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【JAVASCRIPT】算数表达式计算

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为了完成java的作业，先拿js写了一个。暂时只完成了一些简单的运算

效果：

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expression.calcExpression('1%+1');//1.01
expression.calcExpression('.1%+1');//1.001
expression.calcExpression('(5-2)%/0');//Infinity
expression.calcExpression('(4+3)(4-3)');//7
expression.calcExpression('3(4+5)');//27
expression.calcExpression('-3(4+5)');//-27
 

 

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/*
Copyright luojia@luojia.me (https://luojia.me)
*/
 
class expression{
    static throwError(index){
        throw(`expression error @${index}`);
    }
    static compItem(value,index){
        return [value,index];
    }
    static isNumber(item){
        return typeof item[0]==='number';
    }
    static calcExpression(exp){
        let rawcomps=[],
            comps=[];
 
        let signs=new RegExp(/[\(\)\-\+\*\/\%]/y),
            space=new RegExp(/\s+/y),
            number=new RegExp(/(\d+\.*\d*)|(\.\d+)/y);
 
        let lastIndex=0,match;
        do{
            if(((space.lastIndex=lastIndex),true)&&(match=space.exec(exp))){//spaces
                rawcomps.push(match[0]);
                lastIndex=space.lastIndex;
            }else if(((signs.lastIndex=lastIndex),true)&&(match=signs.exec(exp))){//signs
                rawcomps.push(match[0]);
                comps.push(expression.compItem(match[0],match.index));
                if(match[0]==='('){
                    if(comps.length>1){
                        let last=comps[comps.length-2][0];
                        if(last===')'|| expression.isNumber(comps[comps.length-2])){
                            comps.splice(comps.length-1,0,expression.compItem('*',-1));
                        }
                    }
                }
                lastIndex=signs.lastIndex;
            }else if(((number.lastIndex=lastIndex),true)&&(match=number.exec(exp))){//number
                rawcomps.push(match[0]);
                if(match[0][0]==='.')match[0]='0'+match[0];//添加0
                comps.push(expression.compItem(Number(match[0]),match.index));
                lastIndex=number.lastIndex;
            }else{
                expression.throwError(lastIndex);
            }
        }while(lastIndex!==exp.length);
        return expression.expressionParser(comps)[0];
    }
    static expressionParser(comps){//return a compItem
        let groupStack=[],lastWasNumber=false;
        for(let i=0;i<comps.length;i++){//()
            if(expression.isNumber(comps[i])){
                if(lastWasNumber)expression.throwError(comps[i][1]);
                lastWasNumber=true;
                continue;
            }
            lastWasNumber=false;
            if(comps[i][0]==='('){
                groupStack.push(i);
            }else if(comps[i][0]===')'){
                if(groupStack.length===0)
                    expression.throwError(comps[i][1]);
                else if(groupStack.length===1){//use a new parser
                    let newgroup=comps.splice(groupStack[0],i-groupStack[0]+1);
                    newgroup.pop();newgroup.shift();
                    comps.splice(groupStack[0],0,expression.expressionParser(newgroup));
                    i=groupStack[0];
                }
                groupStack.pop();
            }
        }
        if(groupStack.length)expression.throwError(comps[groupStack[groupStack.length-1]][1]);
 
        for(let i=0;i<comps.length;i++){// n%
            if(expression.isNumber(comps[i])){continue;}
            let exp=comps[i][0];
            if(exp==='%'){
                if(!comps[i-1] || !expression.isNumber(comps[i-1]))expression.throwError(comps[i][1]);
                comps.splice(i-1,2,expression.compItem(comps[i-1][0]/100,comps[i-1][1]));
                i--;
            }
        }
        for(let i=0;i<comps.length;i++){// * /
            if(expression.isNumber(comps[i])){continue;}
            let exp=comps[i][0];
            if(exp==='*'||exp==='/'){
                if(i===comps.length || !comps[i-1] || !expression.isNumber(comps[i-1]) || !expression.isNumber(comps[i+1]))
                    expression.throwError(comps[i][1]);
                //calc
                let result;
                if(exp==='*'){
                    result=comps[i-1][0]*comps[i+1][0];
                }else{
                    result=comps[i-1][0]/comps[i+1][0];
                }
                comps.splice(i-1,3,expression.compItem(result,comps[i-1][1]));
                i--;
            }
        }
        for(let i=0;i<comps.length;i++){// + -
            if(expression.isNumber(comps[i])){continue;}
            let exp=comps[i][0];
            if(exp==='+'||exp==='-'){
                if(i===comps.length || !expression.isNumber(comps[i+1]))
                    expression.throwError(comps[i][1]);
                if(i===0||!expression.isNumber(comps[i-1])){//当做正负符号
                    if(exp==='-')comps[i+1][0]=-comps[i+1][0];
                    comps.splice(i,1);
                    i--;
                }else{//计算表达式
                    let result;
                    if(exp==='+'){
                        result=comps[i-1][0]+comps[i+1][0];
                    }else{
                        result=comps[i-1][0]-comps[i+1][0];
                    }
                    comps.splice(i-1,3,expression.compItem(result,comps[i-1][1]));
                    i--;
                }
            }
        }
        if(comps.length!==1){
            console.error([...comps]);
            throw('something wrong');
        }
        return comps[0];//return the value
    }
}

 



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