Week 2 Lecture 2
Custom Data Types
Last lecture
- Control flow
- conditions
- if/else if/else
- while loops
- scans
Today
- Nested loops
- Custom data types
Live lecture code
if statements recap
if(<condition>) {
do_if_true();
} else if(<second_condition>) {
do_if_second_true();
} else {
do_if_both_false();
}
- A condition is a true/false value (1/0)
- We can execute an expression to calculate the condition
my_age > drinking_age-> will evaluate to true/1 if age is greater thandrinking_age
- Conditions are useful in many places, if statements, while loops, etc.
While loops
while(<condition>) {
do_something_over_and_over();
}
- if true, run the body
- at end of body, check condition again
- if true, run the body...
Nested loops
- Simply having a
whileloop within awhileloop - Each time the outer loop runs, the inner loop runs an entire set (the inner loop runs a lot)
Why are nested loops useful?
Why are nested loops useful?
How can we print something like this?
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
#include <stdio.h>
#define ROWS 5
#define COLUMNS 5
int main() {
int i = 0;
while (i < ROWS) {
int j = 1;
while (j <= COLUMNS) {
printf("%d ", j);
j++;
}
printf("\n");
i++;
}
return 0;
}
What about a half-pyramid?
1
12
123
1234
12345
Custom Data Types
Custom data types
- So far, we have used built-in C data types (int, char, double)
- These store a single item of that type
- What if we want to store a group of related data?
int main(void) {
int my_age = 20;
char initial = 'J';
int UNSW_year = 2;
return 0;
}
^ These three things are related...
We can define our own data types (structures) to store a collection of types
Enter the struct
UNSW_student struct
struct UNSW_student {
int age;
int year_number;
double WAM;
}
To use, we simply say:
struct UNSW_student Jake;
struct (structures)
- Are variables made up of other variable(s)
- They have a single identifier
- Can still access the sub-variables
Defining a struct
struct <struct_name> {
data_type identifier;
data_type identifier;
}
Example
struct UNSW_student {
int age;
int year_number;
double WAM;
}
Defining a struct
struct <struct_name> {
data_type identifier;
data_type identifier;
}
Example
struct UNSW_student {
int age;
int year_number;
double WAM;
}
^ Notice, no values... we are only defining.
Full program example
#include <stdio.h>
struct UNSW_student {
int age;
int year_number;
double WAM;
}
int main(void) {
struct UNSW_student Jake;
return 0;
}
But how do I access the actual data...
the . operator
struct coordinate {
int x;
int y;
}
struct coordinate loc;
loc.x
loc.y
DEMO
Another custom data type
The enum
Imagine I wanted to store days of the week
1. int day_of_week = 1;
2. char day_of_week = 'm';
3. #define MONDAY 1
4. #define TUESDAY 2
The problem
- Have to remember that 1 is Monday
- Could accidentally set 8 to
day_of_week
Enums (the solution)
- Store a range or set of possible values
- Assigns a more meaningful name to state
Syntax
enum enum_name { state_1, state_2, state_3... };
Example
enum weekdays { Mon, Tue, Wed, Thu, Fri, Sat, Sun };
Using enums
#include <stdio.h>
enum weekdays { Mon, Tue, Wed, Thu, Fri, Sat, Sun };
int main(void) {
enum weekdays day;
day = Sat; // <-- this is why enums are useful
return 0;
}
Under the hood
#include <stdio.h>
enum weekdays { Mon, Tue, Wed, Thu, Fri, Sat, Sun }
int main(void) {
enum weekdays day;
day = Sat;
printf("The actual value in day is: %d\n, day);
return 0;
}
Advantages over other approaches
- We provide limitations on the possible values (has to be defined in the enum)
- We give a nice label to values (Sat)
- We don't have to remember that 1 is Monday (or was it 0? 🤔)
- Could use
but these can clutter our code if we have many
struct 🤝 enum
enum student_status { Enrolled, Withdrawn, Leave }
struct student {
enum student_status status;
int age;
}
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