Question

Beginner here.
Why is this an endless loop ?

for (p = 0; p < 5; p += 0.5)
{
    printf("p=%2.2f\n",p);
}

Answer 1

You see an endless loop because your p is of an integral type (e.g. an int). No matter how many times you add 0.5 to an int, it would remain 0, because int truncates double/fp values assigned to it. In other words, it is equivalent to a loop where you add zero on each step.

If you make p a float or a double, your problem would go away.

EDIT (Suggested by Oli Charlesworth’s comment)

It is worth noting that using floats and doubles to control loops is discouraged, because the results are not always as clean as in your example. Changing the step from 0.5 (which is 2 to the negative power of 1) to 0.1 (which is not an integral negative power of 2) would change the results that you see in a rather unexpected way.

If you need to iterate by a non-integer step, you should consider using this simple pattern:

// Loop is controlled by an integer counter
for (int i = 0 ; i != 10 ; i++) {
    // FP value is calculated by multiplying the counter by the intended step:
    double p = i * 0.5;
    // p is between 0 and 4.5, inclusive
}