Is there a way to compare multiple strings in C? [ I input them through fgets() ]

int Distance() {
    char from[40], to[40];
    double val;
    double result;

    printf("Enter what you want to convert from: ");
    fflush(stdin);
    fgets(from, 40, stdin);

    printf("Enter what you want to convert to: \n");
    fflush(stdin);
    fgets(to, 40, stdin);

    printf("Enter the numerical value of conversion: \n");
    scanf("%lf", &val);

    (strcmp(from, "cm") == 0 && strcmp(to, "mm") == 0) ? printf("Answer is %lf MilliMeters", val / 10) : 
    (strcmp(from, "mm") == 0 && strcmp(to, "cm") == 0) ? printf("Answer is %lf CentiMeters", val * 10) :
    (strcmp(from, "cm") == 0 && strcmp(to, "km") == 0) ? printf("Answer if %lf KiloMeters", val * 100000) : 
    (strcmp(from, "km") == 0 && strcmp(to, "cm") == 0) ? printf("Answer is %lf CentiMeters", val / 100000) : 
    (strcmp(from, "mm") == 0 && strcmp(to, "km") == 0) ? printf("Anser is %lf KiloMeters", val  * 1000000) : 
    (strcmp(from, "km") == 0 && strcmp(to, "mm") == 0) ? printf("Answer is %lf  MilliMeters", val / 1000000) : 
    printf("Please enter valid conversion units");
}

I am trying to make a unit converter. So I have made different functions for different conversions - like int distance(), int time() and so on.

In my function to calculate distances, I used fgets() to get multi-char inputs like 'cm', 'mm, etc. What i am trying to achieve is If value of fgets in char from is "any string" and fgets in char to is "any other string": print result whose formula is -----(something)

It doesn't compute the result and directly jumps to the final printf("Please enter valid conversion units")

I am still learning at a beginner level, it could be a minor mistake, Please help.

There are multiple issues in the code:

• fgets() stores the trailing newline at the end of the target array, so all comparisons with unit strings will fail.
• you should use an intermediary unit to avoid having to test all combinations.

Here is a modified version:

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

struct unit {
    double factor;
    const char *abbrev, *singular, *plural;
} const units[] = {
    { 1, "mm", "millimeter", "millimeters" },
    { 10, "cm", "centimeter", "centimeters" },
    { 100, "dm", "decimeter", "decimeters" },
    { 1000, "m", "meter", "meters" },
    { 10000, "dam", "decameter", "decameters" },
    { 100000, "hm", "hectometer", "hectometers" },
    { 1000000, "km", "kilometer", "kilometers" },
    { 25.4, "in", "inches", "inches" },
    { 304.8, "ft", "foot", "feet" },
    { 914.4, "yd", "yards", "yards" },
    { 201168, "fur", "furlong", "furlongs" },
    { 1609344, "mi", "mile", "miles" },
};

int get_unit(void) {
    char buf[40];
    while (fgets(buf, sizeof buf, stdin)) {
        buf[strcspn(buf, "\n")] = '\0';   // remove the newline if any
        for (int i = 0, n = sizeof(units) / sizeof(*units); i < n; i++) {
            if (!strcmp(buf, units[i].abbrev)
            ||  !strcmp(buf, units[i].singular)
            ||  !strcmp(buf, units[i].plural)) {
                return i;
            }
        }
        printf("unknown unit\n");
    }
    printf("unexpected end of file, aborting\n");
    exit(1);
}

int main(void) {
    int from, to;
    double val;

    printf("Enter what you want to convert from: ");
    from = get_unit();

    printf("Enter what you want to convert to: ");
    to = get_unit();

    printf("Enter the numerical value of conversion: ");
    if (scanf("%lf", &val) == 1) {
        double result = val * units[from].factor / units[to].factor;
        printf("Answer is %g %s\n", result, result == 1 ? units[to].singular : units[to].plural);
        return 0;
    }
    return 1;
}