Question

我创造了一个重现问题的简单例子（或者更确切地说是我的误解）：

string text = @"eaisjdoaisjdoaisjdai_osjdaisodjasizzi_ojiozaziasjz_";
int[] score = new int[123];

foreach(char letter in text)
{
   int val = score[letter]; //give me the value stored at the index
   score[letter] = val++; //increment it and store it back into the array at the index
}

...

通过上面的调试，正确地为val分配了数组指定索引处的值。但是当递增时，val不会被分配回数组。那是为什么？

图片显示了当从数组中检索val时评估score[letter]的值的即时窗口，分配给val后的gl.LINES的值以及<!doctype html> <head> <title>Triangles</title> <style> html, body { background: #000; height: 100%; margin: 0; } canvas { width: 1280px; height: 720px; position: absolute; margin: auto; top: 0; right: 0; left: 0; bottom: 0; } </style> </head> <body> <script> 'use strict'; const triangleCount = 1e6; const antialias = true; const generateTriangles = (count, width, height) => { const coords = new Float32Array(9 * count); for (var i = 0; i < coords.length;) { const x = Math.random() * 2 - 1; const y = Math.random() * 2 - 1; const z = Math.random() * 2 - 1; const theta = Math.random() * Math.PI; const ax = 10 * Math.cos(theta) / width; const ay = 10 * Math.sin(theta) / height; const bx = 10 * Math.cos(theta + 0.3) / width; const by = 10 * Math.sin(theta + 0.3) / height; coords[i++] = x + ax; coords[i++] = y + ay; coords[i++] = z; coords[i++] = x + bx; coords[i++] = y + by; coords[i++] = z; coords[i++] = x - ax; coords[i++] = y - ay; coords[i++] = z; coords[i++] = x - ax; coords[i++] = y - ay; coords[i++] = z; coords[i++] = x - bx; coords[i++] = y - by; coords[i++] = z; coords[i++] = x + ax; coords[i++] = y + ay; coords[i++] = z; } return coords; }; const vertexShaderSource = ` precision lowp float; attribute vec3 aPosition; uniform float uWobble; void main() { float p = 1.0 / (0.3 * aPosition.z - 1.4 + 0.1 * uWobble); gl_Position = vec4(p * aPosition.x, p * aPosition.y, aPosition.z, 1.0); } `; const fragmentShaderSource = ` precision lowp float; void main() { float z = gl_FragCoord.z; gl_FragColor = vec4(1.2 * z, z * z, z, 1.0); } `; const canvas = document.createElement('canvas'); document.body.appendChild(canvas); canvas.width = canvas.clientWidth; canvas.height = canvas.clientHeight; const gl = canvas.getContext('webgl', {alpha: false, antialias}); const vertexShader = gl.createShader(gl.VERTEX_SHADER); gl.shaderSource(vertexShader, vertexShaderSource); gl.compileShader(vertexShader); const fragmentShader = gl.createShader(gl.FRAGMENT_SHADER); gl.shaderSource(fragmentShader, fragmentShaderSource); gl.compileShader(fragmentShader); const program = gl.createProgram(); gl.attachShader(program, vertexShader); gl.attachShader(program, fragmentShader); gl.linkProgram(program); gl.useProgram(program); const aVertexPosition = gl.getAttribLocation(program, 'aPosition'); gl.enableVertexAttribArray(aVertexPosition); const uWobble = gl.getUniformLocation(program, 'uWobble'); gl.uniform1f(uWobble, 1); const vertices = generateTriangles(triangleCount, canvas.width, canvas.height); const vertexBuffer = gl.createBuffer(); gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer); gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW); gl.vertexAttribPointer(aVertexPosition, 3, gl.FLOAT, false, 0, 0); const render = (timestamp) => { requestAnimationFrame(render); gl.uniform1f(uWobble, Math.sin(0.002 * timestamp)); gl.drawArrays(gl.TRIANGLES, 0, vertices.length / 3); }; window.requestAnimationFrame(render); </script> </body>的递增值

我显然做了一些愚蠢的事情但却无法弄清楚是什么。

Answer 1

这是因为您正在使用后增量运算符，该运算符会在>返回后增加值。
将其更改为预增量运算符++val，它应该可以正常工作。

来自++ operator documentation：


第一种形式（++val）是前缀增量操作。结果   operation是操作数增加后的值。

第二种形式（val++）是后缀增量操作。结果   operation是操作数增加之前的值。

将值赋值给数组中的指定索引无效

1 个答案: