Why doesn't roundf work for me in this case?

Question

This is C. I am a beginner, so sorry for the experts to whom this question may seem trivial. I am trying to round this float to the nearest integer, away from zero. I've also tried rintf based on some other posts on the internet, but it just won't work! I used printf to check the results, and they weren't rounded to the nearest integer.

//Approximate US grade level.
float index = 0.0588 * L - 0.296 * S - 15.8;

float roundf(float index);

Your question is missing critical parts: 1) how `roundf` is used/called 2) how you print the result and conclude. Post a [mcve]. — P.P, Jul 22 '20 at 09:37
"but it just won't work! " lacks needed details. Post a [mcve]. " used printf to check the results, and they weren't rounded to the nearest integer." talks about the how, but code would be better. IAC, sample data is needed too. — chux - Reinstate Monica, Jul 22 '20 at 09:41

RobertS supports Monica Cellio · Answer 1 · 2020-07-22T10:16:50.143

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Note that

float roundf(float index);

is a declaration of a function. It is not a call.

If you use float roundf(float index); as function call inside of index = float roundf(float index); you should get a compiler error, but maybe you are on an uncommon compiler. Thus it can be a reason that it "won't work" as expected.

A correct call would be index = round(index);.

I used printf to check the results, and they weren't rounded to the nearest integer.

Note that floating-point precision isn't the best one in case you want to represent integers with it. A float or double can't represent an even integer value fully accurate. It has only a narrowed and limited precision.

Why not use Double or Float to represent currency?

edited Jul 22 '20 at 10:16

answered Jul 22 '20 at 09:44

RobertS supports Monica Cellio

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"A float or double can't represent an integer precisely/accurately." is unclear. `float` and `double` certainly represent integers +/- 2^24 exactly. – chux - Reinstate Monica Jul 22 '20 at 09:46
@chux-ReinstateMonica Was already at an edit as you commented. :-) – RobertS supports Monica Cellio Jul 22 '20 at 09:48
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OP's formula [`0.0588 * L - 0.296 * S - 15.8`](https://en.wikipedia.org/wiki/Coleman–Liau_index) certainly has nothing to do with currency. Unclear why a link to "Why not use Double or Float to represent currency?" – chux - Reinstate Monica Jul 22 '20 at 09:51
@chux-ReinstateMonica Exactly, but his/her expectation maybe: "I used printf to check the results, and they weren't rounded *to the nearest integer*" – RobertS supports Monica Cellio Jul 22 '20 at 09:52
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There may be a deeper underlying issue in the code that needs to be addressed. The OP wrote(coded) `float roundf (float index)` which is the function header. I think someone needs to explain that syntax for calling a function differs from syntax of function header – Abhay Aravinda Jul 22 '20 at 09:59
@AbhayAravinda Well spotted! But a legit compiler would throw an error in this case and OP said s/he is be able to run the code. But this is also why I voted to close the question, too. I took it into the answer, thank you. – RobertS supports Monica Cellio Jul 22 '20 at 10:07
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"But a legit compiler would throw an error in this case" --> err, no. OK to declare functions in a function. – chux - Reinstate Monica Jul 22 '20 at 10:09
@chux-ReinstateMonica I meant an assignment like that `index = float roundf(float index);`. – RobertS supports Monica Cellio Jul 22 '20 at 10:25

score 0 · Answer 2 · answered Jul 22 '20 at 09:37

Functions in C take an input and return an output in this way: output = function(input);. There may be more than one input for some functions, of course, but the principle is that.

For your case, try index = roundf(index); if you want the result to overwrite the non-rounded value.

Why doesn't roundf work for me in this case?

2 Answers2