Keep creating an infinte loop in my for loop.

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C

Hello, I am creating a program that will display a table with the length and the period of a pendulum. I ask the user for a length and then for an updated/2nd length. I use a for loop to keep displaying the table values. I am allowed to display up to 20 lines at most. I keep getting an infinite loop when I run it. Please help. P.s I am required to create the function and the user can also enter a negative value for the 2nd length, for example -2. Thank you for the help.

What I have tried:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define PI 3.141592

    double pendulum_Period(double length);
int main()
{
    double length;
    double updateLength;
    int i;



    printf("\nThis program displays a table of Pendulum lengths and their respective period.\n"
           "\nPlease enter the length of the pendulum(ft):  ");
    scanf("%lf", &length);

    if( length <= 0 )
    {
        printf("\n\nError: Length must be > 0\n"
               "Please enter the length of the pendulum(ft):   ");
        scanf("%lf", &length);
    }


    printf("\n\nPlease enter the update value for the length(ft):  ");
    scanf("%lf", &updateLength);



     printf("\nLength(ft)\t\t\tPeriod(s)\n"
            "------------------------------------------\n");


    for( i = length; i <= 20; length += updateLength)
    {
        double period = pendulum_Period(length);
        printf("%.6f\t\t\t%.6f\n", length, period);
        printf("------------------------------------------\n");
    }


    return 0;
}


    double pendulum_Period(double length)
    {

        double T;
        double period;
        double gravity;
        gravity = 32.2;
        T = (2 * PI)*(sqrt(length/gravity));
        period = T;

        return period;
    }