#what's the appropriate way to check if a float can be converted to integer

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trail glenBOT
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keen shell
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The float should not loose information when getting converted to int, for example if float you entered is 5.0 it can be converted to int safely, but if float you entered has decimal place values like 5.567 it shouldn't be converted to integer as it will loose the decimal part and converting it to int will make it 5, this is known as narrowing conversion. To avoid this you must use the {} brace initialisation. example of initialisation integer with brace initialisation is

int num{};

now to let compiler warn you about narrowing conversions you must convert a float to an int the below way

float f = 5.623;

// to know whether it is safe to convert it to int or not use brace initialisation

int i{f}; // this is brace initialisation, that means identifier i will try to convert f to int but since you used brace initialisation, compiler will give you an error, that the value can't be converted to int

obtuse steppe
keen shell
obtuse steppe
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e.g. 5.0 can but 1099511627776.0 can't

keen shell
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let me try

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well i think both takes 4 bytes so anything above the value of 4 bytes will not be accepted by the conventional int

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what are your views?

obtuse steppe
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are you asking me what's the appropriate way to check if a float can be converted to integer?

keen shell
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no, i am telling you that conventional int will except the value within the range of -2147483648 to 2147483647, and 10995116277776.0 is out of the range of what a conventional int can accept

obtuse steppe
keen shell
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therefore to convert the float of to an int it must fit the range of int, else the int of larger type must be used.

obtuse steppe
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what's the appropriate way to check if a float CAN be converted to integer

trail glenBOT
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@obtuse steppe

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keen shell
# obtuse steppe what's the appropriate way to check if a float CAN be converted to integer

There are different ways to check if a float value can be converted to an integer in C++. One way is to compare the float value with its truncated or rounded version and see if they are equal. For example, you can use the std::floor or std::ceil functions from the <cmath> header to get the truncated or rounded value of a float¹. Another way is to use the std::modf function from the same header, which splits a float value into its integer and fractional parts and returns the fractional part². You can then check if the fractional part is zero or not. Here is some code that demonstrates both methods:

#include <iostream>
#include <cmath>

bool is_int_trunc(float x) {
    // Check if x is equal to its truncated value
    return x == std::floor(x);
}

bool is_int_round(float x) {
    // Check if x is equal to its rounded value
    return x == std::round(x);
}

bool is_int_modf(float x) {
    // Check if x has zero fractional part
    float frac;
    std::modf(x, &frac);
    return frac == 0.0f;
}

int main() {
    float a = 3.14f; // Not an integer
    float b = 4.0f;  // An integer
    std::cout << "a = " << a << "\n";
    std::cout << "b = " << b << "\n";
    std::cout << "is_int_trunc(a) = " << is_int_trunc(a) << "\n";
    std::cout << "is_int_trunc(b) = " << is_int_trunc(b) << "\n";
    std::cout << "is_int_round(a) = " << is_int_round(a) << "\n";
    std::cout << "is_int_round(b) = " << is_int_round(b) << "\n";
    std::cout << "is_int_modf(a) = " << is_int_modf(a) << "\n";
    std::cout << "is_int_modf(b) = " << is_int_modf(b) << "\n";
    return 0;
}

The output of this program is:

a = 3.14
b = 4
is_int_trunc(a) = 0
is_int_trunc(b) = 1
is_int_round(a) = 0
is_int_round(b) = 1
is_int_modf(a) = 0
is_int_modf(b) = 1

As you can see, the functions return true (1) for b, which is an integer, and false (0) for a, which is not an integer.

I hope this helps you with your C++ question. 😊

I used chat gpt, here is its response.

obtuse steppe
keen shell
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its just an idea, i didn't test the code

obtuse steppe
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you didn't answer my question, so i assume you just pasted this without reading what it says

keen shell
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I cannot comprehend your ORIGINAL question.

obtuse steppe
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i was not asking YOU

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i was hoping for a response from someone who knew

keen shell
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it popped in front of me, blame discord.

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or blame the author of the server who don't ask for any special qualification to answer and question.

obtuse steppe
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you did not know the answer to the question, and it was not addressed to you

keen shell
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so what's the answer?

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to the question

obtuse steppe
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oh my fucking god

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why would i ask the question if i KNEW the answer

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i wasn't testing you

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hell, i wasn't even talking to you

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i don't know why you are here

keen shell
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the only way is if the solution isn't working, and i bet you haven't tested the code.

obtuse steppe
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and that you didn't understand the code from chat GPT

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which, by the way, is incredibly fucking rude

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if i wanted to talk to chat GPT i would talk to chat GPT

keen shell
obtuse steppe
keen shell
obtuse steppe
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because you did not actually know the answer, and the question was not addressed to you

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it was addressed, implicitly, to someone who knew the answer

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second, it was rude for you to paste a question into chat GPT and paste its answer back without even reading that answer

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please do not do that to anyone ever

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that is absolutely infuriating

obtuse steppe
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here's a clue

keen shell
obtuse steppe
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what do you think x == std::floor(x) does if x is 10995116277776.0

obtuse steppe
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i am blocking you now

keen shell
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it is a comparison of x with the floor value of x

obtuse steppe
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please never do this to anyone else

keen shell
# obtuse steppe i am blocking you now

if you have a different idea in mind, then comprehend your question correctly. what you asked was incomplete, seems like it was not directly related to what you wanted to ask

keen shell
sharp hearth
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Could you just include limits.h and check if your float is greater than INT_MAX?

