Week 1 C⚓︎

Syllabus⚓︎

C. Source Code. Machine Code. Compiler. Correctness, Design, Style. Visual Studio Code. Syntax Highlighting. Escape Sequences. Header Files. Libraries. Manual Pages. Types. Conditionals. Variables. Loops. Linux. Graphical User Interface (GUI). Command-Line Interface (CLI). Constants. Comments. Pseudocode. Operators. Integer Overflow. Floating-Point Imprecision.

Notes⚓︎

Machine Code⚓︎

C

#include<stdio.h> //standard io.h

int main(void) //void -no input
{
    printf("hello world\n")
}

The computer can only recognize binary numbers, so we need a compiler（编译器）to translate a higher-level language into a lower-level language.

![[compiler.svg]]

Use a cloud environment (best) URL, or you can use VS Code on your computer.

Some words you should know: GUI CLI

Hello world⚓︎

C

#include<stdio.h>
//pronounced "include standard io.h"

int main(void)
{
    printf("hello world\n")
}

Some little concepts:

#include stdio.h - include standard io.h curly braces - 大括号{} printf - F means formatted semicolon - 分号； syntax - 句法

In the terminal

Bash Session

$ code hello.c
$ make hello
// not need to input hello.c make can look the folder
// make to compile the file from source code to machine code
$ ./hello

and get the 
hello world$(\n)

Library (Header Files)⚓︎

We can use code others have written before via a library. For example, we can find stdio.h here: Manual Pages

C

printf("hello, %s\n", answer)
// %s means a place holder

Case 1

C

#include <stdio.h>

int main(void)
{
    string answer = get_string("What's your name?");
    printf("hello, &s\n", answer);
}

Error: get_string is not declared.

Case 2

C

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    string answer = get_string("What's your name?");
    printf("hello, &s\n", answer);
}

There are so many cases here, and we can design the programs so that they can be more efficient.

Terminal Commands⚓︎

We can learn more about them by Googling.

In the terminal window, some common command-line arguments we may use include:

cd, for changing our current directory (folder)
cp, for copying files and directories
ls, for listing files in a directory
mkdir, for making a directory
mv, for moving (renaming) files and directories
rm, for removing (deleting) files
rmdir, for removing (deleting) directories

Types of variables⚓︎

Types with which you might interact during this course include:

bool, a Boolean expression of either true or false
char, a single character like a or 2
double, a floating-point value with more digits than a float
float, a floating-point value, or a real number with a decimal value
int, integers up to a certain size, or number of bits
long, integers with more bits, so they can count higher than an int
string, a string of characters

The following examples translate Scratch code into C.

Conditionals⚓︎

Some tips: Don't perform unnecessary operations. Focus on how to design a better code structure.

C

if (x < y)
{
    printf("x is less than y\n");
}
else if (x > y)
{
    printf("x is greater than y\n");
}
else
{
    printf("x is equal to y\n");
}

But don't use three ifs, because it will waste time.

Loop⚓︎

Motivation: We want to make VS Code meow like Scratch. How should we do it? Answer: We can make a loop to reduce our code.

C

#include <cs50.h>
#include <stdio.h>

void meow(int n);

int main(void)
{
    int n;
    do
    {
        n = get_int("Number: ");
    }
    while (n < 1);
    meow(n);
}

// Meow some number of times
void meow(int n)
{
    for (int i = 0; i < n; i++)
    {
        printf("meow\n");
    }
}

Comments
Typically, each comment is a few words or more, providing the reader an opportunity to understand what is happening in a specific block of code. Further, such comments serve as a reminder for you later when you need to revise your code.

Functions⚓︎

An inspiring idea is to make the repeating parts into a new function that we can use by including it or writing the function's name.

Here's an example from Mario（循环嵌套）. Notice how printing a row is accomplished through a new function.

C

// Helper function

#include <stdio.h>

void print_row(int width);

//void means no output , int width means we have one input

int main(void)
{
    const int n = 3;
    for (int i = 0; i < n; i++)
    {
        print_row(n);
    }
}

//Row function
void print_row(int width)
{
    for (int i = 0; i < width; i++)
    {
        printf("#");
    }
    printf("\n");
}

Calculator⚓︎

C

// int

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    int dollars = 1;
    while (true)
    {
        char c = get_char("Here's $%i. Double it and give to next person? ", dollars);
        if (c == 'y')
        {
            dollars *= 2;
        }
        else
        {
            break;
        }
    }
    printf("Here's $%i.\n", dollars);
}

Types are very important because each type has specific limits. For example, because of the limits in memory, the highest value of an int can be 4294967295. If you attempt to count an int higher, an integer overflow will result where an incorrect value will be stored in this variable.
The number of bits limits how high and low we can count.
This can have catastrophic, real-world impacts.
We can correct this by using a data type called long.

