Taking a Big Integer Input

Question

I want to create a program which can handle integers greater than what C++ int can store.

I want to first store the integer that the user inputs in a integer array such that each digit of the input number is stored in each array slot,say, 968, it should be stored finally in the array such that arr[0] is 9, arr[1] is 6, arr[2] is 8. Instead, this time user will be inputting a very huge number with say, 1000 digits in it and I have to somehow get it to be stored in an integer array, with each digit of the number in each array element as cited above. So can someone please explain the usage of big int library?

And your question is? instead of revienting the wheel why don't you look into http://gmplib.org/ — NathanOliver, Aug 12 '15 at 13:11
May be this question will be usefull for you: http://stackoverflow.com/questions/12430339/c-large-number-arithmetic — Ilya, Aug 12 '15 at 13:15
@NathanOliver , thanks a lot for that, but then I wanted to do it on my own for the sake of a project which requires these fundamentals clearly understood by working on them first hand. :) — powersource97, Aug 12 '15 at 13:18
http://shygypsy.com/tools/ check this link. good implementation. The downvote is probably for a late answer with a link. — user2736738, Aug 12 '15 at 13:18

6502 · Accepted Answer · 2015-08-12T13:25:44.087

2

You can just use a std::string object.

Each character will be between '0' and '9' and you can use that to do your computations.

For example to compute the sum of the first two digit of two numbers you can do

int sum = (a[0] - '0') + (b[0] - '0');

Hint: it's a bit easier to write the computation algorithms if you keep in num[0] the least significat digit (i.e. what is normally written to the right-most place).

edited Aug 12 '15 at 13:25

answered Aug 12 '15 at 13:14

6502

112,025
15
165
265

But we can't simply operate on strings like that right? We need to convert the character in the string to a integer first isn't it? – powersource97 Aug 12 '15 at 13:17
@powersource97 Correct. – Don Larynx Aug 12 '15 at 13:19
@6502 can you please elaborate on how do you plan to convert that character `1` to an integer `1` ? – powersource97 Aug 12 '15 at 13:24
1

@powersource97: see edit... you can just do `s[i] - '0'` to get the numberic value of `i`-th digit. – 6502 Aug 12 '15 at 13:26
1

@powersource97 Looking at the ASCII table, you simply do '1' - '0'. (Why? use asciitable.com to find out.) – Don Larynx Aug 12 '15 at 13:26

Don Larynx · Answer 2 · 2015-08-12T13:23:20.163

0

Instead, this time user will be inputting a very huge number with say, 1000 digits in it and I have to somehow get it to be stored in an integer array, with each digit of the number in each array element as cited above.

Answer: Allocate dynamic memory.

int variableSize;
char* theNumber;
std::cout << "What is your preferred size? ";
std::cin >> variableSize; /* do some error checking if necessary*/
int* myNumber = new int[variableSize];
std::cout << "Input the number now: ";
std::cin >> theNumber;
/* use a for loop to put each char into its respective array location;
do not forget to convert char to int... */

Prefer to use an std::vector (the preferred way to store dynamic memory)? Here is that approach:

int variableSize;
char* theNumber;
std::cout << "What is your preferred size? ";
std::cin >> variableSize; /* do some error checking if necessary*/
std::vector<int> myNumber(variableSize);
std::cout << "Input the number now: ";
std::cin >> theNumber;
/* use a for loop to put each char into its respective array location;
do not forget to convert char to int... */

edited Aug 12 '15 at 13:23

answered Aug 12 '15 at 13:17

Don Larynx

685
5
15

Eww. That is not the answer. The answer is to use a `std::string` or `std::vector` and avoid doing manual memory operations – NathanOliver Aug 12 '15 at 13:19
@NathanOliver He wants an array, not a `std::vector`. Read the specs. – Don Larynx Aug 12 '15 at 13:20
@Don there is a flaw here. The number is very large and cannot be stored in the `int theNumber` in the first place. – powersource97 Aug 12 '15 at 13:21
And we should guide him into a better approach. If they asked if they what blade to use to cut off their hand to stop it from itching we shouldn't recommend a blade but instead point them to anti itch powder. – NathanOliver Aug 12 '15 at 13:22
Fixed. @NathanOliver I will edit my answer for this alternate approach. – Don Larynx Aug 12 '15 at 13:24

Taking a Big Integer Input

2 Answers2