1

StructuresEtter Chapter 7

• In engineering, we need to process or visualize data– Some of you have done Matlab simulations and

visualizations

• Sometimes we work with a single piece of data– E.g. the temperature reading at this point in time.

• But usually we’d work with a large set of data.– E.g. the temperature reading every minute for the last 8

hours.– This works out to 480 pieces of data.

2

StructuresPages 327 to 332

• In Lecture 8, we solved this problem by using arrays.

• BUT arrays can only store data of a single type!– E.g. you can’t store strings in this array:

double mydata[1024];

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Structures

• Fortunately C gives us a way of representing related information of different types, in a “structure.”

• E.g. hurricanes have names, intensities and categories.– Name: This is a string.– Intensity: Integer.– Category: Intger.

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Structures

struct hurricane{

char name[10];int year, category;

};

• “name”, “year” and “category” are called “members” of the structure “hurricane”

• This statement declares a new type!– IT DOES NOT DECLARE A NEW VARIABLE!

This “;” is very important and often forgotten!

5

Self Help Questions

1. What are the differences between a “variable” and a “type”?

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Answer

1. A “type” tells C what the variable is. E.g. “int” is a type. “double” is a type. After declaring our structure “hurricane”, “struct hurricane” is also a type! Types do not occupy memory and cannot store values.To declare a variable, we always use “type varname;”. So to declare an integer variable, we use “int xl;”. This tells C that “x1” is a variable of type “int”. Variables occupy memory and can be assigned values.

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Structure Variables

• To declare a new variable of the “hurricane” type, we do:

struct hurricane h1;

?

?

?

h1name

year

category

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Structure Variables

• You can declare multiple variables of this type:

struct hurricane h1, my_hurricane;

?

?

?

h1name

year

category

?

?

?

my_hurricane

name

year

category

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Structure Variables

• You can also initialize the members of a structure variable.

struct hurricane h1 = {“Camille”, 1969, 5};

Camille

1969

5

h1name

year

category

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Structure Variables

• Alternatively you can initialize the members using C statements:struct hurricane h1;

h1.name = “Camille”;

h1.year = 1969;

h1.category = 5;

• Notice how we used the “.” operator to access members of a structure variable!

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Structure Example

• We can use scanf or fscanf statements to write to structure variable members.

• We can use printf or fprintf to read them.

• We will now write a program that reads hurricane information from a file, and prints them out.

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Structure Example

#include <stdio.h>

#define FILENAME "storms2.txt"

struct hurricane

{

char name[10];

int year, category;

};

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Structure Example

int main(void)

{

/* Declare variables */

struct hurricane h1;

FILE *storms;

storms = fopen(FILENAME, "r");

if(storms == NULL)

printf("Error opening data file.\n");

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Structure Example

else{

while(fscanf(storms, "%s %d %d", h1.name, &h1.year, &h1.category) == 3)

{printf("Hurricane: %s\n", h1.name);printf("Year: %d, Category: %d\n",

h1.year, h1.category);}fclose(storms);

} /* else */return 0;

}

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Self Help Questions

1. In the fscanf statement:fscanf(storms, "%s %d %d", h1.name,

&h1.year, &h1.category)

- h1.year and h1.category are both passed to fscanf with “&”, but not h1.name. Why?

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ANSWERS

• The “name” member of “hurricane” is defined as char name[10];struct hurricane

{

char name[10];

int year, category;

};

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ANSWERS

• This makes “name” an array of 10 characters.• In an array:

– The name of the array is a pointer to the first item of the array.

• “fscanf” expects pointers to the variable arguments. E.g.:

fscanf(fptr, “%d”, &a);– This is so that fscanf can return the values read using

these variables.– So we use “&” to get the address of the variable.

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ANSWERS

• Since “name” is already a pointer to the first item of the “name” array, there is no need to use “&”. We can just pass in “name” instead of “&name”.– Further question: What does &name give

you?

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Computations with Structures

• We use the “.” operator to access individual members of a structure variable:

h1.name=“Hazel”;

• If we use the structure variable’s name without the “.” operator, then we are accessing the entire array.

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Computations with Structures

• You can do assignments with structures:– Given two structures “h1” and “h2”.

• To make h2 equal to h1, just do:

h2 = h1;

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Computation with Structures

• E.g.struct hurricane h1 ={“Hazel”, 1954, 4}, h2;

………

h2 = h1;

Hazel

1954

4

h1name

year

category

?

?

?

h2name

year

category

Initially:

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Computation with Structures

• E.g.struct hurricane h1 ={“Hazel”, 1954, 4}, h2;

………

h2 = h1;

Hazel

1954

4

h1name

year

category

Hazel

1954

4

h2name

year

category

After h2=h1:

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Computation with Structures

• Other operations like “+”, “-”, “*”, “/”, “>”, “<“, “>=“, “==“ etc. can only be performed on individual structure variable members.

