digitalmars.D.learn - Double value is rounded to unexpected value: 2.5 -> 2 instead of 3
- alex_ca (35/35) Jul 07 2017 Hi,
- =?UTF-8?Q?Ali_=c3=87ehreli?= (23/26) Jul 07 2017 That's because what is printed as 2.5 is actually a little less than
- alex_ca (1/1) Jul 08 2017 Thanks, that was what was happening.
- ag0aep6g (47/88) Jul 07 2017 Works for me when I plug in the exact values:
Hi, I'm having trouble understanding why in some cases a double value will be rounded up and other times down, for the same code. Here's a snippet with code I tried to debug: int getNumberOfStitchesForRowLength(double rowLength) { writeln("input ", rowLength, " ", currentGauge.stitch_gauge, " ", currentGauge.gauge_length); writeln("stitches: ", (rowLength * currentGauge.stitch_gauge) / currentGauge.gauge_length, " -> " , ((rowLength * currentGauge.stitch_gauge) / currentGauge.gauge_length).roundTo!int); double end = 2.5; double start = 0; writeln("I expect: ", ((abs(end-start)*10)/10).roundTo!int); return ((rowLength * currentGauge.stitch_gauge) / currentGauge.gauge_length).roundTo!int; } And here's some output from that: input 2.5 10 10 stitches: 2.5 -> 2 I expect: 3 OR, similarly, I get input 3.5 10 10 stitches: 3.5 -> 3 I expect: 4 However, it works as I would expect for one value: input 1.5 10 10 stitches: 1.5 -> 2 I expect: 2 I would appreciate some ideas for why I see this seeming inconsistency. Or if someone can share how I can further debug this. Thanks! I'm using DMD32 D Compiler v2.074.1 on Windows
Jul 07 2017
On 07/07/2017 11:29 AM, alex_ca wrote:input 2.5 10 10 stitches: 2.5 -> 2 I expect: 3That's because what is printed as 2.5 is actually a little less than that. (Try printing with a format string like "%.20f".) The common way of dealing with this issue is to add 0.5 before the conversion: import std.stdio; import std.conv; void main() { for (double d = 1; d < 2; d += 0.1) { writefln("%s (%.20f): %s", d, d, (d + 0.5).to!int); } } 1 (1.00000000000000000000): 1 1.1 (1.10000000000000008882): 1 1.2 (1.20000000000000017764): 1 1.3 (1.30000000000000026645): 1 1.4 (1.40000000000000035527): 1 1.5 (1.50000000000000044409): 2 1.6 (1.60000000000000053291): 2 1.7 (1.70000000000000062172): 2 1.8 (1.80000000000000071054): 2 1.9 (1.90000000000000079936): 2 Ali
Jul 07 2017
On 07/07/2017 08:29 PM, alex_ca wrote:I'm having trouble understanding why in some cases a double value will be rounded up and other times down, for the same code. Here's a snippet with code I tried to debug: int getNumberOfStitchesForRowLength(double rowLength) { writeln("input ", rowLength, " ", currentGauge.stitch_gauge, " ", currentGauge.gauge_length); writeln("stitches: ", (rowLength * currentGauge.stitch_gauge) / currentGauge.gauge_length, " -> " , ((rowLength * currentGauge.stitch_gauge) / currentGauge.gauge_length).roundTo!int); double end = 2.5; double start = 0; writeln("I expect: ", ((abs(end-start)*10)/10).roundTo!int); return ((rowLength * currentGauge.stitch_gauge) / currentGauge.gauge_length).roundTo!int; } And here's some output from that: input 2.5 10 10 stitches: 2.5 -> 2 I expect: 3 OR, similarly, I get input 3.5 10 10 stitches: 3.5 -> 3 I expect: 4 However, it works as I would expect for one value: input 1.5 10 10 stitches: 1.5 -> 2 I expect: 2 I would appreciate some ideas for why I see this seeming inconsistency. Or if someone can share how I can further debug this.Works for me when I plug in the exact values: ---- import std.stdio; import std.math: abs; import std.conv: roundTo; import std.math; struct Gauge { double stitch_gauge; double gauge_length; } Gauge currentGauge = Gauge(10, 10); int getNumberOfStitchesForRowLength(double rowLength) { writeln("input ", rowLength, " ", currentGauge.stitch_gauge, " ", currentGauge.gauge_length); writeln("stitches: ", (rowLength * currentGauge.stitch_gauge) / currentGauge.gauge_length, " -> " , ((rowLength * currentGauge.stitch_gauge) / currentGauge.gauge_length).roundTo!int); return ((rowLength * currentGauge.stitch_gauge) / currentGauge.gauge_length).roundTo!int; } void main() { foreach (x; [1.5, 2.5, 3.5]) { getNumberOfStitchesForRowLength(x); } } ---- Prints: ---- input 1.5 10 10 stitches: 1.5 -> 2 input 2.5 10 10 stitches: 2.5 -> 3 input 3.5 10 10 stitches: 3.5 -> 4 ---- As expected, right? Could it be that the inputs aren't exactly the numbers you think they are? For example, when you change one the 10s to 9.9999999, it still prints as "10", but the results are different.
Jul 07 2017