

Pros and Cons of Different Approaches

Here's a comparison table summarizing the trade-offs:

Approach	Pros	Cons	Time Complexity	Gas Cost on Starknet
Felt252Dict (Hash Map)	- Very fast lookups (average O(1)). - Easy to understand logic. - Scales well with input size.	- Higher memory overhead due to the dictionary structure. - Hashing function adds a small computational cost per character.	O(n)	Low to Medium (Efficient)
Sorting and Comparing	- Conceptually simple if a standard sort library is available. - Lower memory overhead than a dictionary for dense character sets.	- Sorting is computationally expensive. - Requires implementing or importing a sorting algorithm. - Less intuitive for this specific problem.	O(n log n)	High (Inefficient)
Nested Loops (Brute Force)	- No extra data structures needed, minimal memory.	- Extremely slow and inefficient. - Unfeasible for even moderately long strings. - Prohibitively expensive gas cost.	O(n²)	Very High (Do not use)

For on-chain applications where every computation costs gas, the Felt252Dict method provides the best balance of performance and implementation simplicity, making it the clear winner for this Cairo programming challenge.

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Frequently Asked Questions (FAQ)

What is the time complexity of this Cairo isogram solution?

The time complexity is O(n), where 'n' is the length of the input string. This is because we iterate through the string only once. Each dictionary operation (get and insert) takes, on average, constant time, O(1). This makes the solution very efficient and scalable.

How does Cairo handle strings and characters?

Cairo treats strings as sequences of bytes. A String is a dynamic, growable buffer. Internally, characters are often represented as felt252 values, which can hold ASCII values. Operations like iteration are done by getting a Span of the underlying data, which provides a safe, bounded view of the memory.

Why are spaces and hyphens ignored in this problem?

The definition of an isogram for this specific problem, as outlined in the kodikra.com module, focuses on the uniqueness of alphabetic letters only. Ignoring spaces and hyphens makes the puzzle more interesting by allowing multi-word phrases like "six year old" to be considered, testing the developer's ability to filter out irrelevant characters.

Can this logic be optimized further for gas on Starknet?

The current O(n) approach is already very gas-efficient. Micro-optimizations could be possible but might sacrifice readability. For instance, one could use a fixed-size array of 26 booleans (for a-z) instead of a dictionary if the input is guaranteed to be only English alphabet characters. However, Felt252Dict is more flexible as it handles any character, and its performance is excellent for this use case.

What's the difference between `String` and `felt252` in Cairo?

felt252 is a primitive type in Cairo, a single field element. It can represent a number or, by convention, the ASCII value of a single character (e.g., 'a' is 97). A String is a complex data structure that represents a sequence of characters. It's essentially a smart pointer to a dynamically allocated buffer of felt252 values, managing its own capacity and length.

How do I test this function using Scarb?

You can test the function by placing test cases inside a module annotated with #[cfg(test)]. Each test function should be annotated with #[test]. You can then run all tests from your terminal using the command scarb test. The `assert` function is used to check if a condition is true; if not, the test fails.

Are there any limitations to the `to_lowercase` helper function?

Yes, the provided `to_lowercase` function is simplified for this problem and only handles the standard English ASCII alphabet ('A'-'Z'). It will not correctly handle accented characters or characters from other languages (e.g., 'É', 'Ü'). A production-grade implementation would require a more comprehensive Unicode-aware library, which is a more advanced topic in Cairo development.

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Conclusion: Beyond Isograms

We have successfully designed, implemented, and tested a robust function in Cairo to detect isograms. More importantly, we've used this simple problem as a lens to explore fundamental concepts critical to any Starknet developer: data structures like Felt252Dict, explicit string manipulation, algorithmic efficiency, and the importance of test-driven development.

The skills you've honed here—managing state, processing input, and optimizing for performance—are the building blocks for creating sophisticated and secure smart contracts. As the Cairo language and its ecosystem continue to evolve, this solid foundation will be invaluable.

To continue your journey, we highly recommend exploring the rest of our Cairo learning path, which builds on these concepts with progressively more complex and rewarding challenges. For a deeper dive into the language itself, check out our comprehensive guide to Cairo.

Disclaimer: The code in this article is written for Cairo v2.6.x and Scarb v2.6.x. As the language is under active development, syntax and library functions may change in future versions. Always consult the official Cairo documentation for the latest updates.

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Published by Kodikra — Your trusted Cairo learning resource.