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private long stringToLong(String pNumber) {
long result = 0;
String list = "0123456789";
for (int i = 0; i <= pNumber.length()-1; i++) {
char c = (char) pNumber.charAt(i);
int lastValue = list.indexOf(c);
result= result*10 + lastValue;
}
return result;
}

public static long StringToLong(String s) throws NumberFormatException{
	long n = 0;
	int factor = 1;
	int c;
	for(int i = s.length()-1; i > 0; --i ){
		c = s.charAt(i);
		if(c < 48 || c > 57){
			throw new NumberFormatException(s+" is not a valid number.");
		}
		n += (c-48) * factor;
		factor *= 10;
	}
	
	c = s.charAt(0);
	if(c == '-'){
		return -n;
	}else{
		n += (c-48) * factor;
	}
	return n;
}

long StringToLong(String str)
{
/* code goes here to convert a string to a long */ 
		long result = 0;
		int factor = 1;
		for(int i=str.length()-1;i>-1;i--)
		{
			char c = str.charAt(i);
			int k = c-48;
			result = (factor*k)+result;
			factor = factor * 10;
		}
		
		System.out.println("Therefore the long number is : " + result);
return result;
}

public static long stringToLong(String str){

long result = 0;
int factor =10;
char[] ch = str.toCharArray();
for(int i = 0;i<str.length();i++){
int temp = ch[i]-48;
if(temp>=0||temp<=9){
result = result*factor+temp;

}

}


return result;
}

public class stringtolong {

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
long i = stringToLong("123");
if (i == 123)
System.out.println("success !!");
else
System.out.println("failure !!");
}

static long stringToLong(String s){
long value = 0, longNumber=1;
char counter; //counter to read each character in string
int i=s.length()-1;
while(i>=0)
{
counter = s.charAt(i);
value = value + (counter-48) * longNumber;
longNumber = longNumber * 10;
i--;
}
return value; //returns long value of the given string
}
}

string sx = "567";
int ix = 0;
for (int i = 0; i < sx.Length; i++)
{
ix *= 10;
ix += sx[i] - 48;
}
write ix;

Extarct all the characters from start and add it in long.

logic:

char[] lookupNumber = {'1','2','3'....'8','9'};
string c = "1000";

long l;
while(c!=empty)
{
extractedCharacter = extartFromStartOfString(string c);
l = l*10 + lookup(extractCharacter);
}

int lookup(char c)
{
return binarySearchInLookupNumber (C)
//this would return index.
}

This uses no inbuilt function of any language.

int strlen(const std::string a){
int length=0;
while(a[length])
length++;
//throw exception if string is empty
if(length<1) throw -1;
return length;
}
//look up the ascii code
//and check if any non-numerical char exist
long lookup(char a){
int temp=int(a);
if(temp<48 || temp>57) throw temp;

switch(temp){
case 48:
return 0;
case 49:
return 1;
case 50:
return 2;
case 51:
return 3;
case 52:
return 4;
case 53:
return 5;
case 54:
return 6;
case 55:
return 7;
case 56:
return 8;
case 57:
return 9;
default:
return -1;
:#include "atol.h"

//compute string length
int strlen(const std::string a){
int length=0;
while(a[length])
length++;
//throw exception if string is empty
if(length<1) throw -1;
return length;
}
//look up the ascii code
//and check if any non-numerical char exist
long lookup(char a){
int temp=int(a);
if(temp<48 || temp>57) throw temp;

switch(temp){
case 48:
return 0;
case 49:
return 1;
case 50:
return 2;
case 51:
return 3;
case 52:
return 4;
case 53:
return 5;
case 54:
return 6;
case 55:
return 7;
case 56:
return 8;
case 57:
return 9;
default:
return -1;
}
}

//convert string to long
long stringToLong(const std::string s){
int len=strlen(s);
long temp=0;
long temp2=0;
//flag for negative input
int flag=1;
int i=0;

//check negativity
if(s[0]=='-'){
flag=-1;
i++;
}

for(;i<len;i++)
{
//throw an exception if the number is too large (or too small)
if((LONG_MAX-10*temp)<lookup(s[i])) throw 99;
temp=10*temp+lookup(s[i]);
}
return flag*temp;
}