Structures
Pepper
Much content credited to Dr. Robert
Siegfried and to Dietel C How to
Program
What is a Structure
• A structure is a manufactured data type
consisting of related parts
• A structure will be created in contiguous
memory
• In C, a structure is created on the stack
• Visualize with Excel: a structure would contain
the column headings, and a single row would
be contained in a structure variable.
• As close to a class definition as C gets
Defining A Structure - The Type
• A structure containing the rate of pay and
hours worked might look like this:
struct
int
int
int
} ;
worker {
rate;
hours;
gross;
STRUCTURE TAG
• struct worker is now a new data
type
Defining A Structure (continued)
• Alternatively, we can write:
struct worker {
int rate, hours, gross;
} ;
Declaring A Structure - Make a var
• We can give the structure a name and then declare
variables as structures of this type very easily:
struct workerstuff {
int rate, hours, gross;
} ;
… …
Structure Variable
int main(void)
{
struct workerstuff worker;
// create a variable of type workerstuff
Declaring A Structure - Make a var
• Alternately, make the variable along with structure
definition. This creates both worker and manager
• Workerstuff is optional
struct workerstuff {
int rate, hours, gross;
} worker, manager;
Structure Variables
Using A Structure Variable
• Access parts with dot
• To use a field within the structure, you must
specify both the structure and the field with a
period “.” in between:
scanf("%d",&worker.rate);
scanf("%d",&worker.hours);
worker.gross = worker.rate
* worker.hours;
A simple payroll program
#include <stdio.h>
/*
* a simple structure for payroll
*/
struct workerstuff {
char name[20];
float rate;
float hours;
float gross;
};
/*
* payroll.c - A simple payroll program
*/
int main(void)
{
struct workerstuff worker;
printf("What is the worker\'s rate per "
"hour?\t");
scanf("%f", &worker.rate);
printf("How many hours did the worker work "
"last week?\t");
scanf("%f", &worker.hours);
printf("What is the worker\'s name?\t");
scanf("%s", &worker.name);
printf("%s worked for out %3.1f hours at $"
"%4.2f per hour.\n",
worker.name, worker.hours, worker.rate);
return(0);
}
New Payroll -Array of a Structure
Array of
Remember
#include
#include
a structure like any other
the data type is “struct tag”
<stdio.h>
<ctype.h>
/* # of tax brackets
#define NumBrackets
*/
5
struct brackettype {
float minsalary, taxrate;
};
Parameter of Array of a structure type
/* The prototypes */
void getbrackets(struct brackettype brackets[]);
void getinput(float *hours, float *rate);
float getgross(float hours, float rate);
float gettax(float gross, struct brackettype
brackets[]);
void writecheck(float gross, float tax,
float net, float rate, float hours);
int
calcagain(void);
/*
* A payroll program that allows the user to
* enter the tax brackets
*/
int main(void)
Variable of Array of a structure type
{
float hours, rate;
float gross, tax, net;
struct brackettype
brackets[NumBrackets];
int
again;
/*
* Get the tax brackets before processing any
* payroll records
*/
getbrackets(brackets);
do {
/*
* Get the inputs, calculate the
* gross
*/
getinput(&hours, &rate);
gross = getgross(hours, rate);
/*
* Calculate the tax and subtract it
* to get the net
*/
tax = gettax(gross, brackets);
net = gross - tax;
/*
* Write the paycheck
*/
writecheck(gross, tax, net, rate, hours);
/*
* Does the user want to process another
* record?
*/
again = calcagain();
} while (again);
/* If not, quit */
return(0);
}
/*
* getbrackets() - Input the tax brackets
* We need the minimum gross for the tax
* bracket, and the tax rate itself
*/
void getbrackets(struct brackettype brackets[])
{
int i;
for (i = 0; i < NumBrackets; i++) {
printf("What is the maximum income for"
" bracket #%d?\t", i+1 );
scanf("%f", &brackets[i].minsalary);
printf("What is the tax rate for bracket"
" #%d?\t", i+1);
scanf("%f", &brackets[i].taxrate);
brackets[i].taxrate
= brackets[i].taxrate/100;
}
printf("\n\n\n");
}
/*
* gettax() - Calculate the tax for the
* employee using the tax brackets
*/
float gettax(float gross, struct brackettype
brackets[])
{
int i;
/*
* If the employee doesn't make enough for the
* lowest bracket the tax is zero
*/
if (gross < brackets[0].minsalary)
return(0.0);
/*
* Find the appropriate bracket for the
* employee
*/
for (i = 1; i < NumBrackets; i++) {
if (gross < brackets[i].minsalary)
return(brackets[i-1].taxrate*gross);
}
/* The employee is in the highest bracket */
return(brackets[NumBrackets-1].taxrate*gross);
}
Structures Containing Arrays
• A structure can have an array as a field within
it. Examples of this include character strings.
