Scanf won't notice '\n' char in a program loading only numbers - c

Question

I have been searching for a few days and I have found only one solution that didn't look perfect to me. Our teacher asked us to create a function that would calculate total lenght of distances in between points provided by user.
My idea was to write code this way, using an array of specific type.
The issue is that, I can't come up with any ideas for how to solve the issue with input: He asked us to make the program end once the user doesn't type anything, so I take it for enter - \n sign.
I could use fgets to get the first variable but:
First, I feel like I don't know any other way beside an array for keeping a long decimal number(in a form of a char array with elements making up the number), that the user could put on the input. I don't know if his script doesn't put some "rofl" number in there.
Second, in this case I think that stripping that array off one X would totally break the total structure of this program. I would rather take both X and Y and accept them as char type, but then the function like atof would probably understand only the X and would stop working after the \n sign.
So Y would be left not given. The accepted input numbers should be of double type. Like:
2 2
3 3
-2 4.5
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<math.h>
double lenght(struct point *coordinates, int n);
struct point {
double x;
double y;
};
int main()
{
double x,y,TwiceAsBig=3;
int i=0,l=0;
struct point *coordinates;
coordinates = (struct point*)malloc(sizeof(*coordinates)*3);
//allocation of memory for pointtype array with a pointer
while(scanf("%lg %lg",&x,&y)==2)
{
coordinates[i].x=x;
coordinates[i].y=y;
i++;
if(i==TwiceAsBig)
{
coordinates = (struct point*)realloc(coordinates, 2*i*sizeof(*coordinates));
TwiceAsBig=2*TwiceAsBig;
}
}
printf("\n");
for(l;l<i;l++)
{
printf("%lg %lg\n", coordinates[l].x,coordinates[l].y);
}
//checking on the array if the values were loaded correctly
printf("%lg",lenght(coordinates,i));
}
//function for dinstace in between the points
double lenght(struct point*coordinates,int n)
{
int l=0;
for(l;l<n;l++)
{
printf("%lg %lg\n", coordinates[l].x,coordinates[l].y);
}
int pair=0;
double lenght,distance;
for(int AoP;AoP<n-1;AoP++)
{
distance=sqrt(pow(coordinates[pair+1].x-coordinates[pair].x,2)+pow(coordinates[pair+1].y-coordinates[pair].y,2));
pair++;
printf("%lg: ", distance);
lenght=lenght+distance;
}
return lenght;
}

Answer 1

As for your problem, using fgets to read a whole line, and the possibly use sscanf to parse out the two numbers might work.
The problem with using only scanf is that all the numeric format specifiers reads and skips leading white-space automatically, and newline is a white-space character. That means your scanf call in the loop condition will wait until there's some actual non-space characters being input (followed by a newline of course, which leads to the cycle starting over again).

Answer 2

What about using scanf("%[^\n]%*c", test); to read a full string.
Then parsing the result using sscanf?
Something like this:
char* userinput = (char*) malloc(sizeof(char) * 100);
scanf("%[^\n]%*c", userinput);
double a, b;
sscanf(userinput, "%lg %lg", &a, &b);
printf("sum %lg\n", a+b);
With input "-5.5 3.2" the code produces "sum -2.3".
%[^\n]%*c is a "scanset" which tells scanf to read everything excluding '\n' and once it reaches a newline it reads the newline character and disregards it.
You could even use scansets to check the input to some degree by specifying which type of characters you expect to read.
%[0-9 .\\-] // would read digits from 0-9, 'space', '.' and '-'