C++ 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.

Example: 

String value = 12546
Integer value = 12546
String value = GeeksforGeeks
Integer value = 0

Time Complexity : O(N)

Auxiliary Space : O(1) , as no extra space is needed.

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

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')

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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. 

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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. 

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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: 

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Below is the implementation of the above approach: 

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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!