Python Program To Write Your Own atoi()

The atoi() function in C takes a string (which represents an integer) as an argument and returns its value of type int. So basically the function is used to convert a string argument to an integer.

Syntax:

int atoi(const char strn)

Parameters: The function accepts one parameter strn which refers to the string argument that is needed to be converted into its integer equivalent.

Return Value: If strn is a valid input, then the function returns the equivalent integer number for the passed string number. If no valid conversion takes place, then the function returns zero.

Now let’s understand various ways in which one can create their own atoi() function supported by various conditions:

Approach 1: Following is a simple implementation of conversion without considering any special case. 

• Initialize the result as 0.
• Start from the first character and update result for every character.
• For every character update the answer as result = result * 10 + (s[i] – ‘0’)

Python

# Python program for implementation of atoi

# A simple atoi() function


def myAtoi(string):
    res = 0

    # Iterate through all characters of
    #  input string and update result
    for i in xrange(len(string)):
        res = res * 10 + (ord(string[i]) - ord('0'))

    return res


# Driver program
string = "89789"

# Function call
print myAtoi(string)

# This code is contributed by BHAVYA JAIN

Output

89789

Approach 2: This implementation handles the negative numbers. If the first character is ‘-‘ then store the sign as negative and then convert the rest of the string to number using the previous approach while multiplying sign with it. 

Python

# Python program for implementation of atoi

# A simple atoi() function


def myAtoi(string):
    res = 0
    # initialize sign as positive
    sign = 1
    i = 0

    # if number is negative then update sign
    if string[0] == '-':
        sign = -1
        i += 1

    # Iterate through all characters
    # of input string and update result
    for j in xrange(i, len(string)):
        res = res*10+(ord(string[j])-ord('0'))

    return sign * res


# Driver code
string = "-123"

# Function call
print myAtoi(string)

# This code is contributed by BHAVYA JAIN

Output

-123

Approach 3: This implementation handles various type of errors. If str is NULL or str contains non-numeric characters then return 0 as the number is not valid. 

Output

 -134

Approach 4: Four corner cases needs to be handled: 

• Discards all leading whitespaces
• Sign of the number
• Overflow
• Invalid input

To remove the leading whitespaces run a loop until a character of the digit is reached. If the number is greater than or equal to INT_MAX/10. Then return INT_MAX if the sign is positive and return INT_MIN if the sign is negative. The other cases are handled in previous approaches. 

Dry Run:

Below is the implementation of the above approach: 

Python

# A simple Python3 program for
# implementation of atoi
import sys

def myAtoi(s):
    i, base, sign = 0, 0, 1

    # handle the case where s contains no digits (or input is malformed)
    if not (set(s) & set('0123456789')):
      return 0

    # increment past whitespace (ignore)
    while s[i] == ' ':
        i += 1

    # compute sign (2s complement)
    if s[i] == '-' or s[i] == '+':
        sign = 1 - 2 * (s[i] == '-')
        i += 1

    # loop as long as the next input is a digit
    while i < len(s) and '0' <= s[i] <= '9':

        # test for overflow
        if (base > (sys.maxsize // 10) or
           (base == (sys.maxsize // 10) and
           (ord(s[i]) - ord('0')) > 7)):

          # clamp to +/- maxsize
          if sign == 1:
            return sys.maxsize
          else:
            return -(sys.maxsize) - 1

        base = 10 * base
        base += (ord(s[i]) - ord('0'))
        i += 1

    return base * sign

# Driver Code
s = "   123"

# Functional Code
val = myAtoi(s)

print(val)

# This code is contributed by divyeshrabadiya07, justinalangley

Output

 -123

Complexity Analysis for all the above Approaches:

• Time Complexity: O(n). 
  Only one traversal of string is needed.
• Space Complexity: O(1). 
  As no extra space is required.

Recursive program for atoi().

Exercise:
Write your won atof() that takes a string (which represents an floating point value) as an argument and returns its value as double.

Please refer complete article on Write your own atoi() for more details!