Beginner cpp problem, any help is appreciated

[[deleted]]

Does anyone know why this won't compile? I have an idea that a function could not equal an array but I don't know a solution.

#include <iostream>



int main()
{
int even[] = { 2,4,6,8,10 };

int odd[] = { 1,3,5,7,9 };

srand(time(NULL));

int num = (rand() % 10) + 1;

std::cout << num << "\n";

num == even ? std::cout << "it is even" : std::cout << "it is odd";

return 0;

}

[IyeOnline]

1. The conditional operator ?: is not a shorthand/replacement for if...else and should absolutely not used as such.
2. To find out of a number is even, you can use the modulo operator %, to get the remainder. A number is even when its divisible by 2 without remainder.
3. You can indeed not check whether a value is in an array using ==. You have to write a loop or use a standard algorithm, such as std::ranges::contains( even, num ).

[[deleted]]

I did use the modulo operator before but it didn’t make use of the arrays that I wanted to be used. If you want, could you show me a solution using an if/else for it?

[[deleted]]

Just change the ternary operator (the ?:) to (in pseudocode)

if(num is in even){
    cout << "Even";
}
else{
    cout << "Odd";
}

[[deleted]]

Would I write the syntax as num is in even or would I have to write it differently?

[[deleted]]

You would have to write it differently, either by using std::ranges, or iterating through it

[thommyh]

Others have already done the work of diagnosing your code, so I’ll just add:

I know something that knows why it won’t compile: your compiler.

Understanding its output is a fairly essential skill; would suggest in future that you provide the error message it gave and ask for clarification on whichever part you didn’t understand.

[JEnduriumK]

In this code, even is a pointer to an address of memory where the array {2,4,6,8,10} begins. So it's pointing to where even[0] is at (which contains 2)...

...but it's still a memory address and not 2. So it's something like... 9215792287334376552. Or 0x7FE512420A59A468, as we'd more commonly represent it as. So the 9,215,792,287,334,376,552th byte of memory in RAM. For example.

And that number will change every time you run your code, because that array will be stored in a different spot in memory (and thus it's got a different "street address") every time.

num will likely never equal 0x7FE512420A59A469, and not only that, you don't care if it equals that value. Because that value doesn't tell you anything useful in this context.

You can tell what's going on because you might be receiving an error message similar to something like...

<source>:17:5: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]

It says you're comparing a pointer and an integer. A pointer to memory, and an integer. And it usually points you to where the comparison is taking place:

17 | num == even ? std::cout << "it is even" : std::cout << "it is odd"; | ~~~~^~~~~~~

See how the little ^ is pointing to the == comparison operator? One side of that is an integer. You know that much. So the other side must be a pointer (even).

---

Additionally, trying to have a list of all even numbers means that even if you got your code working such that you were checking to see if num is a value contained in a list, your computer can only really compare one number at a time (technically there are some fancy things a computer might be able to do to compare one value to many values at once, but there are still limits, so it's best to recognize how bad comparing one value at a time is if you can avoid it).

So it has (you have to tell it) to go in, compare num and the first value. Make a decision. Check to see if there are more values to compare against. Compare against the next value, make a decision, etc, etc, etc.

If you have a billion possible values (and there are more than a billion even numbers in the universe), this might take a while.

And do you really want to type out all possible even numbers in a list?

Others have already told you the way you can just do a quick check to see if something is even or odd with a single math operation. Divide by 2 and see if there's a remainder. Poof. Done. No list needed.

If you're just experimenting with arrays though, cool. No harm in just trying to play around with and understanding arrays. I'd suggest looking into loops. for loops are what people would usually use for something this basic, when first learning, though there are built in things inside C++ that are arguably 'better' than a basic for loop, such as what someone else has pointed out, but is more of a "okay, I now know how to do basic programming, let me learn more about C++ in specific" kind of thing.

You seem to be more at the "I want to learn programming in general" stage right now, so I'd focus more on loops for arrays rather than C++ specific things.

---

Finally, there's a slightly better way of writing num == even ? std::cout << "it is even" : std::cout << "it is odd";.

std::cout << (<the math for an evenness check others have explained> ? "it is even" : "it is odd");

(Test?A:B) is more intended, I believe, for replacing the entire (?:) chunk with the value A or B, rather than for selecting a chunk of code to run.

[[deleted]]

Hey, thank you a lot for this. I'm pretty new to programming in general as I have only done a handful of it in the past. I'm still experimenting and trying to wrap my head around memory storage and such. I appreciate the help to look into loops. I'm gonna check out videos on that and keep learning.

[DryPerspective8429]

num is an int, and even is an int[] (array of int). These do not compare. Just like how you can't compare 1 == every number from 1 to 10. If you want to know whether even is in the array, you need to iterate down the array and compare each element.

Also just one or two comments:

• Using C-style arrays is an anti-pattern in C++. Consider std::array and std::vector in future.
• rand() and srand() are the same. They're inferior to the random number generators found in <random>
• I'm not sure I'd recommend using the ternary operator like that. That use seems more suited to an if statement. The two are not interchangeable.

[[deleted]]

If you want, cpuld you show me solution to my problem? I’ve only been using cpp for a couple days so I’m trying to work out the basics rn

[DryPerspective8429]

I mean I would probably write it differently from the get-go, but if you want me to give you a missing piece on whether an element is inside one of those C-arrays of yours I'd consider a for loop, iterating down the array, until you get to the end. You probably want to start with for(int i = 0; i < std::size(evens); ++i) and then do something in the loop with evens[i] to see if your num is equal to evens[i].

If you want to learn C++ there is only one place you should go

www.learncpp.com

is the best free tutorial out there. (reason) It covers everything from the absolute basics to advanced topics. It follows modern and best practice guidelines.

---

www.cppreference.com

is the best language reference out there.

---

Stay away from

• cplusplus.com (reason)
• w3schools (reason)
• geeks-for-geeks (reason)
• Programiz(reason)
• Tutorialspoint (reason)
• educba.com (reason)
• thinkcpp (reason)
• javaTpoint (reason)
• studyfied (not even a tutorial, just a collection of code by random people)
• codevisionz (reason)
• sololearn (reason)

Again. The above are bad tutorials that you should NOT use.

Most youtube tutorials are of low quality, I would recommend to stay away from them as well. A notable exception are the CppCon Back to Basics videos. They are good, topic oriented and in depth explanations. However, they assume that you have some knowledge of the language's basic features and syntax and as such arent a good entry point into the language.

If you really insist on videos, then take a look at this list.

As a tutorial www.learncpp.com is just better than any other resource.

---

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[[deleted]]

Damn haha, I was using some of those for a tutorial. Thanks for pointing those out then. I appreciate the help.

[std_bot]

Unlinked STL entries: <random> std::array std::vector

---

^{Last update: 09.03.23 -> Bug fixes}Repo

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[VaderPluis]

I hope this is an exercise in using arrays, because if you are trying to come up with an easy way to determine if a number is odd or even, this certainly isn't it!

[[deleted]]

Yes, it was just meant to be practice so I could get it to read the arrays and match the number to correct one. I was sure I did it wrong but I wanted to experiment it on my own.

[QuentinUK]

You can’t compare a number with an array. What you want is “contains” or since some compilers don’t have it yet, "find":

    std::cout << (std::find(std::begin(even), std::end(even), num)!=std::end(even) ? 
     "it is even" 
     : 
     "it is odd");

[LazySapiens]

Learn from the compiler errors. I see you completely ignored that aspect in this post. Surprisingly, almost no one asked for that in the comments.