Thread: don't know where to start

I am writing a unit conversion program for a final project in my C-programming class. One of the requirements I have is that I have to have it keep track of only the 20 most recent inputs. I am trying to have it display like this:

Starting Unit Ending Unit Filename
mm ft mm.txt


I am not sure of how to get this started. I have some code written that converts the units, and writes the results to a file, but I don't know where to start for this part. Any help on this would be great.
Here's what I have so far:

Code:

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

void menu(void);
void length(void);
void mm(void); //millimeter function


typedef struct{
        return_end[5];
        return_file[20];
        }return_info;



int main(int argc, char *argv[])
{
	char check[5];
	printf("Welcome to Casey's Conversion Program\n");

	/* loop to rerun program */
	do{
		menu();
		printf("Do you want to continue (y for yes)? ");
		scanf("%s", check);
		
	}while(strcmp("y", check) == 0);
             

	system("PAUSE");	
	return 0;
}

/* Menu Function */
void menu(){
	int start;

	printf("Enter:\n1 for Length Conversion\n");
	printf("2 for Area Conversion\n");
	printf("3 for Volume Conversion\n");
	printf("4 for Time Conversion\n");
	printf("5 for Angle Conversion\n");
	printf("6 for Speed Conversion\n");
	printf("7 for Acceleration Conversion\n");
	printf("8 for Mass Conversion\n");
	printf("9 for Force Conversion\n");
	printf("10 for Pressure Conversion\n");
	printf("11 for Energy Conversion\n");
	printf("12 for Power Conversion\n");
	scanf("%d", &start);

	switch(start){
		case 1:
		  length();
          break;

		default:
			printf("Not valid entry\n");
			break;
	}
}     

/* Length Function */
char length(){
	int start;
	char return_length[5];

	printf("Enter starting unit: \n");
	printf("1 for millimeters\n");
	printf("2 for centimeters\n");
	printf("3 for meters\n");
	printf("4 for kilometers\n");
	printf("5 for inches\n");
	printf("6 for feet\n");
	printf("7 for yards\n");
	printf("8 for miles\n");
	scanf("%d", &start);

	switch(start){
     case 1:
            mm();
	    return_length = mm;
	    break;
     case 2:
            cm();
            return_length = cm;
            break;
     case 3:
            m();
            return_length = m;
            break;
     case 4:
            km();
            return_length = km;
            break;
     case 5:
            in();
            return_length = in;
            break;
     case 6:
            ft();
            return_length = ft;
            break;
     case 7:
            yd();
            return_length = yd;
            break;
     case 8:
            mi();
            return_length = mi;
            break;

		default:
			printf("Not valid entry\n");
			return_length = n/a;
			break;
	}
	return(return_length);
}  

/* mm Function */
return_info mm(void){
	int end;
	double mm;
	double ans;
	FILE *mmptr;
	return_info mm_info[30];
	
mmptr = fopen("mm.txt", "a");
	
	printf("Enter the number of millimeters: ");
	scanf("%lf", &mm);
	
	printf("Enter ending unit: \n");
	printf("1 for millimeters\n");
	printf("2 for centimeters\n");
	printf("3 for meters\n");
	printf("4 for kilometers\n");
	printf("5 for inches\n");
	printf("6 for feet\n");
	printf("7 for yards\n");
	printf("8 for miles\n");
	scanf("%d", &end);
    
	switch(end){
		case 1:
			ans = mm;
			printf("%lf millimeters equals %lf millimeters\n", mm, ans);
			fprintf(mmptr,"%lf mm = %lf mm\n", mm, ans);
			mm_info[] = {"mm","mm.txt"};
			break;

		case 2:
			ans = mm/10;
			printf("%lf millimeters equals %lf centimeters\n", mm, ans);
			fprintf(mmptr,"%lf mm = %lf cm\n", mm, ans);
			mm_info[] = {"cm","mm.txt"};
			break;

		case 3:
			ans = mm/1000;
			printf("%lf millimeters equals %lf meters\n", mm, ans);
			fprintf(mmptr,"%lf mm = %lf m\n", mm, ans);
			mm_info[] = {"m","mm.txt"};
			break;

		case 4:
			ans = mm/1000000;
			printf("%lf millimeters equals %lf kilometers\n", mm, ans);
			fprintf(mmptr,"%lf mm = %lf km\n", mm, ans);
			mm_info[] = {"km","mm.txt"};
			break;

		case 5:
			ans = mm/25.4;
			printf("%lf millimeters equals %lf inches\n", mm, ans);
			fprintf(mmptr,"%lf mm = %lf in\n", mm, ans);
			mm_info[] = {"in","mm.txt"};
			break;

		case 6:
			ans = mm/304.8;
			printf("%lf millimeters equals %lf feet\n", mm, ans);
			fprintf(mmptr,"%lf mm = %lf ft\n", mm, ans);
			mm_info[] = {"mm","ft.txt"};
			break;
			
		case 7:
			ans = mm/1914.4;
			printf("%lf millimeters equals %lf yards\n", mm, ans);
			fprintf(mmptr,"%lf mm = %lf yd\n", mm, ans);
			mm_info[] = {"yd","mm.txt"};
			break;

		case 8:
			ans = mm/1609000;
			printf("%lf millimeters equals %lf miles\n", mm, ans);
			fprintf(mmptr,"%lf mm = %lf mi\n", mm, ans);
			mm_info[] = {"mi","mm.txt"};
			break;
     
		default:
			printf("Not valid entry\n");
	        mm_info[] = {"N/A","N/A"};
			break;
	}
	fclose(mmptr);
}

I think I almost figured it out. I am writing the information I need to a file in this format:

i(counter) starting unit ending unit filename


I then read it in main and only display the first 20.
When I wrote the code, I got an infinite loop.

Code:

Code:

   for (status = fscanf(cntr, "%d %s %s %s",&i, start_unit,end_unit, filename); 
            status != EOF;
         status = fscanf(cntr, "%d %s %s %s",&i, start_unit,end_unit, filename))
         {
                while(i<20){
                            printf("%d\t%s\t\t%s\t\t%s\n",i,start_unit,end_unit,filename);
                            }
                            }

I am trying to get it to display only the 20 most recent inputs. It is for a unit conversion program I am writing for a final project. I want it to display like this:
_________Starting Unit___________Ending Unit____________Filename

1. _________mm___________________in_________________m m.txt

the while loop was used to only display the top 20 lines in the file.