Simple Calculator Help (C++) - c++

Question

I wrote a very simple calculator program in C++ and it works fine! But can someone explain how I can modify this code such that it will be able to add more than 2 numbers at once?
So instead of doing 2 + 2 for example, I want users to be able to do 2 + 2 + 2. But every time I try, it just adds the first two "2"s and gives 4, no matter how many + 2 you type after it.
Here is the code:
#include <iostream>
using namespace std;
// input function
void Input (float &x, float &y);
float a=1.0, b=1.0, result;
char operation;
int main ()
{
cout << "A simple calculator \n\n";
cin >> a >> operation >> b;
Input (a,b);
cout << result << endl;
system ("pause");
return 0;
}
void Input (float &x, float &y)
{
a = x;
b = y;
switch (operation)
{
case '+':
result = x + y;
break;
case '-':
result = x - y;
break;
case '*':
result = x * y;
break;
case '/':
result = x / y;
break;
default:
cout << "Improper operation. Please input a correct calculation operation: \n";
cin >> a >> operation >> b;
Input (a, b);
}
}
Thanks guys!

Answer 1

Your code is thought so you are asking the input for three elements: 2 numbers and their operator (represented as a character).
Let's say you are taking just one shot. If you want to use more operators and numbers you have to develop a simple parser which should continue acquiring input until a end token is found, probably end of line in your case.
To do this I would change your approach concerning data acquisition and calculation. You have to simple options:
Perform your calculations online: That is, to keep a partial result and update it after each input.
Gather all your input in a data structure representing it and then perform all the calculations.
For example, if you decide to implement the first option, this incomplete code may help you:
float Input (float &x, float &y, char operation)
{
float result;
switch (operation)
{
case '+':
result = x + y;
break;
case '-':
result = x - y;
break;
case '*':
result = x * y;
break;
case '/':
result = x / y;
break;
default:
cout << "Improper operation. Please input a correct calculation operation: \n";
}
return result;
}
...
...
string input;
if(!getline( cin, input ) ) return; //Try to read one line.
istringstream lin( input );
float partial = 0.0;
float a;
char op;
while(lin >> op >> a) partial = Input(partial, a, op);
//At this point, partial should contain your final result

Answer 2

Note: I have not tested cases. Like division by zero. etc
float operate(float a, char operand, float b)
{
float result=0.0;
switch(operand)
{
case '+':
result = (a+b);
break;
case '*':
result = (a*b);
break;
case '/':
result = (a/b);
break;
default:
cout<<"Unknown operand"<<endl;
break;
}
return result;
}
int main()
{
float result=0.0;
char operand;
float a,b;
cout<<"enter a number followed by an operation, followed by a number"<<endl;
while(cin>>a>>operand>>b)
result+=operate(a,operand,b);
cout<<result;
return 0;
}

Answer 3

i wrote a little parser a few weeks ago, you can study my code and maybe (hopefully) you find something usefull.
its not c++, but c, so your compiler should be able to compile it.
the parser parses any statements that could be built by following rules:
<expression> ::= <term>
| <term> "+" <term>
| <term> "-" <term>
<term> ::= <factor>
| <factor> "*" <factor>
| <factor> "/" <factor>
<factor> ::= <number>
| "(" <expression> ")"
<number> ::= ["-"] ["0" .. "9"]*
here is the code, it is pretty much self explaining because it follows the rules defined above:
#include <stdio.h>
#include <stdlib.h>
char next;
void getNext(){
next=getchar();
}
int getNum(){ //get a number
int num=0;
if((next<'0')||(next>'9')){
printf("error: expected number; found %c",next);
exit(-1);
}
while((next>='0')&&(next<='9')){
num*=10;
num+=(next-'0');
getNext();
}
return num;
}
int add(int x){
return x+term();
}
int sub(int x){
return x-term();
}
int multiply(int x){
return x*factor();
}
int divide(int x){
return x/factor();
}
int factor(){
int f=0;
if(next=='('){
getNext();
f=expression();
if(next=')')
getNext();
else{
printf("error \")\" expected; found %c",next);
exit(-1);
}
}else{
f=getNum();
}
return f;
}
int term(){
int val;
val=factor();
while((next=='*')||(next=='/')){
char c=next;
getNext();
switch(c){
case '*': val=multiply(val); break;
case '/': val=divide(val); break;
default: printf("error: (*, /) expected; found %c",next);
exit(-1);
}
}
return val;
}
int expression(){
int sign=1;
int val;
if(next=='-'){
sign=-1;
getNext();
}
val=term()*sign;
while((next=='+')||(next=='-')){
char c=next;
getNext();
switch(c){
case '+': val=add(val); break;
case '-': val=sub(val); break;
default: printf("error: (+, -) expected; found %c",next);
exit(-1);
}
}
return val;
}
int main(void) {
getNext();
printf("%d\n", expression());
return 0;
}
example:
$ gcc main.c -o calc
$ ./calc
1+2+3
6
$ ./calc
(1+2+3)/2
3
$ ./calc
3*(1+2)
9
$ ./calc
-10+20
10
$