Declaration Statement Format Difference

In the first example, radius has not been initialized before it's used. Its value is indeterminate. A good compiler with warnings turned on/up will let you know about this.

C is not Excel. In volume = 4.0f / 3.0f * 3.14159 * radius * radius * radius;, the value of the expression 4.0f / 3.0f * 3.14159 * radius * radius * radius is computed immediately, and not when the value of volume is accessed.

You also want to pass the value of volume to printf rather than its address. You've fixed this in the second example.

Best practices notes

You should be checking that scanf succeeded. Otherwise radius could still be uninitialized. A reasonable thing to do is to abort execution of the program, printing an error message to stderr.

Another option would be to print an error message and try again.

#include <stdio.h> int main(void) { float radius, volume; printf("Input your sphere's radius: "); if (scanf("%f", &radius) != 1) { fprintf(stderr, "Invalid input.\n"); return 1; } volume = 4.0f / 3.0f * 3.14159 * radius * radius * radius; printf("Here is its volume: %f", volume); return 0; } 

And you probably want to print a newline after your output.

#include <stdio.h> int main(void) { float radius, volume; printf("Input your sphere's radius: "); if (scanf("%f", &radius) != 1) { fprintf(stderr, "Invalid input.\n"); return 1; } volume = 4.0f / 3.0f * 3.14159 * radius * radius * radius; printf("Here is its volume: %f\n", volume); return 0; } 

There's very little point to using 32-bit floating point numbers with float when you can attain significantly more precision by using the 64-bit double type.

A function

You might implement a volume function that can take a radius as an argument and return the volume. This has the effect of calculating the volume on demand, more in line with your likely original expectation.

double volume(double radius) { return 4.0f / 3.0f * 3.14159 * radius * radius * radius; } 

At which point your main function might be:

int main(void) { double radius printf("Input your sphere's radius: "); if (scanf("%lf", &radius) != 1) { fprintf(stderr, "Invalid input.\n"); return 1; } printf("Here is its volume: %lf\n", volume(radius)); return 0; } 

C language is imperative. So things occurr one after the previous one. In:

 float radius, volume; volume = 4.0f / 3.0f * 3.14159 * radius * radius * radius; printf("Input your sphere's radius: "); scanf("%f", &radius); 

you first do the calculation of volume from an undefined value given to radius because it has not been initialized before use (you do use it when calculating volume) Then you read radius value with scanf() but don't do anything with it. In the first example, when you read radius it is too late, as volume has already been calculated.

When you start in the morning, you first dress, then go to street. If you do in the reverse order, you will get on the street probably naked :)

Was it C a declarative language, as when you use a spreadsheet, both versions of the program would be similar, and each time you modify radius it would show the volume value. But this is not the case.

Think of C as a language in which you write a recipe, first shake the eggs, then put them in the pan, finally serve them in a dish or plate. In the second example you first ask for the radius, then you use it to calculate the volume, and finally you print it.