``````/***************************************************
F = m a Program

Write a program that solves for the unknown variable
in the F = m a equation.  The user enters two
variable = value pairs (e.g "m = 10.0 F = 3.5") and
the program writes the unknown variable and its
value ("a = .35" in our example).

In this solution, we first sort our variables so
that var1 is always before var2 in alphabetical
order, and this makes it easier to figure out which
variable we're solving for.
***************************************************/
#include "si204.h"

int main() {
fputs("The F = m a Program\n", stdout);

char var1;
char c;
double val1;
fputs("Enter <var> = <value> : ", stdout);
var1 = readchar(stdin); // the variable
c = readchar(stdin);    // the equals sign
val1 = readnum(stdin);  // the value

char var2;
double val2;
fputs("Enter <var> = <value> : ", stdout);
var2 = readchar(stdin); // the variable
c = readchar(stdin);    // the equals sign
val2 = readnum(stdin);  // the value

// ensure that var1 is alphabetically before var2
if (var1 > var2) {
// swap var1 and var2
char tvar = var1;
var1 = var2;
var2 = tvar;

// swap val1 and val2
double tval = val1;
val1 = val2;
val2 = tval;
}

// Decide which variable is missing & solve
if (var1 == 'F' && var2 == 'a') {
fputs("m = ", stdout);
writenum(val1 / val2, stdout);
fputs("\n", stdout);
} else if (var1 == 'F' && var2 == 'm') {
fputs("a = ", stdout);
writenum(val1 / val2, stdout);
fputs("\n", stdout);
} else if (var1 == 'a' && var2 == 'm') {
fputs("F = ", stdout);
writenum(val1 * val2, stdout);
fputs("\n", stdout);
}

return 0;
}``````