A basic math programming question

Hi,
I'm trying to learn some basic ideas about programming mathematical functions in C.
I would like the user to be able to input a number with decimals, for instance "2.123"
and then use this input to carry out an equation.
Here is how I've been doing it:
int main (int argc, char * const argv[]) {


/* Variables */
float Xreal;
float Yreal;
float Zreal;
float Xforce;
float Yforce;
float Zforce;
float Tval;
char inXreal[8];
char inYreal[8];
char inZreal[8];
char inZforce[8];


/* Prompts */

/* Get coordinates of real-perspective point */
printf("Xreal value: \n");
gets(inXreal);
Xreal=atoi(inXreal);

printf("Yreal value: \n");
gets(inYreal);
Yreal=atoi(inYreal);

printf("Zreal value: \n");
gets(inZreal);
Zreal=atoi(inZreal);

/* Get Z coordinate of forced-perspective point */
printf("Zforce value: \n");
gets(inZforce);
Zforce=atoi(inZforce);


/* Computation */
Tval=Zforce/Zreal;
Xforce=Tval*Xreal;
Yforce=Tval*Yreal;

/* Output Display */
printf("The point: (%.3f, %.3f, %.3f) was generated with a Tvalue of: %.3f.\n",Xforce,Yforce,Zforce,Tval);
}

This works so long as the input values are whole numbers.
But I would also like to be able to perform these functions using decimals.
I understand that the problem must have something to do with using CHAR and ATOI, but I'm not certain of another way to get user input for decimals, etc.

Can anyone offer some advice?
Thanks!

by the way:
I am working in Xcode 3 on a mac running os x leopard

Since atoi stands for "alphanumeric to integer", it seems unlikely it will work for getting floating point values.

Whatever path you're following in your quest for C knowledge, if it took you past "atoi" it should have taken you past "atof" as well. (And "atof" stands for "alphanumeric to floating point" -- it returns a value of type double.)

>int main (int argc, char * const argv[]) {
If you're not going to use the parameters, don't include them. Also, be sure to return a value from main, even if it's always 0:

Code:

---

int main ( void )
{
  return 0;
}

---

>gets(inXreal);
Never use gets. It's an unsafe function, and it's impossible to make gets safe.

>Xreal=atoi(inXreal);
Even if you use it with integers, atoi is generally a bad idea unless you've validated the string first. I'd recommend strtol, or in the case of floating-point values, strtod.

Hi, thanks for your very quick response.
I suspected that was exactly the problem, and so I checked the index of the book I am using for other ato... functions, but believe it or not, they do not mention atof.

thanks for your help!

prelude,
can you clarify:
I am getting a message in terminal that gets is unsafe...
How could I perform that same function without it?

Printf("Input number: \n");
WHAT GOES HERE?

and, in the context of the above code... where do i put the "return", because my "{" then contains the rest of the program, im not sure where the return zero goes


thanks for your help.
im just getting started

have you read FAQ?
you can use fgets(buffer, sizeof buffer, stdin)

>How could I perform that same function without it?
Sorry about that. I was in so much of a hurry that I forgot to point you toward the fgets function.

>where do i put the "return"
Put the return in main when you're done. A good start is right at the end, just before the closing brace.

Code:

---

int main ( void )
{
  /* Your code here */
  return 0;
}

---

ok, i get the return. thanks

but, im not certain i understand the fgets.
if it is "fgets(buffer, sizeof buffer, stdin)"
what do I put for each: buffer, size of, stdin?

all i want is to have it understand an input such as "2.123"

buffer = where you want the answer to go.

sizeof buffer = size of the buffer. IOW, how many characters are in your char array? (Answer: 8.)

stdin = stdin. You know, the thing where people type, and it shows up on the screen. Short for "standard in". This should literally be the word "stdin".

yes, but how do i implement that?

Once again - have you read the FAQ?
http://faq.cprogramming.com/cgi-bin/...&id=1043284385

I would use scanf

Code:

---

int main()
{
    float a;
    float b;
   
    printf("enter a float:\n");
    scanf("%f", &a);
   
    printf("Your float is %.3f", a);
   
    return (EXIT_SUCCESS);   
   
}

---

>I would use scanf
scanf has its own problems, and conveniently enough, your example exhibits one of them. What happens if I type "I'm a little teapot" instead of "123.456"?

