C Language Programming

All code shows the code inside the main function

Computer thinking

Rolling-Rotating Division

int u = 32;
int v = 32;
//If v=0, at the end of the calculation, u is the greatest common divisor
//v!=0 calculates the remainder of u/v so that u=v=the remainder
while(v!+0)
{
	int temp = u%v;
    u = v;
    v = temp;
}
printf("%d",u);

From Computer to Program to Algorithms

The idea and advantage of computers is to list all the possibilities one by one (enumerate)

Dividing further simplifies operations and improves algorithm efficiency

Program Execution

Interpretation: With a program, that program can try to understand your program and then follow your requirements

Compile: With a program, that program can translate your program into machine language and execute it

Differences between Compilation and Interpretation

• Language is not compiled or interpreted
• Whether a language is compiled or interpreted depends more on how it is commonly executed
• Explanatory languages have special computing power
• Compiled languages have deterministic performance

Case: Find zero

Requirements:

• Have a place to put the number you entered
• Have a way to enter numbers
• The number entered participates in the operation

int price = 0;
printf("Please enter amount (Yuan)");
scanf("%d",&price);
int change = 100 - price;
printf("Find you%d Yuan.\n",change);

optimization

• Easy to modify and maintain
• Reduce meaningless numbers

int price = 0;
const int AMOUNT = 100;
printf("Please enter amount(element)");
scanf("%d",&price);
int change = AMOUNT - price;
printf("Find you%d Yuan.\n"，change);

const

Const is a modifier, preceded by int, const modifier variables cannot be modified once initialized, and can be re-optimized to allow the user to enter the value of that variable instead of a fixed 100

variable

The general form of variable definition is: <type name><variable name>;

In mathematics a=b and b=a have exactly the same meaning, but they have the opposite meaning in programming

All variables should be assigned (initialized) before they are first used

<Type Name><Variable Name>=<Initial Value>;

About scanf

scanf("%d",&price);

Ask the scanf function to read in the next integer and assign the result to price

Time-formatted string in double quotes,%d for reading integers

Beware of &before price, add &before scanf variable

Case: Calculating time difference

int hour1,minute1;
int hour2,minute2;
scanf("%d %d",&hour1,&minute1);
scanf("%d %d",&hour2,&minute2);
int t1 = hour1*60 + minute1;
int t2 = hour2*60 + minute2;
int t = t1 + t2;
printf("The time difference is%d hour%d branch"，t/60,t%60);
//Use t/60 to abstract hours and t%60 to abstract minutes

Case study: average two integers

int a,b;
scanf("%d %d",&a,&b);
//Floating-point numbers are used to represent all numbers with decimal points. When floating-point numbers and integers are combined, c converts integers to floating-point numbers and then performs floating-point operations.
double c = (a+b)/2.0;
printf("%d and%d Average value is%lf"，a,b,c);

data type	Chinese Name	Format string
double	Double	"%lf"
float	Single-precision floating-point	"%f"
int	integer	"%d"

Case study: swapping two variables

int a = 5;
int b = 6;
int t = 0;
t = a;
a = b;
b = t;
printf("a=%d b=%d",a,b);

Compound Assignment

+= -= *= /= %= ++ –

These symbols are because the c language inherits the machine language of the past

count ++;
count += 1;
count = count + 1;

All three of them mean one plus one

A++ denotes adding a previous value

++ a denotes the value after adding one

But after these two operations, the next line, a, is the added value

Judgement Operation

/*if(Conditions are true){
	...;
}*/
// ==Equal!=Inequality>Greater than>Greater than or equal to<Less than<=Less than or equal to
//When the relationship between two values meets the expectation of the relational operator, the result of the relational operation is 1, otherwise 0
printf("%d\n",5==3);
printf("%d\n",5>3);
printf("%d\n",5<3);
//All relational operators have lower priority than arithmetic but higher priority than assignment
7 >= 3+4;
int r = a>0;
//Determine if equal priority is lower than others
//Continuous relational operations run from left to right

Case: Find Zero Calculator

//Initialization
int price = 0;
int bill = 0;
//read in data
printf("Please enter amount");
scanf("%d",&price);
printf("Please enter the face");
scanf("%d",&bill);
printf("You should be found:%d\n",bill-price);

But the program on top of us can't tell if we have enough money, that is, we can't tell if we have enough par space.

We make a logical judgment to optimize the program

//Initialization
int price = 0;
int bill = 0;
//read in data
printf("Please enter amount");
scanf("%d",&price);
printf("Please enter the face");
scanf("%d",&bill);
if(bill>=price)
{
	printf("You should be found:%d\n",bill-price);
}
else
{
	printf("Not enough money:%d\n",price-bill);
}

Case: Size of comparison number

Case 1: Ratio of two numbers

int a,b;
printf("Please enter two positive integers:");
scanf("%d %d",&a,&b);
int max = 0;
if(a>b)
{
	max = a;
}
else
{
    max = b;
}
printf("%d and%d The maximum value is%d",a,b,max);

Case 2: Ratio of three numbers (if nested method)

int  a,b,c;
scanf("%d %d %d",&a,&b,&c);
int max = 0;
if(a>b)
{
	if(a>c)
    {
		max = a;
    }
    else
    {
		max = c;
    }
}
else
{
	if(b>c)
    {
		max = b;
    }
    else
    {
		max = c;
    }
}
printf("Maximum is%d",max);

Case 2 can also be solved by other methods, such as bubble sorting, etc.

Programming