在 JavaScript 中将前缀转换为中缀

Question

我正在尝试编写一个程序，该程序接受前缀表达式并输出中缀表达式。我在下面列出了一些例子来帮助演示我在说什么。我已经在下面粘贴了我的代码，有人可以帮我弄清楚如何在表达式中的 2 个数字之间移动符号吗？请参阅示例 1 以查看我如何尝试获取它的方法，但它不起作用。任何答案都会有所帮助或提示该怎么做。感谢您的帮助！

/* The goal is to take in an expression in prefix notation and output it in infix notation
for example:
+ 1 2 outputs output 1 + 2
+ - 3 4 5 outputs 3 + 4 - 5
% + / - 0 9 3 8 5 outputs 0 % 9 + 3 / 8 - 5 
*/
function convert(input){
var x = input.split(''); // splits each variable and stores it in an array
var output = "";
// these are the valid symbols we can take, we will use these later
var symbols = ['+', '-', '*', '/', '%'];
// lets loop through all the values in x, starting at position 0
for(var i = 0; i < x.length; i++){
if(symbols.includes(x[i])) { // we hit a symbol, lets move it between 2 numbers
/* now we need to figure out where to store the symbol. every 2 spaces starting at index 0
we can insert a symbol (so spots like 1 3 5 7 etc). this loop will help us figure out what spot is empty
, and it will store the symbol at that spot [see example 1 for a visualizaton]*/
for(var j = 0; j < input.length; j+=2){
if(output[j] == " "){
// great, we can save the symbol here 
output = output + x[i];
}
}
}
// otherwise we have a number on our hands
else{
output = output + x[i];
console.log(output);
}
}
}
console.log(convert("+ 1 2"));
/*
example 1
if I have "+ 1 2"
+ is position 0
1 is position 2
2 is position 4
so the whitespace is at position 1 and 3. these are the spots where we can output the symbols
using the original expression + 1 2
position:  value:
-------- | ------
0        |    1
-------- | ------
1        |   " "
-------- | ------
2        |    +
-------- | ------
3        |   " "
-------- | ------
4        |    2
*/

Answer 1

function result_expression(expression, variables) {
let opernads=['+','-','*','/'];
let arr=[...expression.split(" ")];

var len=arr.length;



while(len>0){

let d1=arr[len-1]
let d2=arr[len-2]
let d3=arr[len-3]

if(opernads.includes(d3)){
if(isNaN(d2)){
let tmp=variables[d2]
d2=tmp;
}
if(isNaN(d1)){
let tmp1=variables[d1]
d1=tmp1;
}

let a=d2.toString().concat(d3).concat(d1)
delete arr[len-1]
delete arr[len-2]
delete arr[len-3]

let na=[];

arr[len-3]=eval(a)
arr.forEach(e=>{
if(!(typeof e==='undefined')){
na.push(e)
}
})
arr=[...na]
console.log('arr',arr)
len=arr.length;
// arr=[...newar]
// len=arr.length

}else{

len--
}

if(len==1){
return arr[0]
}

}




}

//let expression="+ 6 * - 4 + 2 3 8";
//let expression="+ 6 * - 4 a b + 2 3 8";
let expression="+ * 6 5 3 2 2";


let variables={a:20,b:1}
let k=result_expression(expression, variables);
console.log('finalresult',k)

Answer 2

只要你只使用简单的表达式，我建议将输入分成两个数组，数字和符号，然后将它们合并在一起。

var symbols = ['+', '-', '*', '/', '%'];
function convert(input) {
var
response = '',
infixes = [],
numbers = [];
// Divide input into two arrays, infixes (or symbols) and numbers
infixes = input.split(' ').filter(function(o) {
if (symbols.includes(o)) {
return true;
} else {
numbers.push(o);
}
});
// Merge arrays
for (let i = 0; i < numbers.length; i++) {
if (infixes[i]) {
response =
response +
numbers[i] + ' ' +
infixes[i] + ' ';
} else {
response =
response + numbers[i] + ' ';
}
}
response = response.slice(0, -1);
return response;
};

此函数适用于您的所有示例，但如果您需要使其更智能，您可以在此处的 codepen 上轻松修改和测试上述函数。