Possible infinite loop

Question

I think my code has an infinite loop. Can someone tell me where I went wrong? The code is supposed to find the number of valid numbers, with a valid number being a number without a digit repeating. For example, 1212 would be a non-valid number because 1 and 2 repeated.

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
int a; int b; int count_validNums = 1; int digit; int last_digit; bool is_valid = true;
vector <int> num_list; 
cout << "Enter numbers 0 < a <= b < = 10000: ";
cin >> a >> b;

// Checks for invalid input
if (a < 0 || b < 0 || a > 10000 || b > 10000) {
    cout << "Invalid input";
    return 1;
}

// Checks every number from the range [a,b]
for (int i = a; i <= b; i++){
    last_digit = i % 10;
    num_list.push_back(last_digit); 
    i = i / 10;
    while (i != 0){
        digit = i % 10;
        if (find(num_list.begin(), num_list.end(), digit) != num_list.end()){
            is_valid = false;
            }
        num_list.push_back(digit);
        i = i / 10;
    }
    if (is_valid) count_validNums++;
}

cout << "They are " << count_validNums << " valid numbers between" << a << " and " << b << endl;
}

And when you used your debugger to run your program, what did you see? This is what a debugger is for. If you don't know how to use a debugger this is a good opportunity to learn how to use it to run your program one line at a time, monitor all variables and their values as they change, and analyse your program's logical execution flow. Knowing how to use a debugger is a required skill for every C++ developer, no exceptions. With your debugger's help you should be able to quickly find all bugs in this and all future programs you write, without having to ask anyone for help. — Sam Varshavchik
– Sam Varshavchik, Commented Jan 26, 2020 at 3:19
Your for loop always ends with i equal to 0 and b always stays the same, thus i will never be greater than b, and your loop never ends. — Jesse
– Jesse, Commented Jan 26, 2020 at 3:30

Igor Tandetnik · Accepted Answer · 2020-01-26 03:30:04Z

1

The inner while loop terminates when i == 0. Then the outer for loop increments it (so i == 1), then the inner loop reduces it to zero again. Then the other loop increments it, then ...

answered Jan 26, 2020 at 3:30

Igor Tandetnik

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Comments

halfer · Accepted Answer · 2020-01-26 12:57:53Z

What is happening to cause the infinite loop is that you are constantly reducing the int i back down to 0. Consider these highlights:

`for(int i = a; i <= b; i++){
    //stuff
    while(i != 0){ //<--this forces i down to 0
        //more stuff
        i = i / 10;
    }
    //final stuff
}`

i here is all one variable, so any changes you make to it anywhere will affect it everywhere else it exists! Instead, you can try saying something like int temp = i; and then perform your operations on temp so that i remains independent, but because your for-loop terminates when i <= b and you are constantly resetting i to 0, it will never reach b.

Also, I noticed that in your check for valid numbers you verify that 0 < a,b < 10000, but later in your for-loop you seem to make the assumption that a <= b will be true. Unfortunately, your test does not ensure this, so the for-loop will immediately terminate for inputs where b < a is true (which your program currently allows) and your program will report answers that are likely incorrect. The same is true when I enter letters as input instead of numbers. You might want to revisit that portion of code.

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