Is it possible to round a double using just a printf statement?

Question

Obviously this is just a fraction of the code.

printf("Please enter a positive number that has a fractional part with three or more decimal places\n");
scanf("%5.2d", &x);
printf("The number you have entered is %5.2d\n" ,x);

Would this automatically round the number I type in? Or is there another way to do this?

Edit:

printf("Please enter a positive number that has a fractional part with three or more decimal places\n");
scanf("%lf", &x);
x = x + 0.05;
printf( "The number you have entered is %5.2lf\n", x);

Iv done this, but Im taking into consideration what someone had said about printf just "changing" the way it reads out. So this is obviously not the right way. Should I implement maybe the pow() function? Will that work with this somehow?

Edit2:

printf("Please enter a positive number that has a fractional part with three or more decimal places\n");
scanf("%lf", &x);
x = x + 0.05;
printf( "The number you have entered is %5.2lf\n", x);

Okay, iv gotten to the point where if i imput a number it will round to a whole number. 35.21 will round to 35, and 35.51 will round to 36 et. etc.

How would I get 35.2178 to round to 35.22, and 35.2135 to round to 35.21. How would I get the certain powers of the decimal to round instead of the whole number?

Solution 1

You really, really should not store "rounded" values in floating point variables. Floating point inaccuracy will ruin this - your 5.10 might become 5.099999999941892 simply because the implementation might not be able to store 5.10 exactly.

As an alternative, read the whole number, multiply it with 100 and convert it to int (which will round it towards zero). That will keep your calculations accurate.

Solution 2

"%.2f" will round a double to 2 digits. A double is not an integer, and %d and %f are not interchangeable.

Solution 3

{

float x;
float rounded_x;

printf("Please enter a positive number that has a fractional part with three or more decimal places\n");
scanf("%f", &x);
rounded_x = ((int)(x * 100 + .5) / 100.0);

printf( "The number you have entered is %.2f\n", rounded_x);
return 0;

}

Thank you to everyone who tried to help! I finally got it

Comments

[Chandler]

I have tried it, and it only works some of the time. Is there another way of going about doing this? I'm relatively new to C.

[palantus]

Then you shouldn't have said it was an integer. Or did you mean "round to an integer"?

[Chandler]

I dont understand, where did I say it was an integer? Does %d not work with this?

[u0b34a0f6ae]

%d is for an integer. Where did you learn it was for doubles? Perhaps you should look into a reference or book.

[hrnt]

You can't use %.2f with scanf.

[pmg]

The format string in scanf() is different than in printf(). The one in your example is just plain wrong. To read a double from stdin, use scanf() with a "%lf" format string, then round the number in any way you prefer.

[Chandler]

yes but how will the rounding occur? By using %5.2lf?

[Chandler]

Im trying to round a number that is 3 decimal places long. Lets say I input 5.092697436 I want that to round to 5.10

[Chandler]

So is that where my problem is? I should have %f instead of %d?

[Chandler]

Alright I tried this, but it keeps spitting .05 back at me. Please enter a positive number that has a fractional part with three or more decimal places 5.698 The number you have entered is 0.05

[Chandler]

should i use the pow() function then in that case?

[Goz]

I thought %f was for doubles and floats. I thought internally printf used doubles, or is that a windows only thing?

[hrnt]

Yes, %f is for doubles and floats with printf (because of the vararg argument promotion rules). For scanf, %f is for floats and %lf is for doubles.

[Chandler]

I've got it to round to whole numbers, I's just curious now as to how to round it to certain decimal places. Would I use the pow() function?

[Martin Beckett]

To round to 2 places multiple by 100 (ie 10^2) chop off the fractional part, then divide by 100 again

[Tim Sylvester]

@Chandler Yes, if the variable you're printing is of type double, you should use "%f".