input reading problem

[chasingxsuns]

i entered this simple code from a book and its not working.

#include <stdio.h>
#include <math.h>

void main()
{
 char input[20];
 double miles, kilometers;

 printf("Enter a value in miles:\n");
 miles=atof(gets(input));

 kilometers = miles*1.609;

 printf("%.2f miles works out to %.2f kilometers", miles, kilometers);
}

its multiplying correctly and everything but it reads the wrong thing. ill put in a number of miles and it always comes out to 37813992.

and this is off topic but what is the point of double variables?

[OnionKnight]

%f won't work for doubles, you'll have to use %lf

A double is a floating point datatype, like float, but unlike float it takes up double the amount of bits, 64, with float being 32 bits.
A double can have 15 decimals compared to a float's 7 and can store numbers in roughly 10 times farther the range of a float.

[chasingxsuns]

#include <stdio.h>
#include <math.h>

void main()
{
 char input[20];
 double miles, kilometers;

 printf("Enter a value in miles:\n");
 miles=atof(gets(input));

 kilometers = miles*1.609;

 printf("%.2lf miles works out to %.2lf kilometers", miles, kilometers);
}

i put that in and its doing the same thing. it only reads 37813992. and i tried changing them to floats but it still did the same thing. any idea what is still wrong?

[OnionKnight]

I compiled it and it works for me with both %f and %lf.

Enter a value in miles:
1
1.00 miles works out to 1.61 kilometers

Are you using and old compiler?
Also, you should use int main instead of void main.

[chasingxsuns]

i use the new version of bloodshed. its free and a lot of source codes from my book dont work with it lol. i use windows so i use void and just start>run>exe my programs. is it possible that its just my compiler? if so..could you possibly give me a link to a decent, free one?

[Dave_Sinkula]

You have one of the best, and it happens to be free.

#include <stdlib.h>

Visit the FAQ. Consider tossing the book.

[chasingxsuns]

ha...wow. it works now and i feel like an idiot. oh well. thanks for the help!

[OnionKnight]

Ah that's right, atof() is declared in stdlib.h and not in stdio.h.
What happened was that since your compiler did not know how the atof() function looks like it assumes that it returns an int, when it actually returns a double. This will cause the compiler to convert this "int" to a double. And usually floating point values when represented as an integer has very high values which can explain the high value in your result.

[Tonto]

>> What happened was that since your compiler did not know how the atof() function looks like it assumes that it returns an int, when it actually returns a double. This will cause the compiler to convert this "int" to a double. And usually floating point values when represented as an integer has very high values which can explain the high value in your result.

How did it find a suitable atof to link with in the first place? Surely, stdio.h doesn't just define some atof returning an int to fool newbies.

[cwr]

%f won't work for doubles, you'll have to use %lf

A double is a floating point datatype, like float, but unlike float it takes up double the amount of bits, 64, with float being 32 bits.
A double can have 15 decimals compared to a float's 7 and can store numbers in roughly 10 times farther the range of a float.

You use %f for doubles and floats with printf, not %lf.

The float passed to printf automatically gets promited to a double, which is why there's only one format specifier (%f). This is not the case with scanf, because you pass a pointer, not the value.