COMP1511

Static arrays

Week 3 Lecture 2

functions/procedures recap

Reusable blocks of code
Callable multiple times
variables within a function are scoped to that function

PI function

Would be annoying to write this every time we need to calculate!

double pi() {
  double sum = 0.0;
  for (int i = 0; i < 1000; i++) {
    sum += (-1.0) * pow(1.0 / 2.0, i) / (i + 1);
  }
  return 4.0 * sum;
}

Forward declaration

int main(void) {
    double calculated_pi = pi();
}

double pi() {
  double sum = 0.0;
  for (int i = 0; i < 1000; i++) {
    sum += (-1.0) * pow(1.0 / 2.0, i) / (i + 1);
  }
  return 4.0 * sum;
}

^ problem! main doesn't know that pi exists yet!

Forward declaration

double pi();

int main(void) {
    double calculated_pi = pi();
}

double pi() {
  double sum = 0.0;
  for (int i = 0; i < 1000; i++) {
    sum += (-1.0) * pow(1.0 / 2.0, i) / (i + 1);
  }
  return 4.0 * sum;
}

^ Solved! We forward declared pi!

Quick functions recap demo

Arrays

So far, we can store a single item in each variable

What if you wanted to store many values?

Number of ice creams eaten

int day_1 = 2;
int day_2 = 3;
int day_3 = 3;
int day_4 = 5;
int day_5 = 7;
int day_6 = 1;
int day_7 = 3;
// Any day with 3 or more scoops is too much! 
if (day_1 >= 3){
    printf("Too much ice cream\n"); 
}
if (day_2 >= 3) {...

Seem repetitive?

Many variables would clutter the program
Many variables would not always be efficient

Data structures

Are common structures (not structs) used to store multiples of data
- Usually (especially in COMP1511) of the same data type
Can scale, easily storing a handful, up to thousands, or more elements of data!

Data structures in COMP1511

We will look primarily at two data structures:

arrays (today)
linked lists (future)
These are very, very powerful data structures you will use forever

Arrays

A collection of data, all of the same type. (homogonous)
We have a single identifier for the entire array
It is a random access data structure, meaning we can access any element in the array at any time

Arrays

We can ready or modify individual elements
It is a contiguous data structure

contigu-what?

Let's visualise arrays

Static arrays have a set size

(which you specify)

int array

This int array will store 5 integers
32bit * 5 elements = 160 bits of memory used

The array declaration syntax

int ice_cream_per_day[7];

Declare + initialise

int ice_cream_per_day[7] = {3, 2, 1, 2, 1, 3, 5};

^ Note you can only do this when you declare, not later!

int ice_cream_per_day[7] = {};

^ Will initialise all elements to 0

int ice_cream_per_day[7] = {3, 2, 1, 2, 1, 3, 5};

Creates:

Accessing elements

int first_day_ice_creams = ice_cream_per_day[0];

Writing elements

ice_cream_per_day[0] = 5;

arrays 🤝 loops

The power of arrays

int ice_cream_per_day[7] = {3, 2, 1, 2, 1, 3, 5};

// read each element
ice_cream_per_day[0];
ice_cream_per_day[1];
ice_cream_per_day[2];
ice_cream_per_day[3];
ice_cream_per_day[4];
ice_cream_per_day[5];
ice_cream_per_day[6];

^ Does this look repetitive?

If only we had a way to count :(

Bad

int ice_cream_per_day[7] = {3, 2, 1, 2, 1, 3, 5};

// read each element
printf("%d\n", ice_cream_per_day[0]);
printf("%d\n", ice_cream_per_day[1]);
printf("%d\n", ice_cream_per_day[2]);
printf("%d\n", ice_cream_per_day[3]);
printf("%d\n", ice_cream_per_day[4]);
printf("%d\n", ice_cream_per_day[5]);
printf("%d\n", ice_cream_per_day[6]);

Good

int ice_cream_per_day[7] = {3, 2, 1, 2, 1, 3, 5};

int i = 0;
while (i < 7) {
    printf("%d\n", ice_cream_per_day[i]);
    i++; // i = i + 1;
}

Demo

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