So, you can't pass "void" to a function. If you don't want to pass anything to a function, then, don't pass anything to the function.
Your original thought was correct, you do indeed need to pass things to the function. Comments:
Code:
#include <stdio.h>
int choice; /*bad and wrong */
float temp; /*bad and wrong */
float convertF(void); /*incorrect prototype for this function */
float convertC(void); /*incorrect prototype for this function */
int main()
{
for( ; ; ) /* possibly the worst way to do this loop */
{
printf( "Enter a number to make a selection:\n" );
printf( "\n 1. Farenheit to Celsius\n 2. Celsius to Farenheit\n 3. Quit\n" );
scanf( "%d", &choice);
if(choice==1)
{
float result;
printf( "Please enter a decimal Farenheit value (e.g. 32.0)\n" );
scanf( "%f", temp );
result=convertF(void); /*you need to tell convertF about the input*/
printf( "\n %f degrees Farenheit equals %f degrees Celsius \n", temp, result);
continue; /* pointless */
}
if(choice==2)
{
printf( "Please enter a decimal Celsius value (e.g. 10.5)\n");
continue; /* similarly pointless */
}
if(choice==3)
{
printf( "Goodbye!\n");
break;
}
}
return(0);
}
float convertF(void) /*convertF must take an input parameter, presumably of type float, if you want to be able to use "temp" below */
{
float Cel;
Cel=(temp - 32)/1.8;
return(Cel);
}
float convertC(void) /*convertC must take an input parameter, presumably of type float, if you want to be able to use "temp" below */
{
float Far;
Far=(temp * 1.8) + 32;
return(Far);
}