我想知道条件语句的正确用法，例如if语句，以避免未定义的行为。让我们从一个示例开始：
uint8_t x = 0;
bool y = false;
bool z = false;
if ((x == 135) and !y and !z) {
//do something
}
else if ((x == 135) and y) {
x = 5;
z = true;
}
else if ((x == 5) and z) {
x = 135;
z = false;
}
else {
//do something
}
现在，通过不将所有3个变量都包含在每个条件中，我是否会得到不确定的行为？是否将所有未说明的条件都放入else语句中？如果是这样，如果我放弃else语句会怎样？我有完全相同的if语句（在更复杂的情况下），而且似乎每次都不会进入正确的语句。
如果有此规定，请启发我？
答案 0 :(得分：5)
通过在每个条件中不包含所有3个变量，我是否会得到不确定的行为？
不完全没有将所有变量都包含在每个条件中的行为。
所有无法解释的情况都会进入else语句吗？
如果条件为false，则执行语句为false（即关键字else之后的语句）。
如果我放弃else语句会怎样？
从if语句之后的语句继续执行。
答案 1 :(得分：2)
根据经验，是的，尝试读取尚未初始化的变量不是一个好主意，因为这样做的行为通常是 undefined 。但这不是事实：所有变量都已初始化。
但是在if(...)条件下，您所需要做的只是评估为true或false。在这一点上，您的代码绝对正确。我会使用&&和||而不是and和or，尽管前者更常见。
有关一组非常全面的未定义构造，请参见What are all the common undefined behaviours that a C++ programmer should know about?
答案 2 :(得分：0)
否，您在这里没有未定义的行为。这听起来像是您没有条件（布尔）逻辑的准确心智模型。 var apprange = function () {
function initAndSetupTheSliders(rangeValue, disabled = false,limitValue) {
var inputs = [].slice.call(document.querySelectorAll('.range-slider input'));
inputs.forEach(function (input) {return input.setAttribute('value', rangeValue);});
inputs.forEach(function (input) {return apprange.updateSlider(input,limitValue, disabled);});
// Cross-browser support where value changes instantly as you drag the handle, therefore two event types.
if(!disabled){
############## Following two lines are not working as it should. ############
inputs.forEach(function (input) {return input.addEventListener('input', eventHandler(input, limitValue, disabled));});
inputs.forEach(function (input) {return input.addEventListener('change', eventHandler(input, limitValue, disabled));});
############# Code end ##################
}else{
inputs.forEach(function (input) {return input.removeEventListener('input', eventHandler(input, limitValue, disabled));});
inputs.forEach(function (input) {return input.removeEventListener('change', eventHandler(input, limitValue, disabled));});
console.log('#### 2');
}
}
function eventHandler(input,limitValue, disabled){
return apprange.updateSlider(input,limitValue, disabled);
}
function updateSlider(element, limitValue, disabled) {
if (element) {
var parent = element.parentElement,
lastValue = parent.getAttribute('data-slider-value');
if (lastValue === element.value) {
return; // No value change, no need to update then
}
parent.setAttribute('data-slider-value', element.value);
var $thumb = parent.querySelector('.range-slider__thumb'),
$bar = parent.querySelector('.range-slider__bar'),
$thumbLine = parent.querySelector('.range-slider__line'),
pct = element.value * ((parent.clientHeight - $thumb.clientHeight) / parent.clientHeight);
pctLimitLine = limitValue * ((parent.clientHeight - $thumb.clientHeight) / parent.clientHeight);
$thumb.style.bottom = pct + '%';
$thumbLine.style.bottom = pctLimitLine + 7 + '%';
$bar.style.height = 'calc(' + pct + '% + ' + $thumb.clientHeight / 2 + 'px)';
$thumb.textContent = element.value + '';
document.getElementById('range').value = element.value;
/* Emoticons Margins */
if(disabled && false){
var sliderHeight = parent.clientHeight;
var multiplier = sliderHeight / 100;
var UpperRegionHeight = (100 - limitValue) * multiplier;
var LowerRegionHeight = sliderHeight - UpperRegionHeight;
document.getElementsByClassName('icon_max_smily')[0].setAttribute("style", "height:"+UpperRegionHeight+"px");
document.getElementsByClassName('icon_high_smily')[0].setAttribute("style", "height:"+(LowerRegionHeight/3)+"px");
document.getElementsByClassName('icon_medium_smily')[0].setAttribute("style", "height:"+(LowerRegionHeight/3)+"px");
document.getElementsByClassName('icon_low_smily')[0].setAttribute("style", "height:"+(LowerRegionHeight/3)+"px");
if(document.getElementsByClassName('icon_max_smily')[1] !== undefined){
document.getElementsByClassName('icon_max_smily')[1].setAttribute("style", "height:"+UpperRegionHeight+"px");
document.getElementsByClassName('icon_high_smily')[1].setAttribute("style", "height:"+(LowerRegionHeight/3)+"px");
document.getElementsByClassName('icon_medium_smily')[1].setAttribute("style", "height:"+(LowerRegionHeight/3)+"px");
document.getElementsByClassName('icon_low_smily')[1].setAttribute("style", "height:"+(LowerRegionHeight/3)+"px");
}
apprange.HighlightSmiley(limitValue, element.value);
}
}
}
function HighlightSmiley(limitValue, selectedValue){
if(selectedValue >= limitValue){
console.log('if');
document.getElementsByClassName('icon_max_smily')[0].querySelector('img').setAttribute("width", "70px");
if(document.getElementsByClassName('icon_max_smily')[1] !== undefined){
