Trying to get an "adjusted" ratio of two numbers - c++

Question

I would like to do this
I have two numbers, A and B, where it can be A greater or smaller than B. What I want to do:
Compare B with A
If their ratio is "ouside" of a range, then take a random number and add it to "A" - Now check if their new ratio meets the condition
0.95 < ratio < 1.05
if not, try with another random.
My problem is that I ran into infinite loops... this is what I do:
float ratio = A/B;
if (ratio < 0.95 || ratio > 1.05) {
do {
// randomly take a negative or positive number
float random_n = ((float)rand())/RAND_MAX - 0.50;
// get an even smaller step
random_n *= 0.1;
// add or subtract the random number (depending on its sign)
A += random_n;
// form the ratio again
ratio = A / B;
cout << "lets see " << A << " " << B << " " << ratio << endl;
}
while (ratio > 0.95 || ratio < 1.05);
}

Answer 1

The condition should be:
while (ratio <= 0.95 || ratio >= 1.05);
You got the "<" and ">" mixed up. ratio is not between 0.95 and 1.05 if it's less or equal than 0.95 or greater or equal than 1.05.
It's easy to avoid such mistakes if you remember how you negate a conjunction:
NOT (0.95 < ratio < 1.05) === NOT (0.95 < ratio AND ratio < 1.05) ====
[...]
The rule is: change every AND to OR and negate all sub-expressions:
[...] === (NOT (0.95 < ratio) OR NOT (ratio < 1.05)) ====
(0.95 >= ratio OR ratio >= 1.05) === (ratio <= 0.95 OR ratio >= 1.05)

Answer 2

Your do..while loop condition should be
while(ratio <= 0.95 || ratio =>1.05)

Answer 3

Well the problem is you're increasing A and never changing B
and still expect ratio to be in (0.95,1.05) . This depends on initial value of A and B to break the loop.
Try:
B=rand()%RAND_MAX;
do {
A=rand()%RAND_MAX;
ratio = A/B;
cout<<"lets see "<< A <<" "<< B <<" "<<ratio<<endl;
}while(ratio <0.95 || ratio>1.05);
Then can divide A and B by power of ten as per your needs.
Also use srand(time(0)) for different random values on successive runs