Using switch statements to call functions - c

Question

I'm trying to write a program in C that uses switch statements to decide which called function to use to convert various values. My instructions were as follows:
"Create a program to convert Fahrenheit to Celsius, Celsius to Fahrenheit, inches to centimeter, and centimeters to inches. Put your choices into a switch statement so they will appear on the screen like below:
Select from the menu below to covert temperature or linear menus:
1 Convert Fahrenheit to Celsius
2 Convert Celsius to Fahrenheit
3 Convert Inches to Centimes
4 Convert Centimeters to Inches
5 Exit the program
Enter your selection:
ake each of the conversion routines a function that you will call from the main program. Each of the selections should be a case in a switch statement. Be sure to comment all your code according to the coding standards.
When I run the program and enter the choice followed by the value to convert, a weird number shows up followed by
ProgramExiting.ProgramExiting.
Here is my code:
#include <stdio.h>
void FahrenheitToCelsiusConversion (double conversionValue) {
double output;
output = conversionValue / (9.0 / 5.0) - 32;
printf("%d", output);
}
void CelsiusToFahrenheitConversion (double conversionValue) {
double output;
output = conversionValue * (9.0 / 5.0) + 32;
printf("%d", output);
}
void InchesToCentimetersConversion (double conversionValue) {
double output;
output = conversionValue * 2.54;
printf("%d", output);
}
void CentimetersToInchesConversion (double conversionValue) {
double output;
output = conversionValue / 2.54;
printf("%d", output);
}
int main(void) {
int conversionChoice;
double conversionValue;
printf("Select from the menu below to convert temperature or linear\n");
printf("menus:");
printf("\n");
printf("1 Convert Fahrenheit to Celsius\n");
printf("2 Convert Celsius to Fahrenheit\n");
printf("3 Convert Inches to Centimeters\n");
printf("4 Convert Centimeters to Inches\n");
printf("5 Exit the Program\n");
scanf("%d", &conversionChoice);
printf("Enter the value you wish to convert:\n");
scanf("%d", &conversionValue);
switch (conversionChoice) {
case 1:
FahrenheitToCelsiusConversion(conversionValue);
case 2:
CelsiusToFahrenheitConversion(conversionValue);
case 3:
InchesToCentimetersConversion(conversionValue);
case 4:
CentimetersToInchesConversion(conversionValue);
case 5:
printf("Program exiting.");
default:
printf("Program exiting.");
}
return 0;
}

Answer 1

You are missing breaks for a start. (so if you pick option 1 it will also perform options 2 3 4 and 5)
conversionValue is a double but you scan it in with %d. This means you aren't converting the value you think you are. Basic debug (printing your input values) would highlight that.
All your prints are trying to print a double with %d (should be %lf for doubles)
Combining output with calculations is bad. i.e. Your convert routines should return the converted value and the print should be outside the convert function.

