This can solve simple mathematical expressions including parentheses, division, multiplication, addition and subtraction respectively with spaces and without spaces. But it cannot detect wrong expressions.

Simple Math Expression.cpp

/*
Md Tahmid Hasan
CSE, BSMRSTU
tahmidhasan3003
*/

#include<bits/stdc++.h>
#include<sstream>

using namespace std;

double stringToDouble(string s)
{
    double temp = 0, multiplicationFactor = 10;
    int n = 1, d = 0;
    for(int i = 0; i < s.length(); i++)
    {
        if(s[i] == '-')
        {
            n = -1;
            continue;
        }
        else if(s[i] == '.')
        {
            multiplicationFactor = .1;
            d = 1;
            continue;
        }

        if(!d)
        {
            temp *= multiplicationFactor;
            temp += (s[i]-'0');
        }
        else
        {
            temp += (s[i]-'0') * multiplicationFactor;
            multiplicationFactor *= .1;
        }
    }
    return temp * n;
}

string toString(double d)
{
    stringstream strs;
    strs << d;
    string str = strs.str();

    return str;
}

string solve(string expr)
{
    int bkt = 0, div = 0, mul = 0, as = 0;
    string tempExp, opnd, opnd2;

    for(int i = 0; i < expr.length(); i++)
    {
        if(expr[i] == ' ')
            continue;                                   //removing spaces
        else if(expr[i] == '(')
        {
            bkt++;
            string subExpr, subExprResult;
            for(i++; bkt != 0; i++)
            {
                if(expr[i] == ' ')
                    continue;
                else if(expr[i] == ')')
                {
                    bkt--;
                    if(bkt == 0)
                    {
                        subExprResult = solve(subExpr); //get the value of parenthesized expression
                        break;
                    }
                }
                else if(expr[i] == '(')
                    bkt++;

                subExpr += expr[i];
            }
            tempExp += subExprResult;
        }
        else if(expr[i] == '/')
        {
            div++;
            tempExp += expr[i];
        }
        else if(expr[i] == '*')
        {
            mul++;
            tempExp += expr[i];
        }
        else if((expr[i] == '+') || (expr[i] == '-'))
        {
            as++;
            tempExp += expr[i];
        }
        else
            tempExp += expr[i];
    }
    expr = tempExp;                                        //update the expression
    //cout<<"After remove brackets & spaces: "<<expr<<endl;

    if(div != 0)                                            //division
    {
        opnd = "";
        tempExp = "";
        for(int i = 0; i < expr.length(); i++)
        {
            if(expr[i] == '/')
            {
                opnd2 = "";
                for(i++; i<expr.length(); i++)
                {
                    if((expr[i] == '/') || (expr[i] == '*') || (expr[i] == '+') || (expr[i] == '-'))
                    {
                        i--;
                        break;
                    }
                    opnd2 += expr[i];
                }
                tempExp += toString(stringToDouble(opnd) / stringToDouble(opnd2));
                opnd = "";
            }
            else if((expr[i] == '*') || (expr[i] == '+') || (expr[i] == '-'))
            {
                tempExp += opnd;
                tempExp += expr[i];
                opnd = "";
            }
            else
                opnd += expr[i];
        }
        expr = tempExp + opnd;
        //cout<<"After division: "<<expr<<endl;
    }

    if(mul != 0)                                    //multiplication
    {
        opnd = "";
        tempExp = "";
        for(int i = 0; i < expr.length(); i++)
        {
            //cout<<expr[i];
            if(expr[i] == '*')
            {
                opnd2 = "";
                for(i++; i < expr.length(); i++)
                {
                    if((expr[i] == '*') || (expr[i] == '+') || (expr[i] == '-'))
                    {
                        i--;
                        break;
                    }
                    opnd2 += expr[i];
                }
                tempExp += toString(stringToDouble(opnd) * stringToDouble(opnd2));
                opnd = "";
            }
            else if((expr[i] == '+') || (expr[i] == '-'))
            {
                tempExp += opnd;
                tempExp += expr[i];
                opnd = "";
            }
            else
                opnd += expr[i];
        }
        expr = tempExp+opnd;
        //cout<<"After multiplication: "<<expr<<endl;
    }

    if(as != 0)                                 //addition & subtraction
    {
        opnd = "";
        double result = 0;
        bool op = false;
        tempExp = "";

        for(int i = 0; i < expr.length(); i++)
        {
            if(expr[i] == '+')
            {
                opnd2 = "";
                for(i++; i < expr.length(); i++)
                {
                    if((expr[i] == '+') || (expr[i] == '-'))
                    {
                        i--;
                        break;
                    }
                    opnd2 += expr[i];
                }
                if(!op)
                {
                    result = stringToDouble(opnd);
                    op = true;
                }
                result += stringToDouble(opnd2);
                //cout<<"After Add: "<<result<<endl;
            }
            else if(expr[i] == '-')
            {
                opnd2 = "";
                for(i++; i < expr.length(); i++)
                {
                    if((expr[i] == '+') || (expr[i] == '-'))
                    {
                        i--;
                        break;
                    }
                    opnd2 += expr[i];
                }
                if(!op)
                {
                    result = stringToDouble(opnd);
                    op = true;
                }
                result -= stringToDouble(opnd2);
                //cout<<"After Sub: "<<result<<endl;
            }
            else
                opnd += expr[i];
        }
        expr = toString(result);
        //cout<<"After addition & subtraction: "<<expr<<endl;
    }
    return expr;
}

int main()
{
    //freopen("Sample Input.txt","r",stdin);
    string expr, result;
    getline(cin, expr);
    cout<<"Input: "<<expr<<endl;
    result = solve(expr);
    cout<<"Result: "<<result<<endl;
    return 0;
}