What is a symbol in Julia?

In Julia, a symbol is a special type of data called Symbol that represents a name or identifier in code. It is denoted by : (colon), followed by the name of the symbol. Unlike strings, symbols are immutable, which means their values cannot be changed once created. This immutability grants symbols a few advantages over strings, especially in certain programming scenarios.

One significant advantage of symbol immutability is its efficiency in comparison operations. When comparing two symbols, Julia simply needs to check their memory addresses to determine if they refer to the same symbol. In contrast, comparing two strings requires examining each character in the strings until a mismatch is found or confirming that the entire strings are identical.

This process can be computationally expensive, especially when dealing with long strings. Therefore, in situations where quick and efficient comparison is crucial, such as searching algorithms, using symbols can significantly improve performance.

Using symbols in Julia

Consider a scenario where we have a collection of items and we want to find a specific item efficiently using a searching algorithm.

Let’s illustrate this with an example of a binary search algorithm using symbols:

# Function to perform binary search using symbols
function binary_search(arr::Vector{Symbol}, key::Symbol)
    low = 1             # Initialize the low pointer to the first index of the array
    high = length(arr)  # Initialize the high pointer to the last index of the array
    while low <= high   # Continue searching until the low pointer is less than or equal to the high pointer
        mid = (low + high) ÷ 2     # Calculate the midpoint index using integer division
        mid_val = arr[mid]         # Get the symbol at the midpoint index
        if mid_val == key          # If the midpoint symbol is equal to the desired symbol (key)
            return mid             # Return the index where the symbol is found
        elseif mid_val < key       # If the midpoint symbol is less than the desired symbol
            low = mid + 1          # Update the low pointer to search the right half of the array
        else                       # If the midpoint symbol is greater than the desired symbol
            high = mid - 1         # Update the high pointer to search the left half of the array
        end
    end
    return -1  # Return -1 if the symbol is not found in the array
end
# Example usage
items = [:apple, :banana, :grape, :orange, :strawberry]  # Sorted array of symbols
search_key = :grape                                      # Symbol we want to find
result = binary_search(items, search_key)                # Perform binary search
if result != -1
    println("Item found at index: ", result)             # If the symbol is found, print its index
else
    println("Item not found.")                           # If the symbol is not found, print a message
end

Code explanation

The code above is explained as follows:

  • Line 2: The binary_search function is defined to perform the binary search. It takes two arguments—arr, a sorted array of symbols, and key, the symbol we want to find within the array.

  • Lines 3–4: The low pointer is initialized to 1, representing the first index of the array, while the high pointer is initialized to the length of the array, representing the last index of the array.

  • Line 6: The while loop continues until low becomes greater than high, i.e., the search range is exhausted.

  • Lines 7–8: The midpoint index mid is calculated using integer division of the sum of low and high. The symbol at the midpoint index, mid_val, is retrieved from the array.

  • Lines 10–16: In the while loop, we have a condition to check if the mid_val is equal to key. If it is, then the desired symbol is found, and the function returns the index mid. If mid_val is less than key, we update the low pointer to search the right half of the array. On the other hand, if mid_val is greater than key, we update the high pointer to search the left half of the array.

  • Line 19: If the symbol is not found in the array, the function returns -1.

  • Lines 23–31: The example usage section demonstrates how to use the binary_search function. It creates a sorted array of symbols (items) and a symbol to search for (search_key). The function is called, and the result is stored in the variable result. Depending on the result of the binary search, either the index of the symbol is printed when found, or a message indicating that the symbol is not found is printed.

Using strings instead of symbols

If we used strings instead of symbols in this binary search algorithm, the process would still work correctly, but it would be less efficient in terms of comparison. For large arrays or repetitive search operations, the performance difference between using symbols and strings can become noticeable. By using symbols in this context, the binary search algorithm can take advantage of faster comparisons and improve the overall efficiency of the searching process.

Remember to use symbols wisely, keeping in mind their immutability and potential memory implications, to make the most of this unique feature offered by the Julia programming language.

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