How to round float number result?

Question

I'm trying to learn C, but currently stuck. I'm trying to get the same output as below, but I'm not sure how to round the last few decimals points in my code to get the same output below.

1. ENTER AN INTEGER
2. 5
3. DIAMETER: 10.000000
4. CIRCUMFERENCE: 31.415900
5. AREA: 78.539750

int main()
{
     float num1, diameter, circumference, area;

    printf("ENTER AN INTEGER\n");
    scanf("%f", &num1);

    diameter = 2 * num1;
    circumference = 2 * 3.14159 * num1;
    area = 3.14159 * (num1 * num1);

    printf("DIAMETER: %f\n", diameter);
    printf("CIRCUMFERENCE: %.6f\n", circumference);
    printf("AREA: %.6f\n", area);


    return 0;
}

The output of my result.

1. ENTER AN INTEGER                                                                        
2. 5                                                                                       
3. DIAMETER: 10.000000                                                                     
4. CIRCUMFERENCE: 31.415899                                                                 
5. AREA: 78.539749

@GeneCode did you try? It doesn't give the answer sought for - as shown by the recently edited question. — Weather Vane, Feb 08 '19 at 17:41
I don't agree with the dup: OP is asking how to round the result. Did the teacher use the same (inaccurate) value for *pi*? That has only 5 decimal places but you want an answer to 6 places. But with 8 significant figures you are the limit of what can be done with `float`. — Weather Vane, Feb 08 '19 at 17:44
Can we please get a clarification about your question? Is it about *how to round your values*, or is it about *why you get the "wrong" values*? — Some programmer dude, Feb 08 '19 at 17:45
If you put the commenter's name like @Someprogrammerdude that will flag him. — Weather Vane, Feb 08 '19 at 17:52

score 1 · Answer 1 · answered Feb 08 '19 at 17:52

1

In typical C implementations, changing float to double and scanf("%f", &num1); to scanf("%lf", &num1); will provide sufficient precision to get the desired output in this case.

In general, you should expect small rounding errors in floating-point operations (which may grow to large errors with complicated sequences of numerous operations).

answered Feb 08 '19 at 17:52

Eric Postpischil

195,579
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after changing from float to double, I got the result I wanted. Thanks! – Rzxd Feb 08 '19 at 18:00

JuMoGar · Answer 2 · 2019-02-08T18:03:41.833

As @Eric Postpischil says, your decimals error are due to operations with float numbers (single precision -32 bits-) have small precision errors. A double number (double precision -64 bits-) will solve your precision problem.

Here you have some information about the three decimal types: Decimal types

A working example code for your chase is:

int main()
{
    double num1, diameter, circumference, area;

    printf("ENTER AN INTEGER\n");
    scanf("%lf", &num1);

    diameter = 2 * num1;
    circumference = 2 * 3.14159 * num1;
    area = 3.14159 * (num1 * num1);

    printf("DIAMETER: %lf\n", diameter);
    printf("CIRCUMFERENCE: %.6lf\n", circumference);
    printf("AREA: %.6lf\n", area);

    return 0;
}

How to round float number result?

2 Answers2