Demonstrate JavaScript floating point math bugs by showing which two-decimal-place numbers between 0.00 and 1.00 inclusive have fractional parts after being multiplied by one hundred.
The original version of this program used the floating-point value, i, as the loop counter of the for-loop.
Apparently, using a floating-point number, along with a loop final-expression that converted the value to a string and back to a float
again in an attempt to carefully increment by 0.01, caused Bad Things™ to happen. E.g., the loop would run for thousands of iterations
rather than exactly one hundred, causing the browser to freeze.
The original code:
for (i = 0.00; i <= 1.00; i = +((i + 0.01).toPrecision(2))) {
// Note the final-expression in the for-loop limits decimal places,
// overcoming float errors from adding 0.01
j = i * 100;
if (Math.round(j) != j) {
console.log('Multiplcation error:', i, j);
}
}Changed in the new version:
- Added an integer loop counter,
n - Check for floating point addition errors, and if found
- Display a message
- Convert the more-precise number from a string back to a float
The last point may be significant, because it avoids converting the float to a string and back to a float again in every iteration. This conversion is done only when necessary.