E.g. h1.category += 1; /* Increment */

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Computation Example

• We will modify our hurricane program to print only the Category 5 hurricanes.– Can just do this by:

if(h1.category == 5)

/* Print hurricane data */

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Computation Example

#include <stdio.h>

#define FILENAME "storms2.txt"

struct hurricane

{

char name[10];

int year, category;

};

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Computation Example

int main(void){struct hurricane h1;FILE *storms;

storms = fopen(FILENAME, "r");if(storms == NULL)

printf("Error opening data file.\n");

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Computation Example

else{

printf("Category 5 hurricanes\n\n");while(fscanf(storms, "%s %d %d",

h1.name, &h1.year, &h1.category) == 3)if(h1.category == 5)

printf("%s\n", h1.name);fclose(storms);

} /* else */}

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Passing Structures to FunctionsPages 332 to 334

• Entire structures can be passed into functions.– Structures are passed by value:

• Members of the actual arguments are copied into corresponding members of the formal parameters.

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Passing Structures to Functions#include <stdio.h>

struct hurricane{

char name[10];int year, category;

};

void print_struct(struct hurricane my_h){

printf("INSIDE print_struct function\n");printf("Contents of parameter my_h: %s, %d, %d\n", my_h.name, my_h.year, my_h.category);printf("Address of parameter my_h: %u\n\n", &my_h);

}

Formal parameter declaration for a structure.

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Passing Structures to Functions

int main(void)

{

struct hurricane h1 = {"Hazel", 1954, 4};

printf("\nContents of h1: %s, %d, %d\n", h1.name, h1.year, h1.category);

printf("Address of h1: %u\n", &h1);

printf("Passing h1 in as argument to print_struct\n\n");

print_struct(h1);

}

Declaration for struct variable h1.

Structure variable is passed in just like any other variable.

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Passing Structures by Reference.

• Ordinarily, structures are passed by value.– This means that you cannot modify the

members of a structure from within a function.

• To be able to modify structure members, you must pass in a pointer to the argument, rather than the argument itself.

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Passing Structures by Reference

Self Help Question• If:

– int *ptr declares a pointer to int.– double *ptr declares a pointer to a double.

– char *ptr declares a pointer to a char.

• How do you:– Declare a pointer to a struct hurricane?

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Passing Structures by Reference

#include <stdio.h>

#include <string.h>

/* This program has a function that modifies a structure passed to it */

struct hurricane

{

char name[10];

int year, category;

};

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Passing Structures by Reference

void modify_structure(struct hurricane *hur)

{

/* Modify the structure values */

strncpy(hur->name, "Grace", 10);

hur->year=1972;

hur->category = 5;

}

Pointer to a struct hurricane.

When accessing structure members from a pointer, use “->” instead of “.”.

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Passing Structures by Reference

int main(void){

struct hurricane h1={"Audrey", 1957, 4};

printf("Contents of h1: %s %d %d\n", h1.name, h1.year, h1.category);printf("\nCalling modify_structure to change the contents of h1.\n\n");

modify_structure(&h1);

printf("Contents of h1: %s %d %d\n\n", h1.name, h1.year, h1.category);

}

Pass in the address of h1, instead of h1 itself.

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Passing Structures by Reference

• One point to note:– When accessing members of a structure

variable, we use the “.” operator.– E.g.

struct hurricane my_hurricane;

my_hurricane.year=1852;

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Passing Structures by Reference

• One point to note:– When accessing members of a pointer to a

structure variable, we use the “->” operator.– E.g.

struct hurricane *ptr = &my_hurricane;

ptr->year=1952;– “ptr->year” is a shortcut for (*ptr).year;

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Returning Structures from Functions.

• A function can return whole structures.• To declare a function that returns an int, we do:

int myfun1(){

…}

• To declare a function that returns a char, we do:char myfun2(){

…}

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Returning Structures from Functions.

• To call these functions:char c;

int num;

num = myfun1();

c = myfun2();

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Returning Structures from Functions.

• Likewise, to declare a function that returns struct hurricane:struct hurrican myfun3(…)

{…

}

• To call myfun3:struct hurricane my_h;

my_h = myfun3();

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Example

#include <stdio.h>

struct tsunami{int mo, da, yr, fatalities;double max_height;char location[20];

};

struct tsunami get_info(void);

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Examplestruct tsunami get_info(void){

/* Declare variables */struct tsunami t1;

printf("Enter information for tsunami in the following order:\n");printf("Enter month, day, year, number of deaths:\n");scanf("%d %d %d %d", &t1.mo, &t1.da, &t1.yr,

&t1.fatalities);printf("Enter location (<20 characters.):\n");scanf("%s", t1.location);

return t1;}

t1 will temporarily store what the user keys in.

t1 is returned here.

