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Converts a non-negative integer to an Excel-style alphabetic column label (A, B, C, ..., Z, AA, AB, ..., ZZ, AAA, etc.).

Purpose

This function generates alphabetic labels commonly used in spreadsheet applications like Excel for column headers. It converts zero-based integer indices to their corresponding alphabetic representation, supporting both uppercase (A-Z) and lowercase (a-z) output. The function handles multi-character labels by extending beyond Z to AA, AB, etc., similar to how spreadsheet columns are labeled.

Source Code

def int_to_alpha(n, upper=True):
    "Generates alphanumeric labels of form A-Z, AA-ZZ etc."
    casenum = 65 if upper else 97
    label = ''
    count= 0
    if n == 0: return str(chr(n + casenum))
    while n >= 0:
        mod, div = n % 26, n
        for _ in range(count):
            div //= 26
        div %= 26
        if count == 0:
            val = mod
        else:
            val = div
        label += str(chr(val + casenum))
        count += 1
        n -= 26**count
    return label[::-1]

Parameters

Parameter Details

n: A non-negative integer representing the index to convert. 0 corresponds to 'A' (or 'a'), 1 to 'B' (or 'b'), 25 to 'Z' (or 'z'), 26 to 'AA' (or 'aa'), etc. Must be >= 0.

upper: Boolean flag to determine case of output. If True (default), returns uppercase letters (A-Z). If False, returns lowercase letters (a-z).

Return Value

Returns a string containing the alphabetic label corresponding to the input integer. For n=0, returns 'A' (or 'a'). For n=25, returns 'Z' (or 'z'). For n=26, returns 'AA' (or 'aa'). The length of the string increases as n grows larger, following the pattern: single letters (0-25), double letters (26-701), triple letters (702+), etc.

Usage Example

# Basic usage
result1 = int_to_alpha(0)  # Returns 'A'
result2 = int_to_alpha(25)  # Returns 'Z'
result3 = int_to_alpha(26)  # Returns 'AA'
result4 = int_to_alpha(51)  # Returns 'AZ'
result5 = int_to_alpha(701)  # Returns 'ZZ'
result6 = int_to_alpha(702)  # Returns 'AAA'

# Lowercase output
result7 = int_to_alpha(0, upper=False)  # Returns 'a'
result8 = int_to_alpha(26, upper=False)  # Returns 'aa'

# Use case: Generate column labels for a table
column_labels = [int_to_alpha(i) for i in range(30)]
print(column_labels)  # ['A', 'B', 'C', ..., 'Z', 'AA', 'AB', 'AC', 'AD']

Best Practices

• Only pass non-negative integers to this function; negative values will cause unexpected behavior
• This function is useful for generating column headers in data tables, spreadsheets, or any UI requiring alphabetic sequential labels
• The function uses a base-26-like system but with a special handling where 'A' starts at 0, not 1
• For large values of n, the function will generate increasingly long strings (e.g., n=18278 produces 'AAAA')
• The algorithm reverses the label at the end, so intermediate string building is done in reverse order

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