CS- Week 4

Pixels

Pixels are the smallest colored dots that make up an image.
If we imagine the image as consisting of zeros and ones, the zeros represent black and the ones represent white:

RGB (Red, Green, Blue) are numbers representing the amount of each color. In Adobe Photoshop we see these settings:

We can see in the image above how the selected amount of red, green and blue changes the color. We can also see in this image that color is not represented by only three values, but by a value made up of special numbers and symbols. For example, the value 255 is represented as FF.

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Hexadecimal

Hexadecimal number system is a number system based on writing numbers using only 16 characters. They are as follows:

0 1 2 3 4 5 6 7 8 9 A B C D E F

In the hexadecimal number system, each column represents 16 levels.
0 – 00 as
1 – 01 as
9 – as 09
10 – as 0A
15 – as 0F
16 – 10 as
255 is expressed as FF because 16 x 15 (or F) equals 240 plus 15 to make 255. This is the largest two-digit number that can be represented in hexadecimal.

The hexadecimal number system allows to express data in a shorter form. Therefore, it is convenient to express information more compactly.

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Memory

If we number memory blocks using the hexadecimal number system, we can imagine them as follows:

It can be difficult to determine whether the block 10 in the figure represents a memory location or the value 10. Therefore, all hexadecimal numbers are usually represented by the prefix 0x:

We assign the value 50 to the integer variable n:

#include <stdio.h>

int main(void)
{
    int n = 50;
    printf("%i\n", n);
}

How the program stores this value in memory can be visualized as follows:

The C language has the following memory manipulation operators:

• & – Gives the address of a value in memory.
• * – Tells the compiler to go to memory location.

If we want to know the memory address of our n learner, we can change our above code as follows:

0 1 2 3 4 5 6 7 8 9 A B C D E F

%p – lets you see the address of a memory location. And &n returns the address of variable n in memory starting with 0x when we run the code.

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Pointers

Pointer is a variable that stores the address of this value in computer memory.

#include <stdio.h>

int main(void)
{
    int n = 50;
    printf("%i\n", n);
}

Where p is a pointer that contains the address of an integer n.

Pointers are usually stored as 8-byte values. p is storing the address of the value 50 in the above image.
We can think of a pointer as an arrow pointing from one location in memory to another:

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String

String is simply an array of characters. For example, the string s = "HI!" can be represented in computer memory as:

A pointer called

s tells the compiler where the first byte of the given value is located:

We can create a

string variable as follows:

#include <stdio.h>

int main(void)
{
    int n = 50;
    printf("%p\n", &n);
}

The code above prints an array of characters starting at position s.

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String Comparison

We compare variable values ​​of

string data type with each other:

int n = 50;
int *p = &n;

In our code above, we give our variables s and t the same "Hi!" even if we give the value "Different" message appears on the screen as a result.
To find out why this happens, we can give the same value to our variables s and t and represent them in computer memory as follows:

So the above code is actually trying to compare the memory locations of the variables s and t, not their values.

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Copying

Let the following code be given:

#include <stdio.h>

int main(void)
{
    char *s = "HI!";
    printf("%s\n", s);
}

In

string t = s, the address of s is copied to t. This does not produce the result we want because the value is not copied - only its address is copied.

s and t are pointing to the same memory blocks. We couldn't copy its value from s to t, instead they became two pointers pointing to a single string.

malloc - allows the programmer to allocate a block of memory of a certain size.
free – asks the computer to free the previously allocated memory block.
We change the code to create a real copy:

0 1 2 3 4 5 6 7 8 9 A B C D E F

malloc(strlen(s) 1) - Adds one to the length of the variable s and allocates space for its character. Then, through the for loop, the values ​​of s are copied to t.

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Garbage Values

If we ask the compiler for a block of memory, there is no guarantee that this memory will be free. The allocated memory may have been used by the computer before, so there is a chance that there are unwanted values:

#include <stdio.h>

int main(void)
{
    int n = 50;
    printf("%i\n", n);
}

When we run this code, 1024 memory locations are allocated for the array, but when we display the values ​​of the array elements using a for loop, we see that not all of them are 0.

Whenever we ask the computer to allocate memory for a variable, it is good practice to initialize it to 0 or some other value.

This article uses CS50x 2024 source.

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