atoi and floating point errors

Hi all! I'm trying to make a really simple program in c which when given 3 values plonks them in a formula and outputs another value.

Here's the code before I get onto the problem

Code:

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

int main()
{
        int wr2;
        int fst2;
        int rst2;
        int gear;
        char wr1[5];
        char fst1[5];
        char rst1[5];

        printf("Please enter wheel radius\n");
        scanf("%d",wr1); 
        wr2=atoi(wr1);
        printf("Please enter number of teeth on front sprocket\n");
        scanf("%d",fst1);
        fst2=atoi(fst1);
        printf("Please enter number of teeth on rear sprocket\n");
        scanf("%d",rst1);
        rst2=atoi(rst1);

        gear=wr2*(fst2/rst2);
        printf("Wheel Radius %d inches, front teeth %d, rear teeth %d gives gear %d inches.\n",wr2,fst2,rst2,gear);
        return(0);
}

When run the program stores the 3 values entered and either outputs 1 of 2 things depending on the coding. If gear is set as an int i get a floating point error, is gear is set as float it always outputs 54 no matter what values are entered. Its a really basic error but as I've just started C I cannot figure out why it wont work. Any help is much appreciated.

Native

Why are you trying to use character arrays for your input?

Your scanf format string is incorrect to get a character. It should be

Code:

("%s",wr1);

to get a character array.

But you could just get floating point numbers with scanf instead of character arrays and then converting to numbers.

Code:

   double wr1 = 0;
   scanf("%f",&wr1);

In the following section of code

Code:

    gear=wr2*(fst2/rst2);

Because you are using Integers you will be throwing away any decimal points.

I would suggest that you convert everything to doubles. This should solve most of your problems.

Jim

will try this, thank you!

ok so it looks like this now

Code:

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

int main()
{
        double wr;
        double fst;
        double rst;
        double gear;

        printf("Please enter wheel radius\n");
        scanf("%f",&wr);
        printf("Please enter number of teeth on front sprocket\n");
        scanf("%f",&fst);
        printf("Please enter number of teeth on rear sprocket\n");
        scanf("%f",&rst);

        gear=wr*(fst/rst);
        printf("Wheel Radius %f inches, front teeth %f, rear teeth %f gives gear %f inches.\n",wr,fst,rst,gear);

        return(0);
}

its giving me a load of zeros, was reading up on integers and read to us %f, im assuming thats why its not workin tho?

Doubles are %lf in printf(). (long float == double).

ah, thanks - will try that instead

scanf() format specifier for double is %lf.
for printf() either float or double is %f but C99 accepts %lf as double.
Question 12.9

scanf doesn't check for numeric overflow, so it's inherently unsuited for getting numeric input.

The best way to get numeric input is to read it in as a string first with fgets or similar, and then use strtod to convert it to a double. If you want a float instead and strtof is available to you (C99 function), then you can use that instead. Otherwise it's not terrible hard to make your own function that uses strtod for the initial conversion.

Code:

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

#define INPUT_SIZE 80

extern int errno;

/*
 * Returns 0 on failure and 1 on success.
 */
int
string_to_double(const char *s, double *result, char **end)
{
    double d;

    errno = 0;
    d = strtod(s, end);

    if (s == *end || ((d == 0 || d == HUGE_VAL) && errno == ERANGE))
    {
        return 0;
    }

    *result = d;
    return 1;
}

/*
 * Returns 0 on failure and 1 on success.
 */
int
string_to_float(const char *s, float *result, char **end)
{
    double d;

    if (!string_to_double(s, &d, end))
    {
        return 0;
    }

    if (d > FLT_MAX || d < FLT_MIN)
    {
        return 0;
    }

    *result = (float)d;
    return 1;
}

int
main(void)
{
    char input[INPUT_SIZE];
    char *end;
    float f;

    if (!fgets(input, INPUT_SIZE, stdin))
    {
        puts("Invalid input!");
        return EXIT_FAILURE;
    }

    if (!string_to_float(input, &f, &end))
    {
        puts("Invalid input!");
        return EXIT_FAILURE;
    }

    printf("Number is: %f\n", f);

    return EXIT_SUCCESS;
}

It worked!
Thank you

here is the finished product

Code:

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

int main()
{
        double wr;
        double fst;
        double rst;
        double gear;
        
        printf("Please enter wheel radius\n");
        scanf("%lf",&wr); 
        printf("Please enter number of teeth on front sprocket\n");
        scanf("%lf",&fst);
        printf("Please enter number of teeth on rear sprocket\n");
        scanf("%lf",&rst);

        gear=wr*(fst/rst);
        printf("Wheel Radius %lf inches, front teeth %lf, rear teeth %lf gives gear %lf inches.\n",wr,fst,rst,gear);
        return(0);
}