Code

				
/*
 * Will Bergen - 2019
 * Pendulum swing period calculator
 * Answer to the following (likely textbook) question:
 * 
 * Input: Prompt user to input the data from the screen in a manner .. 
 * 	- Assume list of pairs (x, N), (N, x) where N is given and x is to be found
 * 	- Pairs are (Length, Period)
 * Output: Print a table of pendulum lengths and swing times like following:
 * 
 * Length (m)		Period (sec)
 * 0.5				-
 * 1.0				-
 * -				10.0
 * -				20.0
 * 0.32				-  				
 * 
 * 
 * NB. pi is available from math.h as M_PI
 *
 * Forumla:
 * T (period) = 2 * pi * sqrt(L (length) / gravity)
 *
 * Solved for L:
 * L = ( (T / (2 * pi) )^2 ) / GRAVITY
 *
 *
 * Quick test via heredoc:
 
  	./a.out << EOF       
		0.5,X
		1.0,X
		X,10.0
		X,20.0
		0.32,X
		EOF

 *
 * Compile against math:
 * gcc swing_table.c -lm -o swing_table
 * */


#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

#define BUF_SIZE 256
#define NUM_PAIRS 5
#define GRAVITY 9.81


double solveForTime(double length)
{
	// printf("Solving for Time\n");
	return 2 * M_PI * sqrt(length / GRAVITY);
}

double solveForLength(double time)
{
	// printf("Solving for Period\n");
	return pow((time / 2 * M_PI), 2) / GRAVITY;
}

int main(void) {

	// Read pairs from stdin
	char buf[BUF_SIZE];
	double lens[NUM_PAIRS];
	double periods[NUM_PAIRS];
	char *ptr;


	// Print instructions:
	printf("Please enter %i pairs of the form:\n", NUM_PAIRS);
	printf("<Length of pendulum>,<Swing Period>\n");
	printf("Make one an X to solve for it\n");
	printf("eg. 0.5,X or X,10 - No Spaces!\n\n");

	// Get Pairs:
	int i = 0;
	for (i; i < NUM_PAIRS; i++)
	{
		printf("Enter pair %i: ", i+1);
		fgets(buf, BUF_SIZE, stdin);
		printf("Pair %i: %s", i+1, buf);
		double len = strtod(buf,&ptr);

		/* Here, ptr points to the next char in buf that's NOT number
		* so if input is '10,50' then ptr -> ','
		* adding 1 to ptr trivially gets around ',', but is very breakable
		* lets instead consume until we've explictly eaten a ','
		* -- This is escessive without other bounds checking
		* -- If we assume no spaces and a single comma, we just increment once...
		* */

		char *next_char = ptr;

		// Check for ptr already pointing at ',':
		if (strncmp(next_char, ",", 1) == 0)
		{
			next_char = next_char + sizeof(char);
		} else {
			// If it doesn't already point at a comma, consume to find:
			while (strncmp(next_char, ",", 1) != 0)
			{
				next_char = next_char + sizeof(char);
			}
			// Comma found, consume:
			next_char = next_char + sizeof(char);
		}
		
		// Convert the string again from updated pointer:
		double period = strtod(next_char,&ptr);

		// One of the vars will be 0.  Solve and append to result arrays:
		// Time is unknown:
		if (period == 0.0)
		{
			periods[i] = solveForTime(len);
			lens[i] = len;

		}

		// Length is unknown:
		if (len == 0.0)
		{
			lens[i] = solveForLength(period);
			periods[i] = period;
		}

		// Reset buf:
		memset(buf, 0, sizeof(buf));
		
	}

	// Display output table with red headings:
	printf("\n\033[1;31mLength (m)\tPeriod (sec)\033[0m\n");
	i = 0;
	for (i; i < NUM_PAIRS; ++i)
	{
		printf("%f\t%f\n", lens[i], periods[i]);
	}


	return 0;
}