C 双型及结果

Question

So I am still training with C and noticed an unusal result while practicing functions.

#include <stdio.h>
#include <math.h>
 void main()
 {


 printf("Input any number for square : ");
  double X;
  scanf("%f", &X);
  double square(double X);
  double n=square(X);
  printf("The square of %f %f:", X, n);
}
double square(double X)
{
return (pow(X,2));
}

Here's the output:

Input any number for square: 21 The square of 0.000000 0.000000:

So, I am not understanding why is it returning zeros while the compilation is totally fine and the semantic looks coherent. I'll appreciate it if you don't go in depth because I think it can be explained simply (I'm still quite new ^^' )

Answer 1

When I try and compile this code I get warnings:

square.c:3:2: warning: return type of 'main' is not 'int' [-Wmain-return-type]
 void main()
 ^
square.c:3:2: note: change return type to 'int'
 void main()
 ^~~~
 int
square.c:9:15: warning: format specifies type 'float *' but the argument has type 'double *' [-Wformat]
  scanf("%f", &X);
         ~~   ^~
         %lf
2 warnings generated.

These are both very relevant to the problem. These are generated with clang using -Wall , but GCC and other compilers have similar methods. clang is very helpful, specific and very good about explaining potential fixes.

Correcting those issues and cleaning up the code results in this:

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

// Declare functions before they are used whenever possible
double square(double X)
{
  // No reason for the extra brackets, just return ...
  return pow(X,2);
}

// main() is supposed to at least return int
int main()
{
  printf("Input any number for square : ");
  double x; // Variables typically lower-case, #define macros are upper-case
  scanf("%lf", &x);

  double n = square(x);

  printf("The square of %lf %lf:", x, n);

  // Formally indicate everything's good (no error = 0)
  return 0;
}

Where now everything works out as expected.