CareerCup

// to-do: 1.take the first char in str for signed/unsigned check
// 2.check for invalid char in str
int my_atoi(const char *str)
{
int i=0;
while ( *str )
{
i = (i<<3) + (i<<1) + ((*str) - '0');
++str;
}

return i;
}

Thanks Yokuki,

This is one of the most awesome tricks I have ever learned! It made my day.

For those wondering how it works:

You have string "364"

Taking chars from left (one by one):

Example: '3'

('3' - '0') - This gives the actual number in the char.

So i becomes 3. (initially i was 0, therefore bit manipulations resulted in 0)

Then we have to somehow make i from 3 to 30 to left shift the number.

Basically, we want to multiply the number by 10.
(i<<3) + (i<<1) does exactly that.

(i<<3) is 8x
(i<<1)is 2x

(i<<3) + (i<<1) becomes 10x.

Well, I guess now its clear to every one.

int myatoi(char* input)
{
int retVal = 0;
int length = 0;
while(true)
{
if(input[length] != '\0')
{
int val = input[length] - '0';
retVal = retVal*10 + val;
length++;
}
else
{
break;
}
}
return retVal;
}

Twist: Handle negative numbers and out of range case

int atoi( char* source )
{
	int ret = 0;
	int negative = 1;
	char* i = source;
	if( *i == '-' ){
		negative = -1;
		i++;
	}
	for( ; *i; i++ )
	{
		if( *source >= '0' && *source <= '9' ){
			int temp = ret*10 + (negative)*(*source-'0');
			if( temp < MININT || temp > MAXINT )
				return 0; // or whatever is your out of range handler
			else
				ret = temp;
		else
			return ret;
	}
	return ret;
}

#define MIN_INT -2147483648
#define MAX_INT 2147483647

int my_atoi(const char str[])
{
if(str == NULL) return 0;

int len = strlen(str);

if(len <= 0) return 0;

int index = 0;

//skip leading spaces
while(str[index] == ' ') index++;

bool isNeg = str[index] == '-';
int outNum = 0;

if(isNeg)
{
index++;
// skip white space after the sign
while(str[index] == ' ') index++;
}

while(index < len)
{
char currentChar = str[index++];
if(currentChar >= '0' && currentChar <= '9')
{
int oldValue = outNum;
int charVal = currentChar - '0';
outNum *= 10;
outNum += charVal;

//overflow underflow detection
if(outNum < oldValue)
{
if(isNeg)
outNum = MIN_INT;
else
outNum = MAX_INT;
return outNum;
}
}
else
break;
}
if(isNeg)
outNum = outNum * -1;
return outNum;
}