C Programming Rounding User's Input

Question

I've just started a class in C Programming, and while I have some background knowledge in JAVA, I'm trying to transition to this programming language. I have a project where I have to round user's input from something like 1.3333 to only two decimal places.

What I have so far is this:

#include <stdio.h>
int main (void)
{
   //v is my variable for the value which the user will input
   //Declaring variable as floating
   float v; 

   printf("Enter your value: \n");
   scanf("%.2f", &v);
   v = 0;

   printf("The rounded version is: %.2f");

   return 0;
}

This is what I have so far based off of what I've read in my book and this link: Rounding Number to 2 Decimal Places in C which my question is different from because it involves user input. My professor does say that I can't use a library function and need to use simple type casts to calculate it. This makes me feel that what I have might be wrong. Would the #include <stdio.h> be considered a library function? Given this information, is my thought process on the right track? If not, then would I do something like divide by variable by 100? Maybe %e for scientific notation?

Thanks ahead of time! Only asking for specific information, not coding or anything. Really want to understand the "hows" and "whys".

Answer 1

First of all #include is a command that you need in order to include and use function that c provides for example for scanf you need to include library.

To round the number in two decimals without using %.2f in scanf you could write:

int x= (v*1000);
if(x%10>6) x=x/10+1 ;
else x= x/10;
printf("%d.%d",x/100,x%100);

Answer 2

I think your professor aims not so much in user input but rather in understanding what happens when converting basic datatypes. Rounding, or at least cutting off digits, without library functions could look as follows:

int main (void)
{
    //v is my variable for the value which the user will input
    //Declaring variable as floating
    float v;

    printf("Enter your value: \n");
    scanf("%f", &v);
    v = float((int)(v*100))/100;

    printf("The rounded version is: %f", v);

    return 0;
}

Input/Output:

Enter your value: 
1.3333333
The rounded version is: 1.330000

Answer 3

Here is a working example that rounds properly without using any library calls other than stdio.

#include <stdio.h>

int main (void)
{
    float v; 

    printf("Enter your value: \n");
    scanf("%f", &v);


    v = (float)((int)(v * 100 + .5) / 100.0);

    printf("The rounded version is: %f\n",v);

    return 0;
}

Output:

jnorton@mint18 ~ $ ./a.out 
Enter your value: 
3.456
The rounded version is: 3.460000