C

// long

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    long dollars = 1;
    while (true)
    {
        char c = get_char("Here's $%li. Double it and give to next person? ", dollars);
        if (c == 'y')
        {
            dollars *= 2;
        }
        else
        {
            break;
        }
    }
    printf("Here's $%li.\n", dollars);
}

Truncation⚓︎

C

// Division with ints, demonstrating truncation

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    // Prompt user for x
    int x = get_int("x: ");

    // Prompt user for y
    int y = get_int("y: ");

    // Divide x by y
    printf("%i\n", x / y);
}

An integer divided by an integer will always result in an integer in C. Accordingly, the above code will often result in any digits after the decimal being thrown away.

C

// Floats

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    // Prompt user for x
    float x = get_float("x: ");

    // Prompt user for y
    float y = get_float("y: ");

    // Divide x by y
    printf("%.50f\n", x / y);
}

Floating point imprecision illustrates that there are limits to how precise computers can calculate numbers.
As you are coding, pay special attention to the types of variables you are using to avoid problems within your code.
We examined some examples of disasters that can occur through type-related errors.

Summing up⚓︎

How to create your first program in C.
How to use the command line.
About predefined functions that come natively with C.
How to use variables, conditionals, and loops.
How to create your own functions to simplify and improve your code.
How to evaluate your code on three axes: correctness, design, and style.
How to integrate comments into your code.
How to utilize types and operators and the implications of your choices.

Problem Set 1⚓︎

Hello, It’s Me ⚓︎

C

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    string name = get_string("What's your name?");
    printf("hello, %s\n",name);
}

Mario-more,⚓︎

C

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    int height;
    do
    {
        height = get_int("Height= ");
    }
    while (height < 1 || height > 8);
    int row = 0;
    while (row < height)
    {
        for (int i = 0; i < height - row - 1; i++)
        {
            printf(" ");
        }
        for (int j = 0; j < row + 1; j++)
        {
            printf("#");
        }
        printf("  ");
        for (int j = 0; j < row + 1; j++)
        {
            printf("#");
        }
        row++;
        printf("\n");
    }
}

Cash,⚓︎

C

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    int changeOwed;
    do
    {
        changeOwed = get_int("Change owed: ");
    }
    while (changeOwed < 0);

    int quarters;
    int dimes;
    int nickels;
    int pennies;

    quarters = changeOwed / 25;
    changeOwed = changeOwed - quarters * 25;
    dimes = changeOwed / 10;
    changeOwed = changeOwed - dimes * 10;
    nickels = changeOwed / 5;
    changeOwed = changeOwed - nickels * 5;
    pennies = changeOwed;

    int k = quarters + dimes + nickels + pennies;
    printf("%i\n", k);

}

Credit,⚓︎

C

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    long checknum;
    // get a number
    checknum = get_long("Number: ");
    // 最终求和变量
    int checksum = 0;
    // 指示变量
    int digit = 1;

    // checksum part
    int reminder;
    int save1 = 0;
    int save2 = 0;
    while (checknum > 0)
    {
        save2 = save1;
        save1 = 0;
        reminder = 0;
        if (digit % 2 == 0)
        {
            save1 = checknum % 10;
            reminder = checknum % 10 * 2;
            checknum /= 10;
            if (reminder >= 10)
            {
                reminder = reminder % 10 + 1;
                checksum += reminder;
            }
            else
            {
                checksum += reminder;
            }
        }
        else
        {
            save1 = checknum % 10;
            checksum += checknum % 10;
            checknum /= 10;
        }
        digit += 1;
    }
    digit -= 1;

    // claim
    if (checksum % 10 == 0)
    {
        if (save1 == 3 && digit == 15)
        {
            if (save2 ==4 || save2 == 7)
            {
                 printf("AMEX\n");
            }
            else
            {
                printf("INVALID\n");
            }
        }
        else if (save1 == 5 && digit == 16)
        {
            if (save2 >=1 && save2 <= 5)
            {
                 printf("MASTERCARD\n");
            }
            else
            {
                printf("INVALID\n");
            }
        }
        else if (save1 == 4 && (digit ==13 || digit == 16))
        {
            printf("VISA\n");
        }
        else
        {
            printf("INVALID\n");
        }
    }
    else
    {
        printf("INVALID\n");
    }
}