C program to determine temperature conversions not working - c

Question

I'm trying to create a program that asks the user what conversion the user wants to do, then scans for the number they want to convert, then does the conversion and spits it out.
Here are the instructions that include desired outputs:
Building on assignment one, create a program to with the choice of convert Celsius to Fahrenheit or Fahrenheit to Celsius using “ifelse” statement to chose which conversion you want to use. Be sure to include the correct header in your .C file. Use the K and R formatting. Name your file “first Initial last name-assingment2.c” ex: “rgalus-assingment2.c” Below is what I expect the program output to look like:
Enter 1 to convert Celsius to Fahrenheit
Enter 2 to convert Fahrenheit to Celsius
Enter 0 to exit
1
10
10 degrees Celsius is equal to 50 degrees Fahrenheit
Enter 1 to convert Celsius to Fahrenheit
Enter 2 to convert Fahrenheit to Celsius
Enter 0 to exit
2
32
32 degrees Fahrenheit equals 0 degrees Celsius
Enter 1 to convert Celsius to Fahrenheit
Enter 2 to convert Fahrenheit to Celsius
Enter 0 to exit
0
“exit the program”
My issue is that while it will scan my choice, the program ends before I can enter a number to convert.
Below is my code:
#include <stdio.h>
int main(void) {
double userConversionChoice;
double CelsiusToFahrenheitResult;
double FahrenheitToCelsiusResult;
double userConversionNumber;
printf("Enter 1 to convert Celsius to Fahrenheit\n");
printf("Enter 2 to convert Fahrenheit to Celsius\n");
printf("Enter 0 to exit\n");
scanf("%d", userConversionChoice);
if (userConversionChoice == 1) {
scanf("%d", userConversionNumber);
CelsiusToFahrenheitResult = (1.8 * userConversionNumber) + 32.0;
printf("%d degrees Celsius equals %d degrees Fahrenheit", userConversionNumber, CelsiusToFahrenheitResult);
}
else {
if (userConversionChoice == 2) {
scanf("%d", userConversionNumber);
FahrenheitToCelsiusResult = (userConversionNumber - 32.0) / 1.8;
printf("%d degrees Fahrenheit equals %d degrees Celsius", userConversionNumber, FahrenheitToCelsiusResult);
}
else {
printf("exit the program");
}
}
return(0);
}

Answer 1

The immediate issues are:
scanf takes pointers to variables (&x), not variables. That's because a normal function call f(x) cannot modify x (arguments are passed by value).
The %d conversion specifier reads an integer (and expects a corresponding int * argument). Your variables are of type double, so either you need to change them to int or use %lf.
Similarly, printf %d expects an int, not a double. The format specifier for double is %f.
You should check the return value when you use scanf to make sure a value was read successfully. scanf returns the number of input items assigned.
Thus:
int userConversionChoice;
if (scanf("%d", &userConversionChoice) != 1) {
fprintf(stderr, "failed to read input\n");
return EXIT_FAILURE; // EXIT_FAILURE is in <stdlib.h>
}
double userConversionNumber;
if (scanf("%lf", &userConversionNumber) != 1) {
printf("%.2f degrees Celsius equals %.2f degrees Fahrenheit", userConversionNumber, CelsiusToFahrenheitResult);
In the long term, I recommend avoiding scanf for user input altogether. It is hard to use correctly, it makes for confusing user interfaces (e.g. if a user just hits enter, your program will sit there seemingly frozen, but actually still waiting for input in scanf), and it's hard to recover from errors.
For more intuitive behavior, read whole lines of input (using e.g. fgets) and convert them afterwards (using e.g. sscanf or strtod / strtol):
char buf[200];
...
printf("enter a number: ");
fflush(stdout);
if (!fgets(buf, sizeof buf, stdin)) {
return 0; // end-of-input was reached
}
double num = strtod(buf, NULL); // convert input to number

Answer 2

userConversionNumber is a double. You are reading into it using %d which is the conversion specifier for integers.
And you are not using the & (address of) operator when reading into it.
scanf("%d", userConversionChoice);
So define userConversionChoice as an int and read the input this way:
scanf("%d", &userConversionChoice);
Behaviour of scanf:
The scanf function returns the value of the macro EOF if an input failure occurs before the first conversion (if any) has completed. Otherwise, the function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.
So it is good practice to check the return value of scanf before further processing.

Answer 3

Your use of scanf (and printf) is incorrect.
The format string must match the type of parameter you are using. You can get away with this on printf() sometimes, but not scanf().
For scanf(), you must also pass the address-of (i.e.: a pointer to) the variable you wish to store into.
#include <stdio.h>
int main(void)
{
int userConversionChoice;
double CelsiusToFahrenheitResult;
double FahrenheitToCelsiusResult;
double userConversionNumber;
printf("Enter 1 to convert Celsius to Fahrenheit\n");
printf("Enter 2 to convert Fahrenheit to Celsius\n");
printf("Enter 0 to exit\n");
printf("Choice> ");
scanf("%d", &userConversionChoice);
if (userConversionChoice == 1)
{
printf("Temperature> ");
scanf("%lf", &userConversionNumber);
CelsiusToFahrenheitResult = (1.8 * userConversionNumber) + 32.0;
printf("%0.2lf degrees Celsius equals %0.2lf degrees Fahrenheit", userConversionNumber, CelsiusToFahrenheitResult);
}
else if (userConversionChoice == 2)
{
printf("Temperature> ");
scanf("%lf", &userConversionNumber);
FahrenheitToCelsiusResult = (userConversionNumber - 32.0) / 1.8;
printf("%0.2lf degrees Fahrenheit equals %0.2lf degrees Celsius", userConversionNumber, FahrenheitToCelsiusResult);
}
else
{
printf("exit the program");
}
return(0);
}
Obviously there's some room here for optimisation, since there's two lots of code inputting the temperature.