Answer 2

As a follow on to your comments, understand that printf will print doubles using "%f" by default, however, to read a double with scanf you must use the l length-modifier, e.g. "%lf". A mismatch in format specifiers results in Undefined Behavior.
Further, as you found, your F->C conversion logic needs... help. The 32 is being subtracted at the wrong point, it should be:
void FahrenheitToCelsiusConversion (double conversionValue) {
double output;
output = (conversionValue - 32) / (9.0 / 5.0);
printf ("%.2f\n", output);
}
(note: the printf format change as well)
Further, in main() you need only a single call to printf (or fputs since no conversions are involved) to print the entire menu. When you terminate a line with a double-quote and the next begins with a double-quote, C will concatenate the strings, e.g.
printf ("\nSelect from the menu below to convert temperature or linear\n"
"menus:\n\n"
" 1 Convert Fahrenheit to Celsius\n"
" 2 Convert Celsius to Fahrenheit\n"
" 3 Convert Inches to Centimeters\n"
" 4 Convert Centimeters to Inches\n"
" 5 Exit the Program\n\n"
"choice: ");
Always, always validate all User Input, this means at minimum validating the return of scanf (and if not exiting after a matching failure -- you must read and discard all offending characters that remain in stdin because when a matching failure occurs, no further characters are removed from the input buffer and any offending characters that caused the matching failure remain unread just waiting to bite you again on your next attempt to read from the input buffer).
At minimum check that the number of conversions expected completed successfully (and you should further check that any numeric input is within range, if applicable), e.g.
if (scanf ("%d", &conversionChoice) != 1 || conversionChoice < 1 ||
conversionChoice > 5) {
fputs ("error: invalid input, or value out of range.\n", stderr);
return 1;
}
printf ("\nEnter the value you wish to convert: ");
if (scanf ("%lf", &conversionValue) != 1) {
fputs ("error: invalid value input.\n", stderr);
return 1;
}
You indicate you have added break statements to your switch statement, but after our discussion of the default fall-through behavior, do you understand why the following works?
switch (conversionChoice) {
case 1:
FahrenheitToCelsiusConversion(conversionValue);
break;
case 2:
CelsiusToFahrenheitConversion(conversionValue);
break;
case 3:
InchesToCentimetersConversion(conversionValue);
break;
case 4:
CentimetersToInchesConversion(conversionValue);
break;
case 5: /* here you CAN use fall-through */
default:
printf("Program exiting.");
break;
}
Putting all the pieces together, you can do something like the following:
#include <stdio.h>
void FahrenheitToCelsiusConversion (double conversionValue) {
double output;
output = (conversionValue - 32) / (9.0 / 5.0);
printf ("%.2f\n", output);
}
void CelsiusToFahrenheitConversion (double conversionValue) {
double output;
output = conversionValue * (9.0 / 5.0) + 32;
printf ("%.2f\n", output);
}
void InchesToCentimetersConversion (double conversionValue) {
double output;
output = conversionValue * 2.54;
printf ("%.2f\n", output);
}
void CentimetersToInchesConversion (double conversionValue) {
double output;
output = conversionValue / 2.54;
printf ("%.2f\n", output);
}
int main (void) {
int conversionChoice;
double conversionValue;
printf ("\nSelect from the menu below to convert temperature or linear\n"
"menus:\n\n"
" 1 Convert Fahrenheit to Celsius\n"
" 2 Convert Celsius to Fahrenheit\n"
" 3 Convert Inches to Centimeters\n"
" 4 Convert Centimeters to Inches\n"
" 5 Exit the Program\n\n"
"choice: ");
if (scanf ("%d", &conversionChoice) != 1 || conversionChoice < 1 ||
conversionChoice > 5) {
fputs ("error: invalid input, or value out of range.\n", stderr);
return 1;
}
printf ("\nEnter the value you wish to convert: ");
if (scanf ("%lf", &conversionValue) != 1) {
fputs ("error: invalid value input.\n", stderr);
return 1;
}
switch (conversionChoice) {
case 1:
FahrenheitToCelsiusConversion(conversionValue);
break;
case 2:
CelsiusToFahrenheitConversion(conversionValue);
break;
case 3:
InchesToCentimetersConversion(conversionValue);
break;
case 4:
CentimetersToInchesConversion(conversionValue);
break;
case 5: /* here you CAN use fall-through */
default:
printf("Program exiting.");
break;
}
return 0;
}
Example Use/Output
$ ./bin/tempconv
Select from the menu below to convert temperature or linear
menus:
1 Convert Fahrenheit to Celsius
2 Convert Celsius to Fahrenheit
3 Convert Inches to Centimeters
4 Convert Centimeters to Inches
5 Exit the Program
choice: 1
Enter the value you wish to convert: 212
100.00
Always compile with warnings enabled, and do not accept code until it compiles cleanly without warning. To enable warnings add -Wall -Wextra to your gcc or clang compile string. (add -pedantic for several additional warnings). For clang, instead you can use -Weverything. For VS (cl.exe on windoze), add /W3 (or use /Wall but you will get quite a few extraneous non-code related warnings). Read and understand each warning. They will identify any problems, and the exact line on which they occur. You can learn a lot by listening to what your compiler is telling you.
While not an error, C generally avoids the use of camelCase or MixedCase variable names in favor of all lower-case while reserving upper-case names for use with macros and constants. It is a matter of style -- so it is completely up to you, but failing to follow it can lead to the wrong first impression in some circles. See e.g. NASA - C Style Guide, 1994 As with anything outside the C-Standard, you are bound to find contra-indications, but generally camelCase or MixedCase are left to java or C++.
If you have any further questions, please just let me know.