Integer & Long

The Log Formula	The Exponent formula
\[\log_{base}{index} = {power}\]	\[base^{power} = index\]
Example : \(\log_{10}{100} = 2\)	Example : \(10^{2} = 100\)

The Log Formula :

$$ \log_{base}{index} = {power} $$

The Exponent formula :

$$ base^{power} = index $$

Counting the number of digits

$$ \text{number of digits, n} = \lfloor \log_{10}(x) \rfloor + 1 \text{, where x is an integer number} $$

int num = 123456789;
int numberOfDigits = (int) Math.floor(Math.log10(num)) + 1;
//Using String Length
int numberOfDigitsWithString = String.valueOf(num).length();

Primitive with parse

parseInt & parseDouble: Used for parsing a String to a primitive int or double.

int i = Integer.parseInt("12345");
int j = Integer.valueOf("1234").intValue();
int k = Integer.valueOf(23).intValue();

Wrapper using valueOf

valueOf is Used for creating instances of wrapper classes from String or primitive to Wrapper

Integer fromPrimitiveInt = Integer.valueOf(25);//From String or primitive to Wrapper
Integer fromString = Integer.valueOf("90");

intValue and Auto unboxing

intValue: Used to retrieve the primitive int value from a wrapper object.

• Not null safe. Throws NumberFormatException when null is used

Integer fromPrimitiveInt = Integer.valueOf(25);//From primitive to Wrapper
Integer fromString = Integer.valueOf("25");// From String
int i1 = fromPrimitiveInt.intValue();

atoi() - String to Int without parseInt

Longer Approach $$ 12345 = 1\times10^4 + 2\times10^3 + 3\times10^2 + 4\times10^1 + 5 \times 10^0 $$

digit = (int) (digit + (x * Math.pow(10, str.length() - 1 - i)));

Left to Right String Processing. The C language atoi() function converts a character string to an integer value.

$$ 12345 = 1\times10 \rightarrow 10+2 \rightarrow (12\times10)+3 \rightarrow (123\times10)+4 \rightarrow (1234\times10)+5 $$

digit = (digit * 10) + x;