String to Integer (atoi)

Question:

Implement atoi which converts a string to an integer.

The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.

If no valid conversion could be performed, a zero value is returned.

Note:

• Only the space character ' ' is considered as whitespace character.
• Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231, 231 − 1]. If the numerical value is out of the range of representable values, INT_MAX (231 − 1) or INT_MIN (−231) is returned.

Approach 1:

Here’s a very naive approach that has a lot of scope for improvement —

//Approach 1
//Runtime: 16ms
//Memory usage: 38.5MB
public int myAtoi(String str) {
 char[] characters = str.toCharArray();
 int startIndex = firstNonWhiteSpace(characters);
 int sign = 1;
 if(startIndex==-1 || characters[startIndex]==’+’){
 startIndex++;
 } else if(characters[startIndex]==’-’) {
 startIndex++;
 sign = -1;
 }
 
 int endIndex = getEndOfFirstIntegerPart(characters, startIndex);
 String numberString = str.substring(startIndex, endIndex);
 try {
 return Integer.valueOf(numberString) * sign;
 } catch(NumberFormatException nfe) {
 if(“”.equals(numberString)){
 return 0;
 }else if(sign==1) {
 return Integer.MAX_VALUE;
 } else {
 return Integer.MIN_VALUE;
 }
 } catch (Exception e) {
 return 0;
 }
 }
 
 private int firstNonWhiteSpace(char[] characters) {
 int index = 0;
 while(index<characters.length && characters[index++]==’ ‘);
 return index-1;
 }
 
 private int getEndOfFirstIntegerPart(char[] characters, int start) {
 for(int index = start;index<characters.length; index++) {
 switch (characters[index]) {
 case ‘0’:
 case ‘1’:
 case ‘2’:
 case ‘3’:
 case ‘4’:
 case ‘5’:
 case ‘6’:
 case ‘7’:
 case ‘8’:
 case ‘9’:
 break;
 default:
 return index;
 }
 }
 return characters.length;
 }

Well, the code works. Yet, there seems to be a long way to go. Here is a different approach you might want to check out. And here is another with a shorter run-time.

Find more posts here.

Cheers & Chao!