Реализация меню выбора арифметических действий

#include <iostream.h>
#include <windows.h>
#include <math.h>
char buflab3[256];
char* lab3(const char* text)
{
    CharToOem(text, buflab3);
    return buflab3;
}
 
int main()
{   
    int a,b,c,u;
 
    long double f,g,h,t,q,r;
 
    cout<<lab3("0..9:=>");
    cin>>a,r ;
    cout<<lab3("0..9:=>");
    cin>>b,q;
    cout<<lab3("Ââåäèòå çíàê +|-|/|*:=>");
    cin>>u; 
    switch (a)
    {
    case 0: cout<<"0 ";break;
    case 1: cout<<"1 ";break;
    case 2: cout<<"2 ";break;
    case 3: cout<<"3 ";break;
    case 4: cout<<"4 ";break;
    case 5: cout<<"5 ";break;
    case 6: cout<<"6 ";break;
    case 7: cout<<"7 ";break;
    case 8: cout<<"8 ";break;
    case 9: cout<<"9 ";break;
    default: cout<<lab3("×èñëî âíå äèàïîçîíà îò 1..9");
    }
            switch (u)
    {
    case 0: cout<<"- ";break;
    case 1: cout<<"+ ";break;
    case 2: cout<<"/ ";break;
    case 3: cout<<"* ";break;
    default: cout<<lab3("ne pravilno: +|-|/|*");
    }
        switch (b)
    {
    case 0: cout<<"0 ";break;
    case 1: cout<<"1 ";break;
    case 2: cout<<"2 ";break;
    case 3: cout<<"3 ";break;
    case 4: cout<<"4 ";break;
    case 5: cout<<"5 ";break;
    case 6: cout<<"6 ";break;
    case 7: cout<<"7 ";break;
    case 8: cout<<"8 ";break;
    case 9: cout<<"9 ";break;
    default: cout<<lab3("1..9");
    }
        if (u="-") || Вот тут не получается задать условие
            f=r-q;
    return 0;
}

char u;

case 0: cout<<'- '; break;