DEV Community: Dave Mcsavvy

What Are You Pointing At???

Dave Mcsavvy — Wed, 21 Sep 2022 06:37:11 +0000

If you really want to kick ass in C, you need to understand how C handles memory

The C language gives you a lot more control over how your program uses the computer memory, let's go behind the scenes and see what happens every time you assign, modify or use a variable.

C code inludes pointers

To best understand pointers, go slowly

Pointers are one of the most fundamental things to understand in
the C programming language. So what’s a pointer? A pointer is
just the address of a piece of data in memory.

Pointers are useful for the following reasons.

Instead of passing around a whole copy of the data, you can just pass a pointer to the original data.
You might want two pieces of code to work on the same piece of data rather than a separate copy.

To illustrate, look at the following code

The program starts with n as 100, so if n is increased by 1 it would now be 101. At least it would be if the code works

Look at the code carefully. Do you think it will work? Why? Why not?

> gcc addone.c -o addone
> ./addone
the value of n is 100
the value of n is now 100

Hmmm, why didn't our function work?

C sends arguments as values

The code broke because of the way C calls functions.

Initially the main() function has a local variable called n that had value 100
When the program calls the add_one() function, it copies the value of n from main() to add_one(). This means that even though they look like the same variables, they are not because when you call a function, you don't send the variable as an argument, just it's value.
When the add_one() function changes the value of n, the function is just changing its local copy. This means when the computer returns to the main() function, the n variable still has its original value of 100

But if that's how C calls functions, how can you ever write a function that updates a value?
It's easy if you use pointers...

Using memory pointers

1. Get the address of a variable

You can find where a variable is stored in memory using the & operator.

int x = 4;
printf("x lives at %p\n", &x);

But one you've got the address of a variable, you may want to store it somewhere. To do that, you will need a pointer variable. A pointer variable is just a variable that stores a memory address. When you declare a pointer variable, you need to say what kind of data is stores at the address it will point to.

int *address_of_x = &x;
printf("x lives at %p\n", address_of_x);

2. Read the contents of an address

When you have a memory address, you will want to read the data that's stored there. You do that with the * operator

int value_stored = *address_of_x;

The * and & operators are opposites. The & operator takes a piece of data and tells you where it's stores. The * operator takes an address and tells you what's stored there.

Because pointers are sometimes called referrences, the * operator is said to dereference a pointer.

3. Change the contents of an address

If you have a pointer variable and you want to change the data at the address where the variable's pointing to, you can just use the * operator again. But this time you need to use it on the left side of an assignment

*address_of_x = 99;

OK, now that you know how to read and write the contents of a memory location(pointer), it's time for you to fix the add_one() function.

Solution

> gcc addone.c -o addone
> ./addone
the value of n is 100
the value of n is now 101

The code works.

Because the function takes a pointer argument, it’s able to
update the original n variable.
That means that you now know how to create functions that
not only return values, but can also update any memory
locations that are passed to them.

BULLET POINTS

Variable are allocated storage in memory
Pointers are just variables that store memory addresses
The & operator finds the address of a variable
The * operator can read the contents of a memory address
The * operator can also set the contents of a memory address

Understand chars in 5 mins or less

Dave Mcsavvy — Tue, 13 Sep 2022 01:28:01 +0000

What is the char type in C and how should it be used.

Before we dive into these, let's have a quick glance at C types.

Types In C

Types are useful to us because they allow us to handle values in a specific way.
In the backend though, C uses types to determine how much memory to allocate for a given value.
C is very particular about memory and it needs to know the type of every value.

The Char type

This special type refers to printable and control character - basically every key on your keyboard!

Declaring Characters

The syntax for declaring a variable as a char looks like this

char character = 'Z';

Note that single quotes '' are used instead of double quotes ""

Under the hood, all char type translate to an integer. So you declare character like this

char c = 48

char c = '0'

You can view the a table of all characters and their integer representation here

Char Operations

C provides a library for handling characters in an exciting way 🔥
You can use that library by including the ctype.h library at the top of your script like this

#include <ctype.h>

Here are a few utilities the library gives you for free...

check is a character is lower case

char c = 'a';

if (islower(c))
    // do something

islower returns 1 if a character is lower case, and 0 if it's not.

check is a character is upper case

char c = 'A';

if (isupper(c))
    // do something

isupper returns 1 if a character is upper case, and 0 if it's not.

check is a character is an alphabet

char c = 'A';

if (isalpha(c))
    // do something

isalpha returns 1 if a character is an alphabet, and 0 if it's not.

check is a character is a digit

char c = '4';

if (isdigit(c))
    // do something

isdigit returns 1 if a character is a decimal number, and 0 if it's not.

check is a character is a digit or alphabet

char c = 'y';

if (isalphanum(c))
    // do something

isalphanum returns 1 if a character is a digit or alphabet, and 0 if it's not.

Bet you can guess what these functions do

ispunc
isspace

You can also convert characters to uppercase and lowercase using toupper and tolower respectively.

char c = 'A';

// Switch case
if (isupper(c))
    c = tolower(c);
else if (islower(c))
    c = toupper(c));

Spoilers
Emojis are also chars!

char damn = '🥵'

Meet Betty, The C code linter

Dave Mcsavvy — Mon, 12 Sep 2022 08:34:31 +0000

I need to make this clear - "betty frustrates me!".

Over the time though, I've learnt to avoid her and I'll tell you how, but first...

Who Is Betty

Betty is actually a very real person 🤯, according to Wikipedia...

Frances Elizabeth Holberton was an American computer scientist who was one of the six original programmers of the first general-purpose electronic digital computer, ENIAC. The other five ENIAC programmers were Jean Bartik, Ruth Teitelbaum, Kathleen Antonelli, Marlyn Meltzer, and Frances Spence.

But seriously, this isn't the Betty we'll be talking about today.
Betty is a Holberton-style C code checker written in perl.

Now there's some explanation needed for the above sentence, so I'll start with...

Holberton

Holberton is a computer science school founded in silicon valley (that's all you get 😴)

Code Checker

Also known as linter.
Lint, or a linter, is a static code analysis tool used to flag programming errors, bugs, stylistic errors and suspicious constructs.

This means that Betty does not have to run or compile your C code before she detects potential errors and wrong styles.

Perl

It's just another programming language like C.
Hold on! So Betty is basically a program in one language trying to detect errors in another language 🤦🏾‍♂️

Now that you know Betty, how do you use Betty?

I'm assuming you already have Betty installed. If you don't, checkout the installation guide here

How to summon Betty

The usage is basically just

betty FILE...

Where FILE... means 1 file or more.
Once you run Betty like this, Betty would enter each file you specified and do a series of checks. Here is a list of the things Betty checks for

Indentation
Length of each line
Placement of braces {}
Placement of spaces
Naming of variables, functions, structs...
Length of functions 🤨
Commenting
Macros & Enums
Header files

You can find exhaustive information on Holberton-style code here, so you don't fall a victim of Betty as I did 😂😭