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Example

int main(void){

struct tsunami my_t;

my_t = get_info();

printf("\nThe information you entered:\n\n");printf("Date (day/month/year): %d/%d/%d\n",

my_t.da, my_t.mo, my_t.yr);printf("Location: %s\n", my_t.location);printf("Fatalities: %d\n\n", my_t.fatalities);

return 0;}

We declare a variable of type struct tsunami, the exact same type as the function get_info.

We call get_info just like any other function.

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Arrays of Structures(Pages 334 to 336)

• Refresher:– To declare an array of 20 ints:

int my_array1[20];

– To declare an array of 20 doubles:double my_array2[20];

– To declare an array of 20 chars:char my_array3[20];

• Question: How do we declare an array of “struct hurricane”?

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Array of Structures

• Assuming we have already declared “struct hurricane”, do declare an array of 20 “struct hurricane”:

struct hurricane my_array4[20];

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Array of Structures

• This creates 20 “struct hurricane” variables, each with:– name– year– category

• All values are un-initialized.

? ? ?

? ? ?

? ? ?

? ? ?

… … …

? ? ?

name year category

0

1

2

3

19

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Array of Structures

• Refresher: – To access individual ints in my_array1:

• myarray1[2] = 5;

– To access individual doubles in my_array2:• myarray2[5] = 3.1;

– To access individual chars in my_array3:• myarray3[2] = ‘c’;

• So:– How to we access individual struct variables?

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Array of Structures

Answer:

my_array4[0].name = “Camille”;

my_array4[0].year = 1969;

my_array4[0].category = 5;

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Example

/* Program ch7_4.c */

#include <stdio.h>#define FILENAME "storms2.txt"

struct hurricane{

char name[10];int year, category;

};

void print_hurricane(struct hurricane h);

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Exampleint main(void){

int max_category=0, k=0, npts;struct hurricane h[100];

FILE *storms;

storms = fopen(FILENAME, "r");if(storms == NULL)

printf("Error opening data file.\n");else{

printf("Hurricanes with Maximum Category:\n");

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Examplewhile(fscanf(storms, "%s %d %d", h[k].name,

&h[k].year, &h[k].category) == 3){

if(h[k].category > max_category)max_category = h[k].category;

k++;} /* while */npts = k;

for(k=0; k<npts; k++)if(h[k].category == max_category)

print_hurricane(h[k]);

fclose(storms);} /* else */

}

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Example

void print_hurricane(struct hurricane h)

{

printf("Hurricane: %s\n", h.name);

printf("Year: %d, Category: %d\n", h.year, h.category);

}

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Case Study(Pages 336 to 340)

• Tsunamis are large destructive waves:– Caused by sudden movements in the sea

floor.

• Wave speeds:– Shallow water: 125 mph.– Deep water: 400 mph.

• Heights:– 30 to 200+ feet high.

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Case Study

• Problem Statement: Print a report giving:– Maximum wave height for tsunamis in a data

file.– Average wave height.– Location of all tsunamis with heights higher

than the average.

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Case Study

• Analysis:– Open file “waves2.txt”

• Tell the user if the file open fails.

– Find largest maximum height.• Set max to 0 first.• Assume the tsunami array is called t:

– If t[k].max_height > max, set max = t[k].max_height.

– Find average• Add t[k] to sum.• Average = sum / # of data points

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Case Study

– Print locations with wave heights larger than the average:

• if t[k].max_height > average, print t[k].location.

• Note: Height data in file is in meters. Report is in feet:– 1 m = 3.28 feet.– z m = z * 3.28 feet.

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Case Study

/* ch7_5.c */

#include <stdio.h>#define FILENAME "waves1.txt"

struct tsunami{

int mo, da, yr, fatalities;double max_height;char location[20];

};

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Case Study

int main(void){

/* Declare variables */int k=0, npts;double max=0, sum=0, ave;struct tsunami t[100];

FILE *waves;

waves = fopen(FILENAME, "r");if(waves == NULL)

printf("Error opening data file.\n");

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Case Study

else{

while(fscanf(waves, "%d %d %d %d %lf %s", &t[k].mo, &t[k].da, &t[k].yr,&t[k].fatalities, &t[k].max_height, t[k].location) == 6)

{

sum += t[k].max_height;

if(t[k].max_height > max)max = t[k].max_height;

k++;} /* while */

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Case Study

npts = k;

ave = sum/npts;

printf("Summary Information for

Tsunamis\n");

printf("Maximum wave height (in feet): %.2f\n", max * 3.28);

printf("Average wave height (in feet): %.2f\n", ave * 3.28);

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Case Study

printf("Tsunamis with greater than average heights:\n\n");

for(k=0; k<npts; k++)if(t[k].max_height > ave)

printf("%s\n", t[k].location);

fclose(waves);} /* else */

}

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Summary

• In this lecture we learnt:– How to aggregate data together in structures.– How to pass structures to functions.– How to return structures in functions.– How to declare arrays of structures.