document.getElementsByClassName('icon_max_smily')[1].querySelector('img').setAttribute("width", "70px");
}
}else{
var limitMultiplier = limitValue/3;
if(selectedValue >= 0 && selectedValue < limitMultiplier){
document.getElementsByClassName('icon_low_smily')[0].querySelector('img').setAttribute("width", "70px");
if(document.getElementsByClassName('icon_low_smily')[1] !== undefined){
document.getElementsByClassName('icon_low_smily')[1].querySelector('img').setAttribute("width", "70px");
}
}else if(selectedValue >= limitMultiplier && selectedValue < 2*limitMultiplier){
document.getElementsByClassName('icon_medium_smily')[0].querySelector('img').setAttribute("width", "70px");
if(document.getElementsByClassName('icon_medium_smily')[1] !== undefined){
document.getElementsByClassName('icon_medium_smily')[1].querySelector('img').setAttribute("width", "70px");
}
}else if(selectedValue >= 2*limitMultiplier && selectedValue < limitValue){
document.getElementsByClassName('icon_high_smily')[0].querySelector('img').setAttribute("width", "70px");
if(document.getElementsByClassName('icon_high_smily')[1] !== undefined){
document.getElementsByClassName('icon_high_smily')[1].querySelector('img').setAttribute("width", "70px");
}
}
}
}
function removeEventHandler(){
}
return {
initAndSetupTheSliders: initAndSetupTheSliders,
updateSlider: updateSlider,
HighlightSmiley: HighlightSmiley,
eventHandler: eventHandler};
}();
始终是 *, *:before, *:after {
box-sizing: border-box;
}
html,
body {
height: 100%;
}
body {
margin: 0;
background: #3D3D4A;
color: white;
min-height: 400px;
font-family: sans-serif;
}
.info {
position: absolute;
top: 0;
left: 0;
padding: 10px;
opacity: .5;
}
.container {
padding-top: 40px;
position: relative;
display: inline-block;
top: 100%;
left: 50%;
transform: translate(-50%, -100%);
padding-bottom: 20px;
}
@media (min-height: 500px) {
.container {
position: absolute;
top: 50%;
transform: translate(-50%, -50%);
padding-bottom: 0;
}
}
.range-slider {
display: inline-block;
width: 40px;
position: relative;
text-align: center;
height: 300px;
max-height: 100%;
}
.range-slider:before {
position: absolute;
top: -2em;
left: .5em;
content: attr(data-slider-value) "%";
color: white;
font-size: 90%;
}
.range-slider__thumb {
position: absolute;
left: 5px;
width: 30px;
height: 30px;
line-height: 30px;
background: white;
color: #777;
font-size: 50%;
box-shadow: 0 0 0 4px #3D3D4A;
border-radius: 50%;
pointer-events: none;
}
.range-slider__bar {
left: 12px;
bottom: 0;
position: absolute;
background: linear-gradient(to left, #ffc000 37%, #c64e13 50%);
pointer-events: none;
width: 18px;
border-radius: 10px;
}
.range-slider input[type=range][orient=vertical] {
position: relative;
margin: 0;
height: 100%;
width: 100%;
display: inline-block;
position: relative;
writing-mode: bt-lr;
-webkit-appearance: slider-vertical;
}
.range-slider input[type=range][orient=vertical]::-webkit-slider-runnable-track, .range-slider input[type=range][orient=vertical]::-webkit-slider-thumb {
-webkit-appearance: none;
}
.range-slider input[type=range][orient=vertical]::-webkit-slider-runnable-track {
border: none;
background: #343440;
width: 18px;
border-color: #343440;
border-radius: 10px;
box-shadow: 0 0 0 2px #3D3D4A;
}
.range-slider input[type=range][orient=vertical]::-moz-range-track {
border: none;
background: #343440;
width: 18px;
border-color: #343440;
border-radius: 10px;
box-shadow: 0 0 0 2px #3D3D4A;
}
.range-slider input[type=range][orient=vertical]::-ms-track {
border: none;
background: white;
width: 18px;
border-color: #343440;
border-radius: 10px;
box-shadow: 0 0 0 2px #3D3D4A;
color: transparent;
height: 100%;
}
.range-slider input[type=range][orient=vertical]::-ms-fill-lower, .range-slider input[type=range][orient=vertical]::-ms-fill-upper, .range-slider input[type=range][orient=vertical]::-ms-tooltip {
display: none;
}
.range-slider input[type=range][orient=vertical]::-webkit-slider-thumb {
width: 30px;
height: 30px;
opacity: 0;
}
.range-slider input[type=range][orient=vertical]::-moz-range-thumb {
width: 30px;
height: 30px;
opacity: 0;
}
.range-slider input[type=range][orient=vertical]::-ms-thumb {
width: 30px;
height: 30px;
opacity: 0;
}
.theme1 {
background: linear-gradient(pink, deeppink);
}
.theme2 {
background: linear-gradient(tomato, red);
}
.theme3 {
background: linear-gradient(yellow, orange);
}语句的可选部分。
<div class="container">
<div class="range-slider">
<input type="range" orient="vertical" min="0" max="100" />
<div class="range-slider__bar"></div>
<div class="range-slider__thumb"></div>
</div>
</div>将被执行。更改其中的else，if或{ ... }的值将不会导致其他任何值被执行，因此首先确定决定。 >