how to divide two variables till get a quotient of 1 inside a loop in c++

Question

I made a program which divides two variables suppose a & b, reads from a text file, but this on a loop coz I wanted to divide these variables 'till it got a quotient of 1, this only applicable for a number that is divisible, and if not divisible it would indicate/prints "Invalid". So far this all Ive got for I am already stuck. Hope someone could give me an advice how to do this.

inside my textfile mytxt.txt. It has one space between two numbers every lines.

heres what ive got so far in my code

  #include <iostream>
  #include <fstream>
  int main() {

   std::ifstream file("mytxt.txt");
    if ( file.eof()) return 0;
     int a, b, cnt=0;

      while (!file.eof()) {
            file>>a>>b;
          loop: 
                 std::cout<<a << " ";
                 a/=b;
          if(a%b == cnt) 
                 goto loop;
                 std::cout<<"\n";
                    }
          file.close();
          system("pause");
}

output of this is

27 9 3
40
1000 200 40
625 125 25 5
Press any key to continue . . .

but it should be like this

27 9 3 1
Invalid
Invalid
625 125 25 5 1

it should have 3/3 to have 1 in the last. What should I have in my code to do this and insert a condition that if its not divisible, it prints "Invalid". thanks in advance. pls excuse my grammar.

Try using a debugger like gdb to see what happens in your code. It's painful, but a very good exercise. — arne, Sep 17 '13 at 10:14
+1. Looks like homework, but still: it's got input, the right amount of code for the problem, actual and expected output. — MSalters, Sep 17 '13 at 10:28
1000 can be divided by 5, so why should it be invalid? (rephrase your question, fix indentation). And how should your code write invalid, when you have no line doing this? — zoska, Sep 17 '13 at 11:02
@zoska, its invalid coz when 40/5=8 den 8/5 would have a remainder of 3 which is not a divisible by 5. — kersey, Sep 17 '13 at 23:45

score 0 · Answer 1 · answered Sep 17 '13 at 10:16

You are printing a before you have divided it... And you don't identify when it's "invalid". If you want to print only "Invalid", I would think that you need to store the values you got as divisors, and then print them only when the value gets to 1.

And you should use while(file>>a>>b) instead of while(!file.eof()), as you will run the loop once more than required if you use the latter.

score 0 · Answer 2 · answered Sep 17 '13 at 10:17

The problem is that you divide a and THEN you check if it is still divisible...so, for example, you print "3", then a := 3/3=1 (which is not divisible by 3) and now your loop finishes without having printed "1"...

However, I'd suggest you to use another 'while' loop instead 'goto'.

score 0 · Answer 3 · answered Sep 17 '13 at 10:26

From your description:

divide these variables 'till it got a quotient of 1

That means looping until a == 1, not while a%b == 0.

if not divisible it would indicate/prints "Invalid"

So check for divisibility with

if (a%b != 0) {
    // it's invalid
}

I would recommend using looping constructs like for and while, rather than hand-crafting your own spaghetti with goto, and also indenting the code to match its structure, to make the code easier for a human to follow.

score 0 · Answer 4 · answered Sep 17 '13 at 10:32

#include <iostream>
#include <fstream>

int main()
{
    std::ifstream file("Input.txt");
    if (file.eof()) return 0;
    int a, b, c, cnt=0;
    while (!file.eof())
    {
        file>>a>>b;
        c = a;
        while(a > 1)
        {
            if (a % b) break;
            a /= b;
        }
        if (a == 1)
        {
            while(c >= 1)
            {
                std::cout << c << " ";
                c /= b;
            }
        }
        else
        {
            std::cout << "Invalid";
        }
        std::cout << std::endl;
    }
    file.close();
}

We use the first while loop to find if it is possible to reach 1. If it is not possible we print Invalid or we iterate till we get 1 and print all the intermediate results.

Output

27 9 3 1 
Invalid
Invalid
625 125 25 5 1

score 0 · Answer 5 · edited May 23 '17 at 11:49

I think this code would do what you want (look at the notes below it for details):

#include <iostream>
#include <fstream>
#include <math.h>

int main() {
    std::ifstream file("mytxt.txt");
    if (file.eof())
        return 0;

    int a, b;
    double tmp;

    while ((file >> a >> b)) {
        if( a == 0 || b == 0 )
            continue;                    

        tmp = log(a) / log(b);
        if (tmp != floor(tmp))
            std::cout << "Invalid";
        else {
            do {
                std::cout << a << " ";
            }while((a/=b) != 1);
            std::cout << "1";
        }
        std::cout << "\n";
    }
    file.close();
    system("pause");
}

First: if you want to check for invalid using the division loop, then you would have to save all values and then check if it reached 1 print them all, otherwise print Invalid.

Second: 'First' is considered a BAD approach, you could use log function instead, check if the logarithm of the a to the base b gives you a non-algebraic integer number then print Invalid. Otherwise go to your division loop and print your number sequence, note that printing 1 is an already-known info that could be omitted.

Third: It worth to add a check on the reading, for example if your file has a new line at its end, you won't exit the loop. So, it is recommended that you put an IO check on the reading stream if(!(file >> a >> b)); you even could make it the main loop check. Also, it is worth to add logical checks on a and b to avoid special values like 0.

Fourth: Generally, using goto statement is considered harmful except in few cases. So, it is better to avoid it as you can. Here are nice articles and discussions about it:

how to divide two variables till get a quotient of 1 inside a loop in c++

5 Answers5