Js 求斐波那契数列第 n 项的值结果总不对

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代码如下:

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>
	<script>
		var num = parseInt(window.prompt('input'));
		var first = 1;
		var second = 1;
		var third;
		if(num > 2){
			for(var i = 0; i < num-2; i++){
				third = first + second;
				first = second;
				second = third;
			}
			document.write(third);
		}
		else{
			document.write(1);
		}
	</script>
</body>
</html>

输出结果第 4 项为 3,第 5 项为 4.调试后发现 first 的结果总是 1,真是搞不懂,请大神帮忙看看!

17 replies • 2019-04-23 13:10:16 +08:00

yngby

Apr 23, 2019

为啥我试了一下没错呢

mango88

Apr 23, 2019 via iPhone

第一项难道不是 1 ？

wa8n

Apr 23, 2019

我这对的

vexjoe

Apr 23, 2019

https://www.google.com/search?q=js+febonaci

wenzichel

Apr 23, 2019

没错的呀！

```javascript
function fb(num) {
var first = 1;
var second = 1;
var third;
if(num > 2){
for(var i = 0; i < num-2; i++){
third = first + second;
first = second;
second = third;
}
return third;
}
else{
return first;
}
}
for(let i=1; i<10; i++) {
console.log(i, fb(i));
}
/*
1 1
2 1
3 2
4 3
5 5
6 8
7 13
8 21
9 34
*/
```

wenzichel

Apr 23, 2019

不支持 markdown 啊，那我怎么删除我上面的这条评论呢

DJQTDJ

Apr 23, 2019

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<script>
var increment = 0;
while(increment < 100) {
var first = 1;
var second = 1;
var third;
if(increment > 2){
for(var i = 0; i < increment-2; i++){
third = first + second;
first = second;
second = third;
}
document.write(increment + ":" + third + "<br>");
}
else{
document.write(increment + ":" + 1 + "<br>");
}
increment++;
}
</script>
</body>
</html>

没问题吖