Write Your Own ATOI in Python

We are given a string that may represent a number and if it is a valid number then we have to convert it into an integer using the Python programming language. atoi() function is used in C programming language and used to convert the string which is passed as the parameter to it into an integer value if the string is a valid integer otherwise it shows the undefined behavior.

Sample Examples

Input 1

string S = "9834"

Output

9834

Explanation

We are given a string that represents a number so we have just got the same output.

Input 2

string S = "09 uy56" 

Output

Invalid Input 

Explanation

The given string is not a valid integer as it contains the whitespaces and lowercase English characters, so we have given the corresponding output.

Input 3

string str = "-987" 

Output

-987

String is Valid

In this approach, we will assume the given string is the valid string and will not contain any whitespaces at the start, middle, or at end of the string.

This string contains only the digits and may contain a '-' character representing the number is the negative number.

In this approach, first, we will create a function that will take a single parameter and return the integer which is the answer.

For a number to be negative there will be a minus sign in front of it, which means we have to check if the character at the zeroth index is '-'' or not.

We will traverse over the string and will maintain a number to store the answer. At each index, we will multiply the current number by 10 to increase one decimal point and then add the current digit to it.

In the end, we will return the final answer, let us see the complete code:

Example

# function to converting the string to an integer 
def atoi(str):
   # Assuming the string is valid 
   neg = 1 # checking for the negative number    
   if (str[0] == '-'):
      neg = -1      
   ans = 0 
   i = 0
   # if the number is the negative number then start from the next index 
   if(neg  == -1):
      i = i + 1    
   while (i < len(str)):
      cur = (int)(str[i])
      ans = ans * 10 + cur
      i = i + 1    
   ans =  ans* neg
   return ans; # returning the answer 
# defining the input and calling the function 
str = "-354663";     
# calling the function 
ans = atoi(str)    
# printing the answer
print("The value of the current number is:" , ans)

Output

The value of the current number is -354663

Time and Space Complexity

The time complexity of the above code is O(N), where N is the number of characters in the given string.

The space complexity of the above code is O(1), as we are not using any extra space.

String Can be Invalid

In this program, we will check if the current string can be invalid, so we will put a condition that if the string contains any character that is not in the range of the '0' to '9'. If any character occurs then we will return an Invalid string as the output otherwise we will follow the steps defined in the previous methods to get the output.

Also, we will use the ord function of the Python programming language to get the ASCII value of the current characters and add them to the number that stores the answer.

Example

# function for converting the string to an integer 
def atoi(str):
   # Assuming the string is valid 
   neg = 1 # checking for the negative number     
   if (str[0] == '-'):
      neg = -1        
   ans = 0 
   i = 0
   # if the number is negative number then start from the next index 
   if(neg  == -1):
      i = i + 1    
   while (i < len(str)):
      # Checking for the base conditions
      # if the current character is not a digit return the invalid answer 
      if((ord(str[i]) > ord('9')) or (ord(str[i]) < ord('0'))):
         print("The given string represents the invalid number")
         return
      cur = (int)(str[i])
      ans = ans * 10 + cur
      i = i + 1    
   ans =  ans* neg
   # printing the answer 
   print("The value of the current number is:", ans);
    
# defining the input and calling the function 
str = "-354 663";     
# Calling the function 
atoi(str)

Output

The given string represents the invalid number

Time and Space Complexity

The time complexity of the above code is O(N), where N is the number of characters in the given string.

The space complexity of the above code is O(1), as we are not using any extra space.

Conclusion

In this tutorial, we have implemented a Python program to convert the number present in the form of a string to an integer. We have traversed over the string and checked if the current string represents a valid number or not. We have used the ord() python function to get the ASCII values of the characters and add them to the answer.