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Section 1: Standard Input and Output

Output

#include <cstdio>  // C STandarD Input Output (I/O). Gives us printf and scanf
// #... statements are called pre-processor statements
int main()
{
    printf( "Hello, World!\n" );
    scanf( "*" );
    return 0;
}

Input

#include <cstdio>
int main()
{
    int num; // <-- Declaring a variable
    printf( "Type a number: " );
    scanf( "%d", &num );  // & means "address of" i.e. where "num" actually is in the RAM
    printf( " Your number is %d.\n", num );
    return 0;
}

Comments

Comments are ways you can write notes to the person reading the code. You can tell what each section does quickly and easily.

printf("this line will be printed\n"); // This is a comment. It finishes at the end of the line
// Comments are ignored by the compiler, and are the exact same as not typing the comment
// You can use comments to write notes to yourself on the importance of certain sections of code.
printf("this also prints\n");/* This is also a comment.
Except, it does not finish at the end of the line, but at the first */
 
// You can also do this:
printf("%d\n", /*comment*/ number);

Section 2: Control Structures

A control structure is not a statement, but it controls other statements i.e. under what conditions do the controlled statements run? Is it run in a loop while the condition is true?

If statements

if(condition)
{
    printf("the condition is true\n");
}
else
{
    printf("the condition is false\n");
}

For loops

Loops do bits of code over and over again. There are two types of loops: For and While.

for(int i = 0; i < 10; ++i)
{
    printf("This loops from 0 to 10. The loop is currently up to %d\n", i);
}
 
/* Structure of a for loop:
 
for ( initialiser; qualifier; iterator )
{
    body_code;
}
 
Initialiser: Gets done before the loop begins. Usually declaring a variable (int i = 0)
Qualifier: Before each time around the loop, check whether this is true. If it is, do the loop (body_code).
           Otherwise, end the loop.
Iterator: Gets done at the end of the loop. Usually adding one to a variable (i++)
 
for ( A; B; C )
{
    D;
}
 
See diagram: Oval = Statement (i.e. do something); Diamond = Check condition (will be true or false) */

While Loops

Loops do bits of code over and over again. There are two types of loops: For and While.

while(condition)
{
    printf("we are still in the loop");
}
 
// The loop will continue while the condition is true. Afterwards, the program will continue to here.
printf("the loop is over");

Section 3: Arrays and Strings

Arrays

If you're writing a program for, say, a database, you want to have a way of quickly making many, many variables at once.
For example, if you were to write a program which asked for 50 people's ages, you wouldn't want something like this:

int age1;
int age2;
int age3;
...
int age50;

Or even this shorthand version:

int age1, age2, age3, age4, age5, ..., age50;

In this case, you'll need to declare not one number, but a whole array of numbers.
The way we do this is using square brackets:

int age[50];  // Means "make 50 'age' variables"

To access (use) the variables, we use the same square bracket notation:

age[12] = 7;
scanf("%d", &age[45]);
// We can access the variable number based on another value:
int n = 6;
age[n] = 4;
if(age[6] == 4)
    printf("age[6] is 4!\n");
 
// NOTE: With arrays, the numbering starts from 0, not 1. So
// int array[10];
// will make 10 variables called "array", numbered array[0], array[1], array[2] ... up to array[9].
// This means that when you type "int array[10];" There will not be an array[10] variable,
// but there will be 10 array[x]'s from 0 to 9
 
// You should now know enough about the syntax of C to understand what the following code does,
// and how and why it works on a fundamental level.
for(int i = 0; i < 50; i++)
    printf("age[%d] is %d\n", i, age[x]);

Variable types

So far, we've looked at variables as numbers (int), but what about other variables (decimals, true/false variables, text)?
For variables such as decimals or true/false's, it's just as easy:

int num;  // An integer (positive or negative whole numbers)
float decimal;  // A decimal number (works the same way as int)
bool variable;  // A boolean is a variable that is either "true" or "false"
variable = false;
variable = true;
if(variable)
{
    // Do something...
}

But about text? Computer scientists call pieces of text "strings". The best way to do this is to use an array of characters.

Section 4: Random etc.

Random Numbers

#include <cstdio>   // C STndarD Input Output (I/O): printf and scanf
#include <cstdlib>  // C STandarD LIBrary: Used for making random numbers
#include <ctime>    // C Time: for time(NULL) - so numbers are different each time the code runs
 
int main()
{
    srand(time(NULL));  // Change the random "seed" so that a different random number is produced.
    num1 = rand();  // Will be any random positive integer.
    num2 = rand() % 7;  /* % is modulo function ( a % b is the remainder of a / b )
                          so a % b will always be a number between 0 and b-1
                          so rand() % x will always be a number between 0 and x-1
                          (Year 11 Maths C students might have heard of this) */
    num3 = rand() % 10 + 1;  // Will be between 1 and 10 (inclusive)
}

For a list of tasks to test your knowledge, see the C++ page, or the Programming Challenges.

… If anything is missing or unclear, bring it up on the forums.