C ++初学者-简单的输出

Question

Here I have simple code to serve as a calculator for integers.

//Calculator, by Michael Lowry

#include <iostream>
using namespace std; 

void main ()
{
    int input = 0;
    int input2 = 0;
    int answer = 0;
    int operation = 0;

    cout << "Enter your first number" << endl;
    cin >> input;

    cout << "Type 1 for addition, 2 for subtraction, 3 for multiplication, or 4 for division" << endl;
    cin >> operation;

    cout << "Enter your second number" << endl;
    cin >> input2;

    if (operation = 1)
    {
        input + input2 == answer;
    }

    if (operation = 2)
    {
        input - input2 == answer;
    }

    if (operation = 3)
    {
        input * input2 == answer;
    }

    if (operation = 4)
    {
        input / input2 == answer;
    }

    cout << "Your answer is " <<
    cout << answer << endl;
    system ("PAUSE");
}

When I enter "1" for all three inputs, I get the output "Your answer is 6121DBCC0". Why is my answer variable all messed up?

Answer 1

Your output goes wrong. You should have

cout << "Your answer is " << answer << endl;

instead of

cout << "Your answer is " << 
cout << answer << endl;

What happens is that you are writing the outstream object cout to output.

Also the comparison operators are wrong, as others have noted. You should have == instead of = in the if -statements and vice versa in the assignment part. Like this:

if (operation == 2)
{
    answer = input - input2;
}

Answer 2

There are several errors: this is assigning a value to the operation variable, not comparing it against something:

if (operation = 1)

it should rather be

if(operation == 1)

furthermore this doesn't assign the result of input+input2 to answer but rather makes an unused comparison evaluation

input + input2 == answer;

and it should rather be

answer = input + input2;

You should change your code accordingly. Finally this:

cout << "Your answer is " << 
cout << answer << endl;

is wrong since you're passing the cout object along (mind the operator<< ). That should have been

cout << "Your answer is " << answer << endl;

Also: main() is supposed to return int .

Thus your code should have looked like:

int main () {
    int input = 0;
    int input2 = 0;
    int answer = 0;
    int operation = 0;

    cout << "Enter your first number" << endl;
    cin >> input; 

    cout << "Type 1 for addition, 2 for subtraction, 3 for multiplication, or 4 for division" << endl;
    cin >> operation;

    cout << "Enter your second number" << endl; 
    cin >> input2;

    if (operation == 1) {
        answer = input + input2;
    }

    if (operation == 2) {
        answer = input - input2;
    }

    if (operation == 3) {
     answer = input * input2;
    }

    if (operation == 4) {
     answer = input / input2;
    }

    cout << "Your answer is " << answer << endl;
    system ("PAUSE");
}

Try it out

Answer 3

First: Use

if (operation == 1)

instead of

if (operation = 1)

because == is for equality, = is for assignment.

Second:

answer = input1 + input2;

instead of

input + input2 == answer;

Do this in all if statements.

Third: Use

cout << "Your answer is "  << answer << endl;

to print your answer.

And remember answer = input / input2 will give you integer division not floating point.

Answer 4

First mistake

If you want to assign value to variable answer you should do:

answer = input1 (required operator here) input2;

In your code such construction:

input - input2 == answer;

Is wrong in 2 ways:

1. If you want assign value to answer use assigning = not comparing == operator.
2. Assigning values goes from right to left, so your desired variable should be on the left side.

Second Mistake

In line if (val = yourConstant) you made very popular mistake - assigning inside if statement. Many languages prohibit such things, because they are hard to detect without debuggining or tests. The code inside if statement will be executed only if yourConstant will be more than 0. Instead please use if (val == yourConstant) or if (yourConstant == val) .

Answer 5

So instead of:

if (operation = 1)

do:

if (operation == 1)

Also change your output to: cout << "Your answer is " << answer << endl;

Another mistake:

input - input2 == answer;

This is wrong. You have to make answer equal to input - input2 . Also there should only be one = , because it is not a condition.

So:

answer = input - input2;

Apply that to all your arithmetic inside the if statements. That should fix it. But try to not use system ("PAUSE"); because it has its problems if you transfer your code. Just a suggestion.