The BEST version to convert a string to float is strtof() - it returns a float, but also tells you if it went OK or not.


http://www.hmug.org/man/3/strtod.php

--
Mats

Prelude,
I see your point. I do want to avoid such inputs. In fact, in my first incarnation I had exactly that problem:
when using atoi instead of atof,
if I input 3.2
it would perform the division on just the 3.

on the other hand, im finding the faq about fgets a bit confusing.
could someone provide me with an example in the context of this program?

thanks to everyone here for their help, as is very obvious, im just getting started...

Quote:

---

Originally Posted by hebali

Prelude,
I see your point. I do want to avoid such inputs. In fact, in my first incarnation I had exactly that problem:
when using atoi instead of atof,
if I input 3.2
it would perform the division on just the 3.

on the other hand, im finding the faq about fgets a bit confusing.
could someone provide me with an example in the context of this program?

thanks to everyone here for their help, as is very obvious, im just getting started...

---

The FAQ is an example in the context of your program.

Code:

---

char inXreal[8];

fgets(inXreal, 8, stdin); // first 8 characters of input are now here

---

True, no argument with that. Just add error checking.

Atoi converts a string to an integer (no decimals, in other words).
Best use strtof or strtod.

aha! thanks. i get it now. sorry.

>could someone provide me with an example in the context of this program?
Sure:

Code:

---

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>

int main ( void )
{
  char buffer[BUFSIZ];

  printf ( "Enter a floating-point number: " );
  fflush ( stdout );

  if ( fgets ( buffer, sizeof buffer, stdin ) != NULL ) {
    char *end;
    double x;

    errno = 0;
    x = strtod ( buffer, &end );

    if ( errno == ERANGE || end == buffer )
      fputs ( "Invalid input\n", stderr );
    else
      printf ( "You entered &#37;f\n", x );
  }

  return 0;
}

---

>Best use strtof or strtod.
Note that strtof is only available in C99 or as an extension.

Slinger: by error checking do you mean "if ((p = strchr(buf, '\n')) != NULL)
*p = '\0';"
(as cited in the FAQ)?

Elysia, from the hmug site, I see the following about the use of srttof:

"strtof(const char *restrict nptr, char **restrict endptr);"
but I am not sure what the implementations of those: const char *restrict nptr, etc are

Is there a strtof actually? I can only see strotod.
But as for the question, the usage goes like this:
strotod(string_you_want_to_convert_here, [1]);
[1] This is a pointer to a pointer (char**) that recieves the address where strtod stopped parsing the string. So if you add junk "10d", it will assign to a pointer to address in the string where "d" resides, so you know that at this point, it stopped parsing.

You can just pass NULL if that was confusing.

>but I am not sure what the implementations of those: const char *restrict nptr, etc are
Don't worry about restrict for now. A lot of experienced C-ites have trouble with the concept, and it's still a very new feature. But to answer your question, the first parameter is the string with your value. The second parameter tells you how far into the string strtof got before it couldn't go on (you can pass NULL to this parameter if you don't care). For example:

Code:

---

#include <stdio.h>
#include <stdlib.h>

int main ( void )
{
  const char *p = "123.456test";
  char *end;
  double x;

  printf ( "Converting \"&#37;s\"\n", p );
  x = strtod ( p, &end );
  printf ( "Found %f with \"%s\" left over\n", x, end );

  return 0;
}

---

>Is there a strtof actually?
In C99 or as an extension, yes. ;) But prior to C99 you can't rely on it being supported.

Quote:

---

Originally Posted by hebali

Slinger: by error checking do you mean "if ((p = strchr(buf, '\n')) != NULL)
*p = '\0';"
(as cited in the FAQ)?

Elysia, from the hmug site, I see the following about the use of srttof:

"strtof(const char *restrict nptr, char **restrict endptr);"
but I am not sure what the implementations of those: const char *restrict nptr, etc are

---

Was not thinking specifics but, scanf does have its limits with user input. If you are needing to check that a number, ie a float, was truly entered then you will need to check that no letters were entered, perhaps by checking against, isalpha? But Preludes recommendations looks good and clean.

