84

I have an array of Strings that represent Binary numbers (without leading zeroes) that I want to convert to their corresponding base 10 numbers. Consider:

binary 1011 becomes integer 11
binary 1001 becomes integer 9
binary   11 becomes integer 3   etc. 

What's the best way to proceed? I've been exploring java.lang.number.* without finding a direct conversion method. Integer.parseInt(b) yields an integer EQUAL to the String...e.g., 1001 becomes 1,001 instead of 9...and does not seem to include a parameter for an output base. toBinaryString does the conversion the wrong direction. I suspect I'll need to do a multistep conversion, but can't seem to find the right combination of methods or subclasses. I'm also not sure the extent to which leading zeros or lack thereof will be an issue. Anyone have any good directions to point me?

  • 4
    Look at Integer#parseInt(String s, int radix) – anubhava Apr 16 '12 at 17:49
  • possible duplicate of converting Binary Numbers in to decimal numbers – Mike Samuel Apr 16 '12 at 17:50
223

You need to specify the radix. There's an overload of Integer#parseInt() which allows you to.

int foo = Integer.parseInt("1001", 2);
  • 1
    Perfection. I completely missed the second line on the parseInt documentation that allows for the radix. Works like a dream. – dwwilson66 Apr 16 '12 at 17:58
  • 1
    Does this work with leading zeros too? Just confirming, although I see no reason why not. – Siddhartha May 25 '17 at 15:22
16

This might work:

public int binaryToInteger(String binary) {
    char[] numbers = binary.toCharArray();
    int result = 0;
    for(int i=numbers.length - 1; i>=0; i--)
        if(numbers[i]=='1')
            result += Math.pow(2, (numbers.length-i - 1));
    return result;
}
  • 11
    -1 for unnecessarily reinventing the wheel. – Matt Ball Apr 16 '12 at 17:57
  • I suppose it is kind of unnecessary. That's what happens when you have a little time between classes. – Hassan Apr 16 '12 at 17:58
  • 6
    this one is helpful for me because I have to do a school project with conversions without using the ones java already has – bucksnort2 Oct 16 '13 at 13:46
  • Did anyone test this before? here number.length minus the index plus 1 is been multiply by 2, if i am not mistaken in bynary you start with 1 and multiply that value by 2 then grab the result and multiply that one by 2 that will be your 3 place and so on – Christopher Cabezudo Rodriguez Jun 23 '15 at 21:00
  • 1
    This code snippet is not working. for loop and calculation of new result variable is not correct. – trylimits Aug 5 '15 at 11:19
6
int foo = Integer.parseInt("1001", 2);

works just fine if you are dealing with positive numbers but if you need to deal with signed numbers you may need to sign extend your string then convert to an Int

public class bit_fun {
    public static void main(String[] args) {
        int x= (int)Long.parseLong("FFFFFFFF", 16);
        System.out.println("x =" +x);       

        System.out.println(signExtend("1"));
        x= (int)Long.parseLong(signExtend("1"), 2);
        System.out.println("x =" +x);

        System.out.println(signExtend("0"));
        x= (int)Long.parseLong(signExtend("0"), 2);
        System.out.println("x =" +x);

        System.out.println(signExtend("1000"));
        x= (int)Long.parseLong(signExtend("1000"), 2);
        System.out.println("x =" +x);

        System.out.println(signExtend("01000"));
        x= (int)Long.parseLong(signExtend("01000"), 2);
        System.out.println("x =" +x);
    }

    private static String signExtend(String str){
        //TODO add bounds checking
        int n=32-str.length();
        char[] sign_ext = new char[n];
        Arrays.fill(sign_ext, str.charAt(0));

        return new String(sign_ext)+str;
    }
}

output:
x =-1
11111111111111111111111111111111
x =-1
00000000000000000000000000000000
x =0
11111111111111111111111111111000
x =-8
00000000000000000000000000001000
x =8 

I hope that helps!

  • I needed -1 converted from binary to decimal, i did this. System.out.println((int)Long.parseLong("11111111111111111111111111111111",2)); – Zeus Feb 28 at 18:36
5
static int binaryToInt (String binary){
    char []cA = binary.toCharArray();
    int result = 0;
    for (int i = cA.length-1;i>=0;i--){
        //111 , length = 3, i = 2, 2^(3-3) + 2^(3-2)
        //                    0           1  
        if(cA[i]=='1') result+=Math.pow(2, cA.length-i-1);
    }
    return result;
}
2
public Integer binaryToInteger(String binary){
    char[] numbers = binary.toCharArray();
    Integer result = 0;
    int count = 0;
    for(int i=numbers.length-1;i>=0;i--){
         if(numbers[i]=='1')result+=(int)Math.pow(2, count);
         count++;
    }
    return result;
}

I guess I'm even more bored! Modified Hassan's answer to function correctly.

0

Fixed version of java's Integer.parseInt(text) to work with negative numbers:

public static int parseInt(String binary) {
    if (binary.length() < Integer.SIZE) return Integer.parseInt(binary, 2);

    int result = 0;
    byte[] bytes = binary.getBytes();

    for (int i = 0; i < bytes.length; i++) {
        if (bytes[i] == 49) {
            result = result | (1 << (bytes.length - 1 - i));
        }
    }

    return result;
}
0

If you're worried about performance, Integer.parseInt() and Math.pow() are too expensive. You can use bit manipulation to do the same thing twice as fast (based on my experience):

final int num = 87;
String biStr = Integer.toBinaryString(num);

System.out.println(" Input Number: " + num + " toBinary "+ biStr);
int dec = binaryStringToDecimal(biStr);
System.out.println("Output Number: " + dec + " toBinary "+Integer.toBinaryString(dec));

Where

int binaryStringToDecimal(String biString){
  int n = biString.length();      
  int decimal = 0;
  for (int d = 0; d < n; d++){
    // append a bit=0 (i.e. shift left) 
    decimal = decimal << 1;

    // if biStr[d] is 1, flip last added bit=0 to 1 
    if (biString.charAt(d) == '1'){
      decimal = decimal | 1; // e.g. dec = 110 | (00)1 = 111
    }
  }
  return decimal;
}

Output:

 Input Number: 87 toBinary 1010111
Output Number: 87 toBinary 1010111
0

For me I got NumberFormatException when trying to deal with the negative numbers. I used the following for the negative and positive numbers.

System.out.println(Integer.parseUnsignedInt("11111111111111111111111111110111", 2));      

Output : -9
-1

I love loops! Yay!

String myString = "1001001"; //73

While loop with accumulator, left to right (l doesn't change):

int n = 0,
    j = -1,
    l = myString.length();
while (++j < l) n = (n << 1) + (myString.charAt(j) == '0' ? 0 : 1);
return n;

Right to left with 2 loop vars, inspired by Convert boolean to int in Java (absolutely horrible):

int n = 0,
    j = myString.length,
    i = 1;
while (j-- != 0) n -= (i = i << 1) * new Boolean(myString.charAt(j) == '0').compareTo(true);
return n >> 1;

A somewhat more reasonable implementation:

int n = 0,
    j = myString.length(),
    i = 1;
while (j-- != 0) n += (i = i << 1) * (myString.charAt(j) == '0' ? 0 : 1);
return n >> 1;

A readable version :p

int n = 0;
for (int j = 0; j < myString.length(); j++) {
    n *= 2;
    n += myString.charAt(j) == '0' ? 0 : 1;
}
return n;

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