Trouble with large numbers while using larger in cpp

Question

While sorting and displaying big numbers you usually end up displaying the large numbers in enotation. I was trying to display the whole number by using the <iomanip> library in cpp and it fails for very large numbers.

 //big sorting
    #include<bits/stdc++.h>
    #include<iomanip>
    using namespace std;
    int main()
    {
        int n;
        cin>>n;
        double arr[n];
        for (int i = 0;i < n; i++)
            cin>>arr[i];

        sort(arr, arr+n);
        cout<<fixed<<setprecision(0);
        for (int i = 0;i < n; i++)
            cout<<arr[i]<<endl; 
    }

Input:

31415926535897932384626433832795
1
3
10
3
5

Expected output:

1
3
3
5
10
31415926535897932384626433832795

Actual output:

1
3
3
5
10
31415926535897933290036940242944

The last digit is getting messed up.

score 1 · Answer 1 · answered Jun 02 '20 at 04:57

1

The double type precision is only 15 decimal digits, so very large whole numbers can't be expressed in double type without loss of precison.

answered Jun 02 '20 at 04:57

Alexander Pivovarov

4,850
1
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could you also give me a solution like should I use long long int or something? – Krishnna Sarrdah Jun 02 '20 at 04:59
Well, there are multiple possible solutions depending on what is your intention. You need to set the requirement - i.e. answer questions like: (a) how large can your numbers be? (b) are the numbers necessarily whole numbers or can be others? (c) If all numbers are integers - can they be negative? – Alexander Pivovarov Jun 02 '20 at 05:03
I'm sorry I should've specified that in the question. – Krishnna Sarrdah Jun 02 '20 at 05:06
1= – Krishnna Sarrdah Jun 02 '20 at 05:09
2

The particular number in your example `31415926535897932384626433832795` is larger than what 64 bit unsigned integer could fit, so `long long` won't be enough. It could fit in 128 bit integer though, but if you consider higher numbers it won't help. Not sure I read your notation correct - 2*10^5 means 20000? Or is it 2 to the power of 100000? If 20000 then why the number in your examples is much larger than that? – Alexander Pivovarov Jun 02 '20 at 05:10
n is just the size of the array – Krishnna Sarrdah Jun 02 '20 at 05:14
what about the numbers themselves? – Alexander Pivovarov Jun 02 '20 at 05:16
If you read the first sentence there carefully - you can see that they specify that each number is between 1 and 10^6 digits (so numbers can be ridiculously large). This is a very unusual thing in most programming tasks - so you probably want to specify it when asking anyone on how to handle such numbers (here or anywhere else). For this task there are many possible solutions, for example: (a) use Python which can handle arbitrary big integers, (b) represent numbers as strings of digits. – Alexander Pivovarov Jun 02 '20 at 05:27
2

@KrishnnaSarrdah *n is just the size of the array* -- First this, `double arr[n];` is not valid C++. Arrays in C++ must have their sizes known at compile time, not runtime. Use `std::vector arr(n);` instead. Second, if `n` was large, you risk exhausting the stack memory using `double arr[n]`. You shouldn't use the fake C++ in declaring arrays -- it is not valid C++ syntax, and can only cause more issues than you may think it solves. – PaulMcKenzie Jun 02 '20 at 05:40
@PaulMcKenzie - after you pointed out that thing I couldn't believe this compiles. Then I found this: https://stackoverflow.com/questions/737240/array-size-at-run-time-without-dynamic-allocation-is-allowed. This was very surprising to me. C++ holds a lot of secrets... – Alexander Pivovarov Jun 02 '20 at 06:05

score 1 · Answer 2 · edited Sep 22 '20 at 07:45

Read How to debug small programs.

Enable all warnings and debug info when compiling your C++ code (with GCC, use g++ -Wall -Wextra -g).

Read of course the documentation of your favorite debugger.

I am a happy user of GDB.

Consider using static program analysis tools such as the Clang static analyzer or Frama-C++.

You might want to add `-pedantic` flag in the list of suggested compiler options. Without it the compiler won't catch the issue with declaring `double arr[n]` with `n` unknown at compile time. — Alexander Pivovarov, Jun 02 '20 at 06:00

Trouble with large numbers while using larger in cpp

2 Answers2

Read more about C++, perhaps the C++11 standard n3337.

Read How to debug small programs.

Read of course the documentation of your favorite debugger.

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