Problem is that it leaves crap in the input buffer which you need to clean (if it fails). That's why I like the fgets/strtod/strtol approach instead.

>That's why I like the fgets/strtod/strtol approach instead.
This has the same problem, though it's subtler. In practice, fgets is best used as the foundation of a more robust, but application-specific, line reading function.

Ok, so my current implementation looks like this:

Code:

---

/* Variables */
        float Xreal;
        float Yreal;
        float Zreal;
        float Xforce;
        float Yforce;
        float Zforce;
        float Tval;
        char inXreal[8];
        char inYreal[8];
        char inZreal[8];
        char inZforce[8];

       
/* Prompts */
       
        /* Get coordinates of real-perspective point */
        printf("Xreal value: ");
        fgets(inXreal, 8, stdin);
        Xreal=strtof(inXreal, NULL);
       
        printf("Yreal value: ");
        fgets(inYreal, 8, stdin);
        Yreal=strtof(inYreal, NULL);
       
        printf("Zreal value: ");
        fgets(inZreal, 8, stdin);
        Zreal=strtof(inZreal, NULL);
       
        /* Get Z coordinate of forced-perspective point */
        printf("Zforce value: ");
        fgets(inZforce, 8, stdin);
        Zforce=strtof(inZforce, NULL);

/* Computation */
        Tval=Zforce/Zreal;
        Xforce=Tval*Xreal;
        Yforce=Tval*Yreal;
       
/* Output Display */
        printf("The point: (%.3f, %.3f, %.3f) was generated with a Tvalue of: %.3f.\n",Xforce,Yforce,Zforce,Tval);
       
        return(0);
}

---

It builds. But, I think I may be seeing the effect of using NULL?? I'm not sure. When I run it, if each of the user input are the same length: for example, 2.343 5.425, etc., then it will perform normally. however, if i make one of the inputs have a different number of characters (length?) from the previous, it will skip the next prompt, and go right to the final output with incorrect values.

What is this about?

Thanks. I must say, this is the most helpful forum I've ever used. I'm new to programming, but not new to computing (mainly Maya, etc.) and you have all been wonderful.

Everything has its ups and downs, but it's easier to check for errors and no need with annoying syntax to clear input buffers. IMHO, of course.

>Everything has its ups and downs, but it's easier to check for errors
>and no need with annoying syntax to clear input buffers.
Don't read it as me knocking fgets. I think it's a good solution to the gets/scanf problem, but "use fgets!" seems to have become something religious among some C programmers, and we don't want to mislead rookies into thinking that all they need to do for great code is use fgets. ;)

Cheers to that! :D

>and we don't want to mislead rookies into thinking that all they need to do for great code is use fgets.

I, for one, don't think that's all I need.

actually, even with atof I have problems if the input values for Xreal, Yreal, Zreal, Zforce have different numbers of decimals from one another.

?

A thought without looking over your shoulder: since your buffers are, for whatever reasons, only eight characters long, you can only get seven characters (plus the \0) from the input. So: if you type "12345.678" the first fgets will peel off "12345.6", while the second fgets will then peel off "78\n" -- hence it won't ask for input here, since there's already perfectly good input sitting in the input stream. You can fix that by changing 8 to, say, 25 (or some other number that people are probably not going to type that many digits).

yes indeed!
thanks.
ok one last question for me today:
if instead of asking for Xreal, then Yreal, then...
I wanted to ask for the input in the form of either:
(23, 3489, 348.387) or if that is too difficult because of the other characters,
23 3489 348.387

I know in PASCAL it would be something like
Readln(Xreal, Yreal, Zreal)

for 23 3489 348.347 type input

I would probably do something like this:

Code:

---

char input_buffer[80];
int check_input;

fgets(input_buffer, sizeof input_buffer, stdin);
check_input = sscanf(input_buffer, "(%f,%f,%f)", &Xreal, &Yreal, &Zreal);
while (check_input != 3) {
  //bad input do it again
}

---

Assuming your prompt gets people to type the numbers in with () around them and commas in between. The "formatted input" functions ?scanf work reasonably well, if you know you have formatted input.

Great. Thanks.