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Also if you need to check for any decimal portion, you could maybe use fmodf(x, 1.0f) == 0.0f

clever eagle
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I think this won't work, because the range of float contains "holes" in its spanning of the integer numbers (assuming both in 32 bits) (example: 16777217)

idk if it "won't work", because functions that operate on double (64 bits) might hide tricky behaviors

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(some "integer"-ish float indeed overflow the range of integers, but you also have hole phenomenon: some int just cann't be represented ; you have the same phenomenon when using long and double (64 bits))

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@obtuse steppe there is this function : https://cplusplus.com/reference/cmath/rint/

according to the spec, it might yield an error when the conversion is inexact
maybe if you manage to find an example implementation of this method that raises an error, you could see how they detect a conversion is inexact and mimic it

clever eagle
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@obtuse steppe in Hanson's book, they seem to have a more pedant approach:

you represent your numbers in a base that is a power of 2 (they take 8) and compute the expansion of the number. Doing this, you can check for overflows every time you make an operation.

maybe that approach might work good

obtuse steppe
obtuse steppe
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rint rounds to an integral-valued float, which is probably not ever inexact in practice

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if the conversion to integer is inexact (i.e. truncates) it's possible to detect that by just checking ==
the problem is just knowing if the conversion is UB

clever eagle
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yes yes, I was not saying to use rint drectly ; just to check how they verify for exactness

obtuse steppe
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i see what you're saying, but if there even are any implementations that do that, they would be checking if the result is exact within the floating type

clever eagle
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for sure yes

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the initial message has been deleted so I dn't have the initial question, but what is your definition of "a float without decimal part" ?

clever eagle
obtuse steppe
clever eagle
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I understood from your initial question that you already have something that would be an integral-valued float

obtuse steppe
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i have a float. i want to convert to some integer type. this may be UB if it is out of range. i would like a way to know ahead of time whether the conversion would be UB.

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i already know how to detect truncation, but that is no help to me if the conversion raises a floating point exception

clever eagle
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as far as I can read the C spec, the only reason for UB is numbers going outside of the range of the integral type

obtuse steppe
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e.g. if INT_MAX is 2147483647 is not representable in the float type, it may be rounded up when you convert it

clever eagle
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ah yeah right ; I forgot about why I said "this wouldn't work" above 😄

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but yes, you cann't rely neither on float -> int promotion because of the holes for example, or others

obtuse steppe
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like if x is a floating point variable then x > 2147483647 may be false even though x is actually greater

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;cpp { printf("%f", (float)2147483647); }

brazen tulipBOT
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Program Output
2147483648.000000
abcq2#0000 | 36ms | c++ | x86-64 gcc 10.1 | godbolt.org
clever eagle
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then I don't see better than trying to decompose the float in some base and recreate the integer, catching overflows as they might appear (Hanson does it in his book iirc)

mint perch
# obtuse steppe e.g. if INT_MAX is 2147483647 is not representable in the float type, it may be ...

You can do some stuff to make sure that the bounds are correct, which is what this library looks like it does:https://www.reddit.com/r/cpp/comments/rsldwl/converting_float_to_int_can_be_undefined_behavior/

There's also this solution which gets the exponent to check bounds https://stackoverflow.com/questions/25857843/how-do-i-convert-an-arbitrary-double-to-an-integer-while-avoiding-undefined-beha

Edit: I am incredibly blind and didn't realize I was in the C forum instead of the C++ forum, ignore me

Reddit
From the cpp community on Reddit: Converting float to int can be un...

Explore this post and more from the cpp community

Stack Overflow
How do I convert an arbitrary double to an integer while avoiding u...

Let's say I've got a function that accepts a 64-bit integer, and I want to call
it with a double with arbitrary numeric value (i.e. it may be very large in
magnitude, or even infinite):

void DoSom...

obtuse steppe
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... i can probably sprintf and then parse the result kekw but that seems... not ideal

clever eagle
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(I'm trying to find back the book, because I don't have it neither)

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you could also just convert to double, check in doubles, and go back

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but that assumes int are represented entirely with double

obtuse steppe
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true, but that doesn't work for larger integer types

clever eagle
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yes indeed

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(and I couldn't find in the spec where there is a guarantee for double to entirely span int anyway)

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(we definitely should create a "unfair C compiler" that exhibit every possible edge cases 🤣 would be fun)

obtuse steppe
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especially in C++

obtuse steppe
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when i made the post i was half hoping there was a function that would just convert a float to the nearest integer in some integral type, which seems to be definitely not the case

clever eagle
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there is also issues when the numbers get too large

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If the value to be stored in *exp is outside the range of int, the behavior is unspecified.

shut roost
obtuse steppe
shut roost
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snprintf can report errors when parsing fails

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I mean... sscanf

clever eagle
obtuse steppe
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could use the log function

mint perch
obtuse steppe
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i.e. log(x) / log(2)

clever eagle
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yeah I guess

shut roost
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the perfect student project - bruteforce all floats and see which ones can be safely and 100% accurately converted to int32_t

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with sscanf

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❤️

clever eagle
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meeeeh, tbh... if you think about it... searching a int that works between 2^32 and 0 only involves something like 32 checks, using a dichotomic search

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haha 😄

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(I mean, in 32 checks, you can likely find a window of size 2 around the solution 😄 haha)

shut roost
clever eagle
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yeah

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that doesn't discard anything 😄 just lowers the bound by 1 😄 haha

ripe rover
shut roost
stiff cloak
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!warn @keen shell We don't allow AI generated answers here. If you don't know enough to know the answer yourself you don't know enough to understand why the AI answer is wrong.

trail glenBOT
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success Ayush was warned

keen shell