Find Excel column number from column title

Try it on GfG Practice

We have discussed Conversion from column number to Excel Column name. In this post, reverse is discussed.

Given a column title as appears in an Excel sheet, return its corresponding column number.

column  column number
  A  ->  1
  B  ->  2
  C  ->  3
  ...
  Z  ->  26
  AA ->  27
  AB ->  28 

Examples: 

Input: A
Output: 1
A is the first column so the output is 1.

Input: AA
Output: 27
The columns are in order A, B, ..., Y, Z, AA ..
So, there are 26 columns after which AA comes.

Approach: The process is similar to binary to decimal conversion. 
For example, to convert AB, the formula is 26 * 1 + 2. 

As another example, 

To convert CDA,
3*26*26 + 4*26 + 1
= 26(3*26 + 4) + 1
= 26(0*26 + 3*26 + 4) + 1

So it is very much similar to converting binary to decimal keeping the base as 26. 
Take the input as string and the traverse the input string from the left to right and calculate the result as follows: 

result = 26*result + s[i] - 'A' + 1

The result will be summation of 
Result &= \sum_{i=0}^{n} a[n-i-1]*(26^{(n-i-1)})      .

Implementation:

// C++ program to return title to result
// of excel sheet.
#include <bits/stdc++.h>

using namespace std;

// Returns result when we pass title.
int titleToNumber(string s)
{
    // This process is similar to 
    // binary-to-decimal conversion
    int result = 0;
    for (const auto& c : s)
    {
        result *= 26;
        result += c  - 'A' + 1;
    }

    return result;
}

// Driver function
int main()
{
    cout << titleToNumber("CDA") << endl;
    return 0;
}

// Java program to return title 
// to result of excel sheet.
import java.util.*;
import java.lang.*;

class GFG
{

// Returns result when we pass title.
static int titleToNumber(String s)
{
    // This process is similar to 
    // binary-to-decimal conversion
    int result = 0;
    for (int i = 0; i < s.length(); i++)
    {
        result *= 26;
        result += s.charAt(i) - 'A' + 1;
    }
    return result;
}
    
// Driver Code
public static void main (String[] args) 
{
    System.out.print(titleToNumber("CDA"));
}
}

// This code is contributed 
// by Akanksha Rai(Abby_akku)

# Python program to return title to result 
# of excel sheet. 

# Returns result when we pass title. 
def titleToNumber(s):
    # This process is similar to binary-to- 
    # decimal conversion 
    result = 0; 
    for B in range(len(s)): 
        result *= 26; 
        result += ord(s[B]) - ord('A') + 1; 

    return result; 

# Driver function 
print(titleToNumber("CDA")); 

# This code contributed by Rajput-Ji

// C# program to return title 
// to result of excel sheet. 
using System;

class GFG
{

// Returns result when we pass title. 
public static int titleToNumber(string s)
{
    // This process is similar to 
    // binary-to-decimal conversion 
    int result = 0;
    for (int i = 0; i < s.Length; i++)
    {
        result *= 26;
        result += s[i] - 'A' + 1;
    }
    return result;
}

// Driver Code 
public static void Main(string[] args)
{
    Console.Write(titleToNumber("CDA"));
}
}

// This code is contributed by Shrikant13

<script>

// JavaScript program to return title 
// to result of excel sheet.

// Returns result when we pass title
function titleToNumber(s)
{
    // This process is similar to 
    // binary-to-decimal conversion
    let result = 0;
    for (let i = 0; i < s.length; i++)
    {
        result *= 26;
        result += s[i].charCodeAt(0) - 'A'.charCodeAt(0) + 1;
    }
    return result;
}
// Driver Code
document.write(titleToNumber("CDA"));

// This code is contributed by avanitrachhadiya2155

</script>

2133

Complexity Analysis: 

• Time Complexity: O(n), where n is length of input string.
• Space Complexity: O(1). 
  As no extra space is required.