Java: How to round a float to two, four, or other specified digits after the decimal point.

Java: How to round a float to two, four, or other specified digits after the decimal point.

I thought it was easy and never cared about it. I suddenly used it today and found that the system didn't have such a function. the method used by colleagues is to convert it to string first, then several digits, and then to float type (such as: string. valueof (c ). substring (0, String. valueof (c ). indexof (". ") + 3): I felt so uncomfortable, so I found a book and I couldn't find it. I was dizzy. I found some methods on the Internet. as shown in the following figure (both of the following are two bits. If you want to take other bits, modify them yourself ):

(1): This method is convenient for me.

Float a = 123.2334f;
Float B = (float) (math. round (A * 100)/100; (here 100 is two decimal places. If you want other digits, such as four digits, here two 100 digits are changed to 10000)

(2): This method is also simple, but it must be converted to the float type:
Import java. Text. decimalformat;
String A = new decimalformat ("####,########"). Format (100.12345 );

(3): this can also be used.

Float Ft = 134.3435f;
Int scale = 2; // set the number of digits
Int roundingmode = 4; // indicates rounding. You can select other value-based methods, such as tail and so on.
Bigdecimal BD = new bigdecimal (double) ft );
BD = BD. setscale (scale, roundingmode );
FT = BD. floatvalue ();

Here, I also wrote a small algorithm. The problem is,

"Write a program in Java and use a series of 4*(1-1/3 + 1/5-1/7 + 1/9-1/11 + ....) to obtain the approximate circumference rate. and calculate the number of The number of entries in the sequence before obtaining 3.14159?"

Here I used the number of digits after the decimal point. I wrote a paragraph myself and hope you can correct it.

Package com. Zhou;
Import java. Math. bigdecimal;
Public class yuanzhou {
Static int Index = 1;

Public static void main (string [] ARGs ){
// Todo auto-generated method stub
Double Total = 1.00;
Float F = 3.14159f;
For (INT n = 3; n <1000000; n + = 2 ){
If (n-1) % 2 = 0 & (n-1) % 4! = 0 ){
Total = total-1/(double) N;
Index ++;
}
If (n-1) % 4 = 0 ){
Total = total + 1/(double) N;
Index ++;
}

Bigdecimal BD = new bigdecimal (total * 4 );
BD = BD. setscale (5, bigdecimal. round_down );
Float Ft = BD. floatvalue ();
System. Out. println (FT );
If (FT = f ){
Break;
}
}

System. Out. Print (INDEX );
}

}