Task D. Calc3:Squares. how to construct a proper loop for my assignment? C++

问题

I need help with a coding assignment.

Write an even better calculator program calc3.cpp(Using iostream only) that can understand squared numbers. We are going to use a simplified notation X^ to mean X^2. For example, 10^ + 7 - 51^ should mean 10^2 + 7 − 51^2. Example: When reading input file formulas.txt

5^;
1000 + 6^ - 5^ + 1;
15^+15^;

the program should report:

$ ./calc3 < formulas.txt
25
1012
50625

A hint: To take into account ^, don’t add or subtract new numbers right away after reading them. Instead, remember the number, read the next operator and if it is a ^, square the remembered number, then add or subtract it.

What I have so far:

#include <iostream>
using namespace std;
int main() 
{
int numbers,firstnum;
char operators ; // for storing -+;^ from the txt file
int sum = 0;
cin >> firstnum; // stores the 1st integer of the expression found in the txt file. 
while(cin >> operators >> numbers)
 { // while loop for storing characters and int 

    if(numbers>=0 and operators =='+')//adding
    {
        firstnum+=numbers;
    }
    else if (numbers>=0 and operators == '-')//subtracting
    {
        firstnum-=numbers;
    }
    if (operators==';') // semicolon:ends the loop for each expression
    {
    cout<<firstnum<<'\n';
    firstnum=0+numbers;
    }

 }  
sum=firstnum; // for the last expression not picked up by the while-loop
cout<<sum;

}

I need help constructing a while loop so the '^' doesn't break the loop.

回答1:

In the terminology of parsing, ^ has higher precedence than + and -. That is, ^ must be applied first before additions and subtractions are evaluated. You shouldn't add/subtract a number before knowing whether the next token after the number is a ^; because if it is, then you need to perform the squaring operation first.

Consider, for example, the following input file:

1 + 3^;

Right now, your program will read 1 and store this in firstnum. Then your program reads + (storing this in operators) and 3 (storing this in numbers). Then your program reaches the numbers>=0 and operators =='+' case, so firstnum+=numbers; is evaluated and firstnum becomes 4. But, this is a mistake, because the next token is a ^, so the 3 should have been squared first.

Consider this other example:

4^ + 1;

Your program currently reads 4 into firstnum, ^ into operators, and then tries to parse + as an integer. This fails, causing the fail bit to be set on cin and cin >> operators >> numbers evaluates to false.

One special case that you might want to handle is when an expression begins with a -:

-9^;
-10^ + 11^;

Also, you might want to ask for clarification as to how the following should be handled:

1^ + -2^;
1^ - -2^;
2^^^;

来源：https://stackoverflow.com/questions/49101581/task-d-calc3squares-how-to-construct-a-proper-loop-for-my-assignment-c

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