So far

• Welcome and introductions
• Getting started

Today

• Memory
  • Data (variables + constants)
  • Expressions

A brief recap

Out first program

#include <stdio.h>

int main(void) {
    printf("Hello, world\n");
    return 0;
}

How do computers store data?

• Alone, a single bit can't do much...
• What if we group them together?

Different types of memory

• RAM (memory)
  • this is mostly what we care about in 1511
• HDD/SDD (persistant data)
  • Tapes?

A: Variables

• A label for a piece of memory
• "variable" because the value in memory can change
• A certain number of bits required to store that data type
• Stores a specific type of data

To make a variable, you need:

• It's type
• It's name

Some data types

• int -> an integer, a whole number (1, -5, 100)
• char -> a single character ('a', 'V', ' ')
• double -> a floating point number (3.14159)

Each type has different memory requirements

• int -> 32 bits in C, 4 bytes
• char -> 8 bits, 1 byte
• double -> 64 bits, 8bytes

And therefore limits...

• int -> -2,147,483,648 to 2,147,483,647
• char -> -128 to 127 chars
• double -> -2,147,483,648 to +2,147,483,647

Variable names

• Surprisingly important...
• Should describe what it's storing
• You can pick whatever you want (mostly)
• in C, always use lowercase letters
  • name is different to nAme
• seperate words by underscores first_name

int

• A whole number, with no fractions or decimals
• Most commonly uses 32 bits (which is also 4 bytes)
• This gives us exactly 2 different possible values
• Exact ranges from -2147483648 to 2147483647

char

• A char type is used to store a single character
• chars have to be wrapped in single quotes, like: 'a'
• Each char is associated with an integer
• We can convert chars to ints, and back
• 'a' and 'A' are different characters!
• chars are just ints under the hood...

Variables syntax

To declare a variable, you use:

<type> <name>;

• int age;
• char first_initial;
• double pi

Printing variables using printf

• We can print variables to our terminal!
• We describe the format of how we want text printed, then the actual values.
• To print out a variable value, we use format specifiers with printf

• The format specifier (%) indicates WHERE a value will output within the format string.

  int my_age = 13;
  printf("I am %d years!", my_age);
  

• %c for chars
• %d for ints “decimal integer”
• %lf for “long floating point number” (a double)
• printf needs to know what type it should expect in what order, because...

You can have multiple variables:

int diameter = 5;
double pi = 3.141;
printf("The diameter is %d, pi is %lf", diameter, pi);

#include <stdio.h>

int input;
printf("Please enter your age: ");
scanf("%d", &input);

Constants

• A value that will never change
• More efficient to store a constant (less memory)
• Different syntax
• We use UPPERCASE to signify it's a constant

#define <NAME> <value>

#define PI 3.1415

int age = 28;
int current_year = 2023;
int year_born = current_year - age;

printf("You were born in %d", year_born);

chars are just ints playing dress-up

char letter = 'b';
letter = letter + 1;
printf("%c\n", letter);

^^ Will print 'c'

Don't forget your limits

If we add two large ints together, we might go over the maximum value, which will actually roll around to the minimum value and possibly end up negative

• (Check out Ariane 5 explosion), a simple error like this caused a rather large problem: https://www.bbc.com/future/article/20150505-the-numbers-that-lead-to-disaster)

• Boeing 787 had to be rebooted every 248 days (2³¹ -hundredths of a second)
• https://www.engadget.com/2015-05-01-boeing-787-dreamliner-software-bug.html
• In a less destructive example, the video Gangham Style on YouTube maxed out the views counter : https://www.bbc.com/news/world-asia-30288542

Doubles :(

• No such thing as infinite precision
• We can’t precisely encode a simple number like ⅓
• If we divide 1.0 by 3.0, we'll get an approximation of ⅓
• The effect of approximation can compound the more you use them