random and pointers doesn't work

Question

Hello I'm trying to increase float to Pointer But somehow The program prints all the time 0.00000. The number should be drawn between 12.01 to -13.00.

My code -

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main()
{
    float* num = (float*)malloc(sizeof(float));

    srand(time(NULL));

    *num = rand() % 1300 + 1201 / 100.00;

    printf("%f",num);
    system("PAUSE");

    free(num);
}

I would love if someone could help me fix it thanks.

Answer 1

note the * if you want to print the number while you're printing the address :

printf("%f",*num);

Answer 2

You need to print the value that num points to:

printf("%f", *num);

Answer 3

Two things:

1. Don't cast your malloc in C (unrelated to your problem but a good idea nonetheless)
2. Enable compiler warnings

Doing both of these things will help you avoid problems. Compiling with gcc -Wall I get:

warning: format '%f' expects argument of type 'double', but argument 2 has type 'float *' [-Wformat=] printf("%f",num);

which would have answered your question. You are using a %f format specifier but passing a pointer, not a float or double. You need to dereference your pointer:

printf("%f", *num);

Answer 4

These are errors:

• *num = rand() % 1300 + 1201 / 100.00;

  *num is in the [12.01, 1311.01] range. If you need a number in the [12.01, 13.00] range, change the assignment:

  *num = 12.01 + (rand() % 100) / 100.0;
  

• printf("%f", num); should be printf("%f", *num);

Also it's a good idea to enable extra warnings during compilation. E.g. with -Wall -Wextra

clang -Wall -Wextra filename.c
warning: format specifies type 'double' but the argument has type 'float *' [-Wformat]

Same with gcc

gcc -Wall -Wextra filename.c

warning: format '%f' expects argument of type 'double', but argument 2 has type 'float *' [-Wformat=]

If you are playing with malloc/free you should check for allocation failures.

Memory allocation is not guaranteed to succeed, and may instead return a null pointer. If there's no check for successful allocation implemented, this usually leads to a crash of the program, due to the resulting segmentation fault on the null pointer dereference.

So you could change your code in this way:

float *num = malloc(sizeof(float));
if (num)
{
  // your code
}
else
{
  // handle failure
}

Anyway it's better to just use a simple float.

Answer 5

In addition of fixing printf("%f", *num);, you need to check your math!

If you really want result between 12.01 and -13.00,

*num = (rand() % 2501 - 1300) / 100.00;

Answer 6

You program invokes undefined behaviour because num is not a float but a pointer to a float and your trying to print it as a float by %f conversion specifier in printf call. Also, don't cast the result of malloc and explicitly mention void in the paramater list of main.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void) {
    float *num = malloc(sizeof *num);
    srand(time(NULL));
    *num = rand() % 1300 + 1201 / 100.00;
    printf("%f", *num);   // note *num instead of num
    system("PAUSE");

    free(num);
}