PL
JavaScript - Math.log2() - przykład metody z dokumentacją
6 points
Math.log2() ta metoda zwraca logarytm o podstawie 2 liczby.
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// Logarithm with base 2:2
// x y3
console.log( Math.log2( 1 ) ); // 04
console.log( Math.log2( 2 ) ); // 15
console.log( Math.log2( 4 ) ); // 26
console.log( Math.log2( 8 ) ); // 37
console.log( Math.log2( 1024 ) ); // 108
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console.log( Math.log2( -1 ) ); // NaN10
console.log( Math.log2( 0 ) ); // -Infinity11
console.log( Math.log2( +Infinity ) ); // +Infinity12
Metodę Math.log2()przedstawia poniższy wykres:
| Składnia | Math.log2(x) |
| Parametry | x - liczba całkowita lub zmiennoprzecinkowa w zakresie 0 do +Infinity (wartość prymitywna). |
| Wynik |
Jeżeli Jeżeli Jeżeli |
| Opis |
|
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if (Math.log2 == null) {2
Math.log2 = function(x) {3
/*4
1. Using logarithm definition:5
b^y = x <=> log_b(x) = y6
Where: b - base7
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2. We can create system of equations:9
y = log_b(x)10
x = b^y11
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3. Which can be expressed as:13
x = b^log_b(x)14
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4. We can transform equation:16
log_2(x) = log_2(e^log_e(x)) = log_2(e) * log_e(x) = log_e(x) * log_2(e)17
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5. It can be written in JavaScript:19
log_2(e) <=> Math.LOG2E 20
log_e(x) * log_2(e) <=> Math.log(x) * Math.LOG2E21
*/22
return Math.log(x) * Math.LOG2E;23
};24
}25
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// Usage example:27
// x y28
console.log( Math.log2( 1 ) ); // 029
console.log( Math.log2( 2 ) ); // 130
console.log( Math.log2( 4 ) ); // ~231
console.log( Math.log2( 8 ) ); // 332
console.log( Math.log2( 1024 ) ); // 1033
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console.log( Math.log2( -1 ) ); // NaN35
console.log( Math.log2( 0 ) ); // -Infinity36
console.log( Math.log2( +Infinity ) ); // +Infinityxxxxxxxxxx1
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<html>3
<head>4
<style> #canvas { border: 1px solid black; } </style>5
</head>6
<body>7
<canvas id="canvas" width="400" height="400"></canvas>8
<script>9
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var canvas = document.querySelector('#canvas');11
var context = canvas.getContext('2d');12
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// logarithm chart range14
var x1 = 0;15
var x2 = 10;16
var y1 = -5;17
var y2 = 4;18
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var dx = 0.005;20
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var xRange = x2 - x1;22
var yRange = y2 - y1;23
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function calculateLogarithm(base, x) {25
var a = Math.log(x);26
var b = Math.log(base);27
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return a / b;29
}30
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function calculatePoint(base, x) {32
var y = calculateLogarithm(base, x);33
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// wykres zostanie odwrócony w poziomie z powodu odwróconych pikseli obszaru roboczego35
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var nx = (x - x1) / xRange; // normalized x37
var ny = 1.0 - (y - y1) / yRange; // normalized y38
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var point = {40
x: nx * canvas.width,41
y: ny * canvas.height42
};43
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return point;45
}46
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function drawChart(base, color, thickness) {48
var point = calculatePoint(base, x1);49
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context.beginPath();51
context.lineWidth = thickness;52
context.strokeStyle = color;53
context.moveTo(point.x, point.y);54
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for (var x = x1 + dx; x < x2; x += dx) {56
point = calculatePoint(base, x);57
context.lineTo(point.x, point.y);58
}59
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point = calculatePoint(base, x2);61
context.lineTo(point.x, point.y);62
context.stroke();63
}64
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console.log('log_2(x) <- red');66
console.log('log_e(x) <- green');67
console.log('log_10(x) <- blue');68
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// base color thickness70
drawChart( 2, '#ff001b', 2 ); // red71
drawChart( Math.E, '#159600', 0.8 ); // green72
drawChart( 10, '#0000ff', 0.8 ); // blue73
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console.log('x range: <' + x1 + '; ' + x2 + '>');75
console.log('y range: <' + y1 + '; ' + y2 + '>');76
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</script>78
</body>79
</html>