Question

I just started learning C and am not sure what the issue is here.

EDIT

#include <stdio.h>

int main (int argc, const char * argv[]) {

struct student {
    int age;
    char gender;
    char course[30];
};

defineNewStudent("Jarryd", 24, 'M', "Software Engineering");

return 0;
}

void defineNewStudent(char studentName[20], int age, char gender, char course[30])
{
student studentName[30];
studentName.age = age;
studentName.gender = gender;
studentName.course = course[20];

printf("%s is %d.\n Gender: %c.\n Course: %s.\n", studentName, studentName.age,  studentName.gender, studentName.course);
}

I have a warning

warning: implicit declaration of function ‘defineNewStudent’

I am trying to take the passed in argument and use it to name the struct, how is this done?

What is this warning about and what are the consequences?

Thanks

Answer 1

The error seems to be because the struct student is only defined inside the function main. You’re trying to use it in the defineNewStudent function, but that struct is not defined there. Define the struct at the global scope.

The warning is because you’re trying to call the defineNewStudent function before you actually declare it, and the compiler still doesn’t know about it at that point. You can declare the function before you try to use it.

// define the student struct at global scope
struct student
{
    int age;
    char gender;
    char course[20];
};

// declare the function (it can still be defined later)
void defineNewStudent(char studentName[20], int age, char gender, char course[20]);

int main (int argc, const char * argv[])
{
   defineNewStudent("Jarryd", 24, 'M', "Software Engineering");

   return 0;
}

void defineNewStudent(char studentName[20], int age, char gender, char course[20])
{
    // you already have a variable called studentName,
    // you can't have two variables with the same name
    struct student studentData;
    studentData.age = age;
    studentData.gender = gender;
    studentData.course = course;

    printf("%s is %d.\n Gender: %c.\n Course: %s.\n", 
         studentName, 
         studentData.age, 
         studentData.gender,
         studentData.course
    );
}

EDIT:

to define an instance of a struct in C you need the struct keyword, so in the example it should be

struct student studentData;
studentData.age = age;
studentData.gender = gender;
studentData.course = course;

If it were c++, you could drop the struct keyword:

student studentData;
studentData.age = age;
studentData.gender = gender;
studentData.course = course;

optionally in C, you can typedef the struct to an alias to make it easier to define instances of it:

typedef struct student
{
    int age;
    char gender;
    char course[20];
} StudentStruct;

// define it like this in the function:
StudentStruct studentData;