Store the contents of a pair of brackets/parenthises in a variable-CodePudding

In some variable called a, I want to store the contents of a pair of brackets from a variable called equation. I have an array called opening which marks the index of the opening brackets.

let equation = '2(x(4-5x)(4-x)) 10'
let opening = [9,3,1,0]
let a = ''

Attempt 1

    for(let x = equation[opening[0]]; x != ')'; x  ) {
    a  = x;
    }

this lead to an infinite loop.

Attempt 2

    for(let x = equation[opening[0]]; x < equation.length; x  ) {
        if (x == ')') {break;}
        a  = x;
    }

this did not lead to an infinite loop but the variable a remained an empty string.

I want the result to be a = ‘4-x’

CodePudding user response：

Put the a = x in the brackets before break

CodePudding user response：

Alright. This was fun enough that I wrote an HTML page with JavaScript...

https://highdex.net/parse_eq.htm

You can view the source and see all the code, but in case I ever take that page down, I'll put just the JavaScript function in here too...

//this function takes an equation and breaks it into parts based on parentheses.
//it returns an array of the parts, where and equation like "4 * (8 - 1) / 4" would come back like...
//    index 0: "4 * [1] / 4"
//    index 1: "8 - 1"
//Any number shown in brackets is a reference to the index that holds what goes in that place, so you can rebuild the equation if you want.
function parseEquation(srcEq) {
    let parts = [""];
    let nextIndex = 0; //holds the next index we'll use inside those brackets
    let currentIndex = 0; //holds the current index we're adding characters to
    let backToArray = [0]; //holds the index that we need to go back to when the parentheses close
    let srcLen = srcEq.length;
        
    for (let i = 0; i < srcLen; i  ) {
        let currentChar = srcEq[i];
            
        if (currentChar == "(") { //in this case we need to put the placeholder into the current index that references the next index, and then bump up the current index
            if (parts.length -1 < currentIndex) {
                parts.push("");
            }
            nextIndex  ;
            parts[currentIndex]  = "["   nextIndex   "]";
            backToArray.push(currentIndex);
            currentIndex = nextIndex;
        }
        else if (currentChar == ")") { //in this case we need to drop the current char back down a level
            currentIndex = backToArray.pop();
        }
        else { //in this case we just add the current character to the current array at the current index
            if (parts.length -1 < currentIndex) {
                parts.push("");
            }
            
            parts[currentIndex]  = currentChar;
        }
            
    }
        
    return parts;
}