Using goto statement for loop

Question

#include <stdio.h>

int main() {

    char choice;
    float x,y;

    start:
    printf("[c] Converts Celsius -> Fahrenheit\n[f] Converts Fahrenheit -> Celsius\n\n\n");
    printf("Enter Choice: ");
    scanf("%c",&choice);
    if (choice!='c' || choice!='f' || choice!='x') {
            printf("Wrong Choice: Try Again!");
            goto start;

        } if (choice!=x)
          printf("Input Value: ");
          scanf("%f",&x);
                if (choice =='c')
                    y = 1.8 * x + 32
                else
                    y = (x-32) * (5/9)
          printf("Result: %.2f",y);
          exit:

    return 0;
}

My instructor posted this but when I tried it, it have errors, need help to fix it.

Using `goto` is considered okay in some certain situations. Using it instead of a *loop* is not one of those situations. Why don't you use loops? — Some programmer dude, Jan 24 '16 at 13:09
One obvious problem is the `if` statement that checks `choice`. The condition is clearly always true: It can't be equal to `c`, `f`, and `x` all at the same time, and the test is that if it's not equal to *any* of them, do something. You can replace it with `if (1)` if that's really your intent. — Tom Karzes, Jan 24 '16 at 13:09
And a general tip regarding reading single characters using [`scanf`](http://en.cppreference.com/w/c/io/fscanf): Almost always you want to skip leading white-space, or you will read possible newlines which often is not wanted. Try doing `scanf(" %c", &choice);`, note the space in `" %c"`. — Some programmer dude, Jan 24 '16 at 13:11
Instantly get a new instructor. Such code should be reason to get fired. There are rare acceptable reasons to use `goto`; this is definitively none of them. — too honest for this site, Jan 24 '16 at 13:18
@Olaf perhaps the assignment was "what is wrong with this code and fix it". — Bart Friederichs, Jan 24 '16 at 13:19
@BartFriederichs: Iff(!) that is true, it still is bad idea, as it implies using `goto` is bad juju. (I know many people think so) — too honest for this site, Jan 24 '16 at 13:23
@Olaf it might be implying `goto` is bad in this case, which it is. Nothing wrong with that, I'd say. — Bart Friederichs, Jan 24 '16 at 13:24
The title was nonsense. `goto` is no computational statement. — too honest for this site, Jan 24 '16 at 13:55

score 2 · Answer 1 · answered Jan 24 '16 at 13:13

There are quite some things wrong with this code:

Using goto instead of a while loop is not the way to make this.
Your second if (if (choice != x)) is missing braces. This way only the first statement is executed if true. The rest is always executed.
You are missing semicolons ; after your calculations.
Your boolean logic in the first if is incorrect.
In your second if you are comparing against a variable, instead of a fixed value.
Your Fahrenheit-to-Celsius arithmetic is incorrect.

Hope these tips help you. I won't post the correct code, that's up to you as you are still learning ;-).

Vlad from Moscow · Answer 2 · 2016-01-24T14:16:35.683

First of all you should forget that there is the goto statement in C.

All programs that contained a goto statement and I saw including some IBM's source code had numerous bugs related to goto statements.

The goto statement makes programs difficult to read and more difficult to maintain and modify them.

Using the goto statement is the same as traffic violatins.:)

Instead of this code snippet

start:
printf("[c] Converts Celsius -> Fahrenheit\n[f] Converts Fahrenheit -> Celsius\n\n\n");
printf("Enter Choice: ");
scanf("%c",&choice);
if (choice!='c' || choice!='f' || choice!='x') {
        printf("Wrong Choice: Try Again!");
        goto start;

    }

that at least has an invalid condition in the if statement

if (choice!='c' || choice!='f' || choice!='x')

instead of valid

if (choice!='c' && choice!='f' && choice!='x')

You could write for example

enum { Celsius = 'c', Fahrenheit = 'f', Exit = 'x' };
char choice;
int valid_input;

do
{
    printf( "[c] Converts Celsius -> Fahrenheit\n"
            "[f] Converts Fahrenheit -> Celsius\n"
            "[x] Exit\n\n" );

    printf( "Enter Choice: " );

    if ( scanf( "%c ", &choice ) != 1 ) choice = Exit;

    valid_input = choice == Celsius || choice == Fahrenheit || choice == Exit;

    if ( !valid_input ) puts( "Wrong Choice: Try Again!\n" );
} while ( !valid_input );

Your program contains numerous bugs. Insetad of to list them I will show how the program could be written

#include <stdio.h>

int main( void )
{    
    enum { Celsius = 'c', Fahrenheit = 'f', Exit = 'x' };
    char choice;
    int valid_input;

    do
    {
        printf( "[c] Converts Celsius -> Fahrenheit\n"
                "[f] Converts Fahrenheit -> Celsius\n"
                "[x] Exit\n\n" );

        printf( "Enter Choice: " );

        if ( scanf( "%c ", &choice ) != 1 ) choice = Exit;

        valid_input = choice == Celsius || choice == Fahrenheit || choice == Exit;

        if ( !valid_input ) puts( "Wrong Choice: Try Again!\n" );
    } while ( !valid_input );


    switch( choice )
    {
        float x, y;

        case Celsius: case Fahrenheit:
            printf( "Input Value: " );
            if ( scanf( "%f", &x ) == 1 )
            {                
                if ( choice == Celsius )
                    y = 1.8 * x + 32;
                else
                    y = ( x - 32 ) * ( 5.0f / 9.0f );
                printf( "Result: %.2f\n", y );
            }                
        case Exit:
            puts( "Bye!" );
            break;
    }

    return 0;
}

if to enter sequentially

c 20 x

then the output will look like

[c] Converts Celsius -> Fahrenheit
[f] Converts Fahrenheit -> Celsius
[x] Exit

Enter Choice: c
Input Value: 20
Result: 68.00
Bye!

Guys, I put a wrong Title. It should be TEMPERATURE CONVERTER USING GOTO STATEMENT not in loop. Sorry. — Adrian Pete Lagare, Feb 14 '16 at 13:55

Using goto statement for loop

2 Answers2