Calculate using int and output float?

Question

//findSlope(twoPoints).exe
//finding the slope of line AB, using coordiantes of point A and B.

#include <iostream>

int main()
{
    int a, b, c, d;
    float answer;

    std::cout << "The X coordiante of A: ";
    std::cin >> a;
    std::cout << "\nThe Y coordiante of A: ";
    std::cin >> b;
    std::cout << "\nThe X coordiante of B: ";
    std::cin >> c;
    std::cout << "\nThe Y coordiante of B: ";
    std::cin >> d;
    std::cout << "\nThe slope of line AB = " << std::endl;

    answer = (b-d)/(a-c); 

    std::cout.setf(std::ios::fixed);
    std::cout.precision(3);

    std::cout << answer << std::endl; 
    //alternative= std::cout << fixed << setprecision(#) << answer << std::endl;

    std::cout.unsetf(std::ios::fixed);

    return 0;
}

I am learning C++ and I tried to code a program that calculate the slope using the coordinates of two points.

I understand that if I use float for variables I declared for the coordinates, the result of the calculation would output as float with decimals. However, I wonder if I may still use int for user input so that I can ensure the inputs are integers.

Extra question: Would it be possible to convert a float presented in the form of "#.##" to "# #/#"? More like how we do mathematics IRL.

You should learn about cast – Thomas Ayoub Sep 08 '16 at 13:49 — Thomas Ayoub, Sep 08 '16 at 13:49
You want fractions instead of decimals right? – Hayt Sep 08 '16 at 13:49 — Hayt, Sep 08 '16 at 13:49
http://en.cppreference.com/w/cpp/language/static_cast – Matt Sep 08 '16 at 13:50 — Matt, Sep 08 '16 at 13:50

score 2 · Accepted Answer · edited May 23 '17 at 10:30

2

You can use implicit conversion to double:

answer = (b-d)/(a-c*1.0);

Or explicit cast:

answer = (b-d)/(a-(float)c);

Bonuses:

for the fraction part: Converting decimal to fraction c++
Why does integer division result in an integer?

edited May 23 '17 at 10:30

Community

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answered Sep 08 '16 at 13:51

Thomas Ayoub

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`1.0` is a literal of type `double`, not an implicit conversion or cast. Both of your examples work due to implicit conversions, which occur for different reasons. – Peter Sep 08 '16 at 13:58

exploser · Answer 2 · 2016-09-08T14:01:24.963

You can use int for user input, but to precisely calculate anything that contains a division operator /, you'll need to cast to floating point types.

It's usually considered a good practice in C++ to use static_cast for that (although you still may use c-style (float) syntax).

For example:

answer = static_cast<float>(b - d) / (a - c);

Here, you convert (b - d) to float and then divide it by integer, which results in a float.

Note that the following wouldn't work correctly:

answer = static_cast<float>((b - d) / (a - c));

The reason is that you first divide an int by another int and then convert the resulting int to a float.

P. S. float is really inaccurate, so I would advise to use double instead of float in all cases except where you want to write faster code that does not depend on mathematical accuracy (even though I'm not sure it would be faster on modern processors) or maintain compatibility with an existing library that uses float for some of its functions.

There is no division by integer in your first example. The result of the expression `a-c` is converted to `float` before doing the division. And using `double` everywhere is really poor advice. — Peter, Sep 08 '16 at 14:00

Calculate using int and output float?

2 Answers2