Thread: Structures and instances.

Hi everyone,

I'm writing a program that creates 2 structures, with the second having an instance of the first structure. I then create an instance of the second structure which the values are then in putted via the console, the program then creates a second instance of the second structure and copies all of the values from the user in putted structure.

My program copies all of the values just fine except for the string and the the double.
Can anyone spot any obvious flaws with my program?

Thanks a lot
Ryan

Code:

int main()
{
	struct Struct1
	{
		short int shortInt;
		long int longInt;
		char string[64];
		double floating;
	};

	struct Struct2
	{
		short int shortInt;
		long int longInt1;
		long int longInt2;
		struct Struct1 instance;
	};

	struct Struct2 My_Struct;

	printf("Please enter a short integer: ");
	scanf("%d", &My_Struct.shortInt );

	printf("Please enter a long integer: ");
	scanf("%d", &My_Struct.longInt1 );

	printf("Please enter a second long integer: ");
	scanf("%d", &My_Struct.longInt2 );
	
	printf("Please enter a short integer value, an instance of Struct1: ");
	scanf("%d", &My_Struct.instance.shortInt );

	printf("Please enter a long integer value, an instance of Struct1: ");
	scanf("%d", &My_Struct.instance.longInt );

	printf("Please enter a string, an instance of Struct1: ");
	scanf("%s", &My_Struct.instance.string );

	printf("Please enter a floating point number, an instance of Struct1: ");
	scanf("%f", &My_Struct.instance.floating );

	struct Struct2 My_Struct_2 = {
									My_Struct.shortInt, My_Struct.longInt1, My_Struct.longInt2,
									My_Struct.instance.shortInt, My_Struct.instance.longInt,
									My_Struct.instance.string[64], My_Struct.instance.floating
							     };

	printf("The values that were entered and copied are: \n%d \n%d \n%d \n%d \n%d \n%s \n%f \n ",
		My_Struct.shortInt, My_Struct.longInt1, My_Struct.longInt2,
		My_Struct.instance.shortInt, My_Struct.instance.longInt,
		My_Struct.instance.string[64], My_Struct.instance.floating);
}

Good heavens, you should pay attention to your compiler:

Code:

ryan.c: In function ‘main’:
ryan.c:21: warning: implicit declaration of function ‘printf’
ryan.c:21: warning: incompatible implicit declaration of built-in function ‘printf’
ryan.c:22: warning: implicit declaration of function ‘scanf’
ryan.c:22: warning: incompatible implicit declaration of built-in function ‘scanf’
ryan.c:22: warning: format ‘%d’ expects type ‘int *’, but argument 2 has type ‘short int *’
ryan.c:25: warning: format ‘%d’ expects type ‘int *’, but argument 2 has type ‘long int *’
ryan.c:28: warning: format ‘%d’ expects type ‘int *’, but argument 2 has type ‘long int *’
ryan.c:31: warning: format ‘%d’ expects type ‘int *’, but argument 2 has type ‘short int *’
ryan.c:34: warning: format ‘%d’ expects type ‘int *’, but argument 2 has type ‘long int *’
ryan.c:37: warning: format ‘%s’ expects type ‘char *’, but argument 2 has type ‘char (*)[64]’
ryan.c:40: warning: format ‘%f’ expects type ‘float *’, but argument 2 has type ‘double *’
ryan.c:44: warning: missing braces around initializer
ryan.c:44: warning: (near initialization for ‘My_Struct_2.instance’)
ryan.c:46: warning: missing initializer
ryan.c:46: warning: (near initialization for ‘My_Struct_2.instance.floating’)
ryan.c:51: warning: format ‘%d’ expects type ‘int’, but argument 3 has type ‘long int’
ryan.c:51: warning: format ‘%d’ expects type ‘int’, but argument 4 has type ‘long int’
ryan.c:51: warning: format ‘%d’ expects type ‘int’, but argument 6 has type ‘long int’
ryan.c:51: warning: format ‘%s’ expects type ‘char *’, but argument 7 has type ‘int’
ryan.c:42: warning: unused variable ‘My_Struct_2’
ryan.c:52: warning: control reaches end of non-void function

Your specific question comes from lines 44-46, where they talk about missing initializers. Once you start trying to initialize your char array with a single char, it all goes downhill from there.

You cannot assign array.
When you have
char a[10] = "hello";

It's array initialization not assignment.
char a[10];
a = "hello"; // compile error

You could just simply assign struct.

struct my_struct a,b;
// init b
a = b;

But take note that it's the same as memcpy(a,b,sizeof(struct my_struct) );
If there're pointers in struct, only pointer values are copied.

Apologies , but i still dont understand what is wrong with the string, the value printed is <null> on the console.

Bayint Naung, Showed me a more efficient way of copying the instances, that's all I understood though. My book has a sample program in it and declares the array of string the exact same way i have.

Code:

#include<stdio.h>
#include<stdlib.h>
int main()
{
	struct Struct1
	{
		short int shortInt;
		long int longInt;
		char string;
		double floating;
	};

	struct Struct2
	{
		short int shortInt;
		long int longInt1;
		long int longInt2;
		struct Struct1 instance;
	};

	struct Struct2 My_Struct, My_Struct_2;

	printf("Please enter a short integer: ");
	scanf("%d", &My_Struct.shortInt );

	printf("Please enter a long integer: ");
	scanf("%d", &My_Struct.longInt1 );

	printf("Please enter a second long integer: ");
	scanf("%d", &My_Struct.longInt2 );
	
	printf("Please enter a short integer value, an instance of Struct1: ");
	scanf("%d", &My_Struct.instance.shortInt );

	printf("Please enter a long integer value, an instance of Struct1: ");
	scanf("%d", &My_Struct.instance.longInt );

	printf("Please enter a string, an instance of Struct1: ");
	scanf("%s", &My_Struct.instance.string );

	printf("Please enter a floating point number, an instance of Struct1: ");
	scanf("%f", &My_Struct.instance.floating );

	My_Struct_2 = My_Struct;
	
	printf("The values that were entered and copied are: \n%d \n%d \n%d \n%d \n%d \n%s \n%f \n ",
		My_Struct_2.shortInt, My_Struct_2.longInt1, My_Struct_2.longInt2,
		My_Struct_2.instance.shortInt, My_Struct_2.instance.longInt,
		My_Struct_2.instance.string, My_Struct_2.instance.floating);
}

I Seem to have fixed the problem without realising in terms of the string, although my double value still prints as some ridiculous negative number regardless of what i enter, is this a formatting problem?

Yes, Read the documentation of the functions you used if you are not sure.
scanf format specifier for double is %lf , not %f.

In the scanf...
for int, use format "%d"
for long use format "%ld"
for double use format "%lf"

Same for the printf.

Code:

	struct Struct1
	{
		short int shortInt;
		long int longInt;
		char string;
		double floating;

char string; should be char string[somesize], as in char string[64];

char string is only 1 character ... if you want to put "hello" or something meaningful in there you need to initialize an array to hold the characters entered.