Chapter four: A new loop

Today, I'll talk about one of the most important structures in programming:

This series is taking a 24 hour break. Yes, no post on Wednesday.

The loop

You know what a loop is in real life, right?

Good.

A loop in programming does the same thing, except it does it based on a boolean value.

Loop #1: While.

This loop has no magic. Just does the code inside {} while bool is true:

while (bool) {

}

For example:

while (true) {
    printf("Linux is better\n");
}

Will output:

Linux is better
Linux is better
Linux is better
Linux is better
Linux is better
Linux is better
Linux is better
Linux is better
Linux is better
Linux is better
...

Let me introduce you to a new function, getchar().
This reads a char from STDIN and returns it.

So let's make a program that reads input:

#include <stdio.h>

int main()
{
    char c;
    while ((c = getchar()) != EOF) 
    {
        printchar(c);
    }
}

This introduces a few new concepts.

When you put an assignment in (), you can use it to compare.

So

char c = 65;
while (c != EOF)  
{
    printchar(c);
    c = getchar()
}

is equal to

char c;
while ((c = getchar()) != EOF) 
{
    printchar(c);
}

Loop #2: For loops

These are a little more complex.

They have the syntax

for (assignment; bool; change){}

The assignment is usually int i = 0.

This sets i to 0;

The bool is a boolean, usually dealing with i.

For example: i < 10.

And change is how to change the variable.

For example: i++.

Now is a ripe time to introduce to two new operators:

++

This changes tha double/int by one.

There are two forms:

• ++i
• i++

What's the difference?

Well, i++ adds one to i, but returns i.

For example:

int i = 1
printf(i++);
printf(i)

Will say:

1
2

While ++i will say:

2
2

Now, back to the for loop:

To print all the numbers between 1-100, you would use:

for (int i = 0; i < 100; i++) // Needs 0
    printf(i);

Loop #3: Do while.

This loop is a bit overlooked.

To know why this loop exisits, we have to look a little close at the while loop.

If you have this code:

int i = 0;
while (i != 0) {
    super_important();
}

Will it run. No. (We can't tell if it will halt, though.)

However, if you use a d-w (do-while), it will run.

The D-W syntax is:

do {


} while (bool)

Again, bool is a boolean condition.

So you can complete your homework, I need to teach you one more thing:

The % operator.

This computes the remainder between two numbers.

It's commonly used to check if a number is even.

if (num % 2 == 0)

Homework

Now, make a program that computes the Collatz conjucture

The Collatx conjucture states:

If a number is even, divide it by two.
If not, triple it and add one.
If you do this enough, it will become one.

Impliment this in C.

Please progmpt the user for input and print out the number after each cycle:

8
42
1

or

17
52
26
13
40
20
10
5
16
8
4
2
1

So long!
Part One: The Segfault menace

Part Two: The data wars

Part Three: Revenge of the if

Part Four: A new loop

Part Five: The Empire points back

Part Six: Return of the function