Maximum Number of Decimal Digits In Binary Floating-Point Numbers

I’ve written about the formulas used to compute the number of decimal digits in a binary integer and the number of decimal digits in a binary fraction. In this article, I’ll use those formulas to determine the maximum number of digits required by the double-precision (double), single-precision (float), and quadruple-precision (quad) IEEE binary floating-point formats.

The maximum digit counts are useful if you want to print the full decimal value of a floating-point number (worst case format specifier and buffer size) or if you are writing or trying to understand a decimal to floating-point conversion routine (worst case number of input digits that must be converted).

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The Safe Range For PHP’s base_convert()

PHP’s base_convert() is a useful function that converts integers between any pair of bases, 2 through 36. However, you might hesitate to use it after reading this vague and mysterious warning in its documentation:

base_convert() may lose precision on large numbers due to properties related to the internal “double” or “float” type used.

The truth is that it works perfectly for integers up to a certain maximum — you just have to know what that is. I will show you this maximum value in each of the 35 bases, and how to check if the values you are using are within this limit.

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Decimal/Binary Conversion of Integers in App Inventor

To complete my exploration of numbers in App Inventor, I’ve written an app that converts integers between decimal and binary. It uses the standard algorithms, which I’ve just translated into blocks.

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App Inventor Computes With Big Integers

I recently wrote that App Inventor represents its numbers in floating-point. I’ve since discovered something curious about integers. When typed into math blocks, they are represented in floating-point; but when generated through calculations, they are represented as arbitrary-precision integers — big integers.

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Testing Some Edge Case Conversions In App Inventor

After discovering that App Inventor represents numbers in floating-point, I wanted to see how it handled some edge case decimal/floating-point conversions. In one group of tests, I gave it numbers that were converted to floating-point incorrectly in other programming languages (I include the famous PHP and Java numbers). In another group of tests, I gave it numbers that, when converted to floating-point and back, demonstrate the rounding algorithm used when printing halfway cases. It turns out that App Inventor converts all examples correctly, and prints numbers using the round-half-to-even rule.

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Numbers In App Inventor Are Stored As Floating-Point

I am exploring App Inventor, an Android application development environment for novice programmers. I am teaching it to my kids, as an introductory step towards “real” app development. While playing with it I wondered: are its numbers implemented in decimal? No, they aren’t. They are implemented in double-precision binary floating-point. I put together a few simple block programs to demonstrate this, and to expose the usual floating-point “gotchas”.

App Inventor Blocks To Display 0.1 to 17 Digits
App Inventor Blocks To Display 0.1 to 17 Digits

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Number of Bits in a Decimal Integer

Every integer has an equivalent representation in decimal and binary. Except for 0 and 1, the binary representation of an integer has more digits than its decimal counterpart. To find the number of binary digits (bits) corresponding to any given decimal integer, you could convert the decimal number to binary and count the bits. For example, the two-digit decimal integer 29 converts to the five-digit binary integer 11101. But there’s a way to compute the number of bits directly, without the conversion.

Sometimes you want to know, not how many bits are required for a specific integer, but how many are required for a d-digit integer — a range of integers. A range of integers has a range of bit counts. For example, four-digit decimal integers require between 10 and 14 bits. For any d-digit range, you might want to know its minimum, maximum, or average number of bits. Those values can be computed directly as well.

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Using Integers to Check a Floating-Point Approximation

For decimal inputs that don’t qualify for fast path conversion, David Gay’s strtod() function does three things: first, it uses IEEE double-precision floating-point arithmetic to calculate an approximation to the correct result; next, it uses arbitrary-precision integer arithmetic (AKA big integers) to check if the approximation is correct; finally, it adjusts the approximation, if necessary. In this article, I’ll explain the second step — how the check of the approximation is done.

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strtod()’s Initial Decimal to Floating-Point Approximation

David Gay’s strtod() function does decimal to floating-point conversion using both IEEE double-precision floating-point arithmetic and arbitrary-precision integer arithmetic. For some inputs, a simple IEEE floating-point calculation suffices to produce the correct result; for other inputs, a combination of IEEE arithmetic and arbitrary-precision arithmetic is required. In the latter case, IEEE arithmetic is used to calculate an approximation to the correct result, which is then refined using arbitrary-precision arithmetic. In this article, I’ll describe the approximation calculation, which is based on a form of binary exponentiation.

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Nondeterministic Floating-Point Conversions in Java

Recently I discovered that Java converts some very small decimal numbers to double-precision floating-point incorrectly. While investigating that bug, I stumbled upon something very strange: Java’s decimal to floating-point conversion routine, Double.parseDouble(), sometimes returns two different results for the same decimal string. The culprit appears to be just-in-time compilation of Double.parseDouble() into SSE instructions, which exposes an architecture-dependent bug in Java’s conversion algorithm — and another real-world example of a double rounding on underflow error. I’ll describe the problem, and take you through the detective work to find its cause.

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A Closer Look at the Java 2.2250738585072012e-308 Bug

Java’s decimal to floating-point conversion routine, the doubleValue() method of its FloatingDecimal class, goes into an infinite loop when converting the decimal string 2.2250738585072012e-308 to double-precision binary floating-point. I took a closer look at the bug, by tracing the doubleValue() method in the Eclipse IDE for Java (thanks to Konstantin Preißer for helping me set that up). What I found was that our initial analysis of the bug was wrong; what actually happens is that doubleValue()’s correction loop oscillates between two values, 0x1p-1022 and 0x0.fffffffffffffp-1022.

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