如何在结构中打印变量？

Question

#include <stdio.h>
#include <math.h>
#define G 9.81

typedef struct
{
    double weight;
    double drag;
    double time;
} USER_INPUT;

double calculateVelocity(USER_INPUT);

int main(int argc, char **argv)
{
    USER_INPUT userInput;
    double velocity;

    printf("Please enter weight, drag and time: ");
    scanf("%lf %lf %lf", &userInput.weight, &userInput.drag, &userInput.time);

    velocity = calculateVelocity(userInput);

    printf("At t = %f , the parachutist with weight %d kg\nand a drag coefficient %8.2f kg/s \n will have a velocity of %8.2f m/s^2", &userInput.time, &userInput.weight, &userInput.drag, velocity);

    return 0;
}

double calculateVelocity(USER_INPUT data)
{
    double velocity;
    // TODO compute velocity
    return velocity;
}

In the main function, I want to display the result. How can I print variables defined in the structure? I tried %f , which returns 0.000000, and %d returns a random number.

Answer 1

Well not sure about all the physics or which equation so I kind of faked that, but I got your program to compile and run so it should be pretty easy for you to fix it from here. Feel free to ask questions about the pointer stuff and why addresses were and weren't used in different places.

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

#define G 9.81
#define P 1.00 // ?
#define A 1.00 // ?

typedef struct {
    double weight;
    double drag;
    double time;
} userInput_t;

double calculateVelocity(userInput_t *); // Terminal velocity?

double calculateVelocity(userInput_t *data) {
   return sqrt((2 * data->weight * G)/(P * A * data->drag));
}

int main(void) {

    userInput_t userInput;

    printf("Please enter weight, drag and time: ");
    scanf("%lf %lf %lf", &userInput.weight, &userInput.drag, &userInput.time);


    double velocity = calculateVelocity(&userInput);

    printf("\nAt t = %f, the parachutist with weight %f kg\n"
           "and a drag coefficient %f kg/s\n"
           "will have a velocity of %f m/s^2\n", 
              userInput.time, userInput.weight, userInput.drag, velocity);
}

Answer 2

int printf(const char *format, ...); is NOT int scanf(const char *format, ...);

scanf works with memory addresses of variables, while printf with variables itselves.

It's UB trying to printf ("%d", memory_addres_of_variable);

because the right way to print memory addresses is with %zu starting from C99 and later, and %lu with ANSI C 90.

That's why you aree seeing a random number.

Answer 3

The issue is not related to struct . You are passing addresses of variables instead of their values to printf . So the solution is simple: Remove the & before variable names in the printf call.

This is a common mistake: scanf needs the addresses of variables to be able to alter their values, while printf just takes the values. Unfortunately even the prototypes do not make it evident:

int printf(const char *format, ...);
int scanf(const char *format, ...);

And you need to use %f for double variables (derived from the type called f loat), %d is used for int ( d ecimal).

The result:

printf("At t = %f , the parachutist with weight %f kg\nand a drag coefficient %8.2f kg/s \n will have a velocity of %8.2f m/s^2", userInput.time, userInput.weight, userInput.drag, velocity);

References:

• http://en.cppreference.com/w/cpp/io/c/fprintf
• http://en.cppreference.com/w/c/io/fscanf

Answer 4

You are doing mistake while printing,

Please note '&' is used to get the address of any variable, not the value.So when you are printing:

printf("At t = %f , the parachutist with weight %d kg\nand a drag coefficient %8.2f kg/s \n will have a velocity of %8.2f m/s^2", &userInput.time, &userInput.weight, &userInput.drag, velocity);

you are actually printing the adderss of variables:

&userInput.time (address of (userInput.time)), &userInput.weight(address of (userInput.weight)), &userInput.drag (address of (userInput.drag)).

You want to print their values, not address, hence remove '&' while printing:

ie;

printf("At t = %f , the parachutist with weight %d kg\nand a drag coefficient %8.2f kg/s \n will have a velocity of %8.2f m/s^2", userInput.time, userInput.weight, userInput.drag, velocity);