Double being returned as jumbled numbers

Question

New to programming in general, creating a program in C with VisualStudio that would take a floating point from the user and return if it is negative or not, then get it as an integer, and finally return the first digit of the integer. Code looks something like this:

double extractDigit1(x){
     double userFloatValue = x;
     (userFloatValue >= 0) ? printf(" %f is a positive value.", userFloatValue) :
                             printf(" %f is a negative value.", userFloatValue);
}

int main(){
     double userValue;
     scanf_s("%lf", &userValue);
     extractDigit1(userValue);
}

Entering 1234.345 gets me:

"1202590843.000000 is a positive value."

instead of:

"1234.345 is a positive value."

If you don't specify the type of the function argument, it is assumed to be `int`. You should change your function to `void extractDigit1(double x)` — Bodo, Jul 08 '19 at 15:03
I'm not sure `double extractDigit1(x)` is what you intended to write, because here `x` will be an integer. — ForceBru, Jul 08 '19 at 15:04
This code should'nt compile. What IDE/Compiler/environnment/OS do you use? — Jabberwocky, Jul 08 '19 at 15:05
@Jabberwocky it is valid C89 garbage... (less the includes). The only platform that has `scanf_s` is... _________. — Antti Haapala -- Слава Україні, Jul 08 '19 at 15:10
Try to enable all warnings and read them! ... in `gcc` you may use: `gcc -Wall source.c` — Sir Jo Black, Jul 08 '19 at 15:10
@AnttiHaapala right. Thanks, I completely forgot about such garbage, I've never used it and I haven't seen it in the last 25 years. — Jabberwocky, Jul 08 '19 at 15:12
@Jabberwocky just tested with gcc, and `-std=c89' no warnings if the include is there. — Antti Haapala -- Слава Україні, Jul 08 '19 at 15:15
Using a ternary operator instead of an `if` statement is an interesting stylistic choice. And I use "interesting" euphemistically. Don't do that. — William Pursell, Jul 08 '19 at 17:06

Mathieu · Accepted Answer · 2019-07-08T17:02:37.310

3

The root problem is

double extractDigit1(x) is not a correct function prototype, it should be double extractDigit1(double x)

Wiht what your write, x was considered as int which is not compatible with the rest of code.

Note that

Your functions return nothing whereas their prototypes tell so

It's an undefined behavior that can lead to anything

Finally

You use scanf_s function, with is almost like scanf but handle strings differently and is less portable (see Difference between scanf and scanf_s). I advice you not to use it.

Corrected code:

#include <stdio.h>

double extractDigit1(double x){
    double userFloatValue = x;
    (userFloatValue >= 0) ? 
        printf(" %f is a positive value.", userFloatValue) :
        printf(" %f is a negative value.", userFloatValue);
    return 0;
}

int main(){
   double userValue;
   scanf("%lf", &userValue);
   extractDigit1(userValue);
   return 0;
}

edited Jul 08 '19 at 17:02

answered Jul 08 '19 at 15:05

Mathieu

8,840
7
32
45

Thank you very much this worked. I'll accept your answer in like 6 mins. – flapjacks2341 Jul 08 '19 at 15:09
2

`scanf_s` is a microsoft function that take more parameters than `scanf`. In `scanf_s`, buffer sizes have to be specified to prevent buffer overflow. So functions are not interchangeable. – Mathieu Jul 08 '19 at 15:15
1

IIRC, more parameters are required only for format specfiers like `%[`, `%c` and `%s`, right? How is the `scanf_s` a problem in the OP's case? – Spikatrix Jul 08 '19 at 15:16
@Spikatrix [Indeed](https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/scanf-s-scanf-s-l-wscanf-s-wscanf-s-l?view=vs-2019)! – Mathieu Jul 08 '19 at 15:21
But then how is that a problem as your answer states? – Spikatrix Jul 08 '19 at 16:26
1

@Spikatrix It is not, I was persuaded that `scanf_s` was requiring the buffer size in all cases. The root error is just the `x` variable defined as an `int` since not typed. That said, I discourage the `scanf_s` usage. – Mathieu Jul 08 '19 at 16:30
1

Yes but your answer stating it as a problem "_You mix `scanf_s` and `scanf`: you called the first like the second_" is not right as the OP's `scanf_s` is perfectly valid – Spikatrix Jul 08 '19 at 16:31
Again you're right, I was correcting when you tell me ;-) – Mathieu Jul 08 '19 at 17:03

Jin Thakur · Answer 2 · 2019-07-08T15:12:09.683

-2

double userFloatValue = x;
int my_var = (int)userFloatValue ;
         (userFloatValue >= 0) ? printf(" %d is a positive value.",   my_var ) :
                                 printf(" %d is a negative value.",  my_var ) ;

Use scanf rather than scanf_s scanf("%f", &a);

edited Jul 08 '19 at 15:12

answered Jul 08 '19 at 15:05

Jin Thakur

2,711
18
15

1

then get it as an integer don't you think it should be int – Jin Thakur Jul 08 '19 at 15:13

Double being returned as jumbled numbers

2 Answers2