How to use strtol to extract numbers from user input - c

Question

I'm trying to build a Linear Equation calculator in C. My source code is below. I know this is a coding question but to explain my issue, I need to explain a little math here. I have the user input the points in the format (x,y) and want to convert the equation into one of the following ways:
Slope-intercept -> y = mx + b
Standard form -> ax + by = c
Point-slope form -> y - y1 = m(x - x1)
y is the point on the y-axis of the coordinate plane, x is the point on the x-axis on the coordinate plane, m is the slope of a line, b is the y-intercept of the first equation, and a, b, and c are variables in the second equation.
My issue is that I'm having trouble finding a way to extract numbers from a coordinate. In my source code, I have a 2 dimensional array that holds 2 strings with a certain size. I don't know if i should use strtol() or something else. For example, take the point (34,89). I want to extract the 34 and 89 and plug it in to the values for finding the slope (which would be (y2 - y1) divided by (x2 - x1). So it would be (89 - y) / (34 - x). As for the second point, I want to be able to do the same thing. I would like to avoid making 4 variables for each point if possible. I would also like to avoid asking the user to enter the x value of the first point then the y value of the first point etc. etc. I want to be able to ask the user for a point, including both x and y values. Near the end of the code is unfinshed, so if i have errors, I'll figure them out as I go. The problem is near the beginning and middle.
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#define MAXC 56
//void convert_equation(); Function is not defined or used. Uncomment when ready to define
void equation_format();
void print_dots(int n);
char points[2][MAXC];
int main(void) {
printf("\033[0;32mInitializing Linear Equation Calculator ");
fflush(stdout);
print_dots (3);
putchar('\n');
printf("\nEnter the 1st point on the line, format (x,y)\n>>> ");
fflush(stdout);
fgets(points[0], sizeof points[0], stdin);
/** trim \n from end of string read with fgets()
*
* use: stringvar[strcspn (stringvar, "\n")] = 0;
*
* strcspn returns the length of stringvar that does NOT include \n
* this allows you to simply overwrite the \n with \0 (or plain old 0)
*/
points[0][strcspn(points[0], "\n")] = 0;
printf("Enter the 2nd point on the line, format (x,y)\n>>> ");
fflush(stdout);
fgets(points[1], sizeof points[1], stdin);
points[1][strcspn(points[1], "\n")] = 0;
equation_format();
return 0;
}
void equation_format() {
char option;
while (1) {
printf("\n\nWhat format would you like the equation in?\n1. Slope-Intercept\n2. Standard Form\n3. Point-slope\n>>> ");
fgets(&option, 2, stdin);
switch(option) {
case(1):{
int slope, yintercept;
break;
}
case(2):{
break;
}
case(3):{
break;
}
default: {
printf("Invalid input. Try again");
print_dots(3);
}
}
break;
}
}
void print_dots(int n) {
if (n < 0)
n = -n;
while (n--) {
sleep(1);
putchar('.');
fflush(stdout);
}
}
This is a little hard to explain so if I'm not being clear, please tell me. Thank you!

Answer 1

I want to be able to ask the user for a point, including both x and y values.
After fgets(), parse the string with a helper function, tailored to your needs.
Robust code also detects when the input is ill-formed.
// return 2 on success
// return -1 on failure.
int parse_2_int(const char *s, long *x, long *y) {
errno = 0;
char *endptr;
*x = strtol(s, &endptr, 0);
if (s == endptr || errno) return -1; // no conversion or overflow
s = endptr;
while (isspace((unsigned char) *s) { // skip spaces
s++;
}
if (*s != ',') return -1; // Missing ,
errno = 0;
*y = strtol(s, &endptr, 0);
if (s == endptr || errno) return -1;
s = endptr;
while (isspace((unsigned char) *s) { // skip spaces
s++;
}
if (*s != '\0') return -1; // junk at the end
return 2;
}
Usage
long x,y;
if (fgets(points[1], sizeof points[1], stdin)) {
if (parse_2_int(points[1], &x, &y) == 2) {
printf("Success %ld, %ld\n", x,y);
} else
printf("Trouble with <%s>\n", points[1]);
}
}

Answer 2

"My issue is that I'm having trouble finding a way to extract numbers from a coordinate.".
There are several way to convert a string to a number, strtol() and atoi() are either able to do this. Regarding user input, it is recommended that you keep the instructions to any user input as simple as possible. For example, rather than instructing a user to input "(34,89)", consider leaving the parenthesis off: "\nEnter the 1st point on the line, format: x,y\n>>> "
With that said, after user inputs are entered as say "56,78" and "3,7" and read into the string variables point1[0] and point1[1] respectively, it is a simple process to parse these into int (or long) members of a point struct using strtok():
typedef struct {
int x;
int y;
}POINT;
POINT p[2] = {0};
First, parse the string into char *, and convert:
char *tok = NULL;
tok = strtok(point[0], ",");
if(tok)
{
p[0].x = atoi(tok);
tok = strtok(NULL, ",");
if(tok)
{
p[0].y = atoi(tok);
)
And similar for p[1]. Then you can use the member pairs for x and y as arguments in the equation.
If strtol() is favored over atoi(), then adjust to replace:
p[0].x = atoi(tok);
with
p[0].x = strtol(tok, &ptr, 10);
As shown in the following:
typedef struct {
long x;
long y;
}POINT;
POINT p[2] = {0};
char *ptr = NULL;
p[0].x = strtol(tok, &ptr, 10);
//test return value for good results, and handle error if necessary
if(*ptr)
printf("Unable to convert '%s'.", tok);
else
...