• A Dean’s List program could use this to
include the grades that comprised our students’
g.p.a.
Reminder typedef
• There are times when it is useful to define one's
own data types. You can do this with the typedef
statement.
• Syntax:
typedef DataType DataTypeName;
• Examples
typedef int
typedef int
IntegerType;
*IntPtr;
• Remember that a struct’s datatype
is two words: “struct tag”
Structure Typedef – 2 methods
• 1: Typedef the struct tag :
struct examrec
{char first[namelen];
int
exam[numexams];};
typedef struct examrec Exam
• 2: Typedef the entire struct definition (no tag):
typedef struct
{
char first[namelen];
int
exam[numexams];
} Exam;
• Variable Creation with either method:
Exam anExam;
Example: Find a Student’s Exam Average
• Find a student’s exam average, given the
scores on 4 exams.
• Initial algorithm:
1.Find the student’s exam scores
2.Calculate the student’s average
3.Print the scores and average
Pointer to a structure->
• If we are working with a pointer to a structure
and we wish to reference one of the fields, we
can write:
*(myStructPtr.myField)
or
myStructPtr -> myField
• This second form is consider far better form.
avggrade.c
#include
#define
#define
<stdio.h>
namelen
numexams
15
4
typedef
struct
{
char
firstname[namelen], lastname[namelen];
int exam[numexams];
} examrec;
void readstudent(examrec *student);
float findaverage(examrec student);
void writestudent(examrec student, float
average);
/*
* AvgGrade() Averages the grades on n exams
*/
int
main(void)
{
examrec
student;
float
average;
/* Read the students name and test scores */
readstudent(&student);
/* Find the average */
average = findaverage(student);
/* Print the results */
writestudent(student, average);
return(0);
}
/*
* ReadStudent() Read the input about the
*
student
Pass by Ref
*/
void readstudent( examrec *student)
{
int
i;
printf("First name\t?");
scanf("%s", student -> firstname);
SAME ACCESS
printf("Last name\t?");
scanf("%s", (*student).lastname );
for (i = 0; i < numexams; i++) {
printf("Enter grade for exam #%d\t?",
i+1);
scanf("%d", &(student->exam[i]));
}
}
/*
* FindAverage() Returns the average of n
*
exam scores
Pass by Value, no ptr
*/
float findaverage( examrec student)
{
int
i, sum = 0;
for (i = 0; i < numexams; i++)
sum += student.exam[i];
return((float) sum/numexams);
}
/*
* WriteStudent() Print the data about the
*
student including the
*
average
*/
void writestudent(examrec student, float average)
{
int i;
printf("%s %s scored :\n", student.firstname,
student.lastname);
for
(i = 0; i < numexams; i++)
printf("%d\t", student.exam[i]);
printf("\n\twhich resulted in an average of"
"%3.1f\n", average);
}
Pass Structure by Ref or Value
• Note: By Val when passing a function an array
inside a struct?
• If definition: struct z {char x[10];}; then a
copy of x array is passed – by value
• If definition: struct z {char * x;}; then address
of x is passed – by ref
Union
• Same definition syntax as structure but holds
only one of the values.
• Used when memory is tight or for casting
union number {
int x; int * p;};
will hold one piece of data that you can access
as either the name x, (considered an int) or as the
name p, (considered a pointer) .
ENUM
• Define constant words mapped to numbers for
hard coded lists
• quickly declare a range of constant values
Definition: enum days { SUN, MON, TUE, WED,
THUR, FRI, SAT};
Declaration: enum days day; (day should only take
on one of the 7 values)
Same as
#define SUN 0;
Use: day = SUN;
Tutorials
• http://www.tutorialspoint.com/cprogramming/c_s
tructures.htm
• http://denniskubes.com/2012/08/20/is-c-pass-byvalue-or-reference/
• http://www.programiz.com/c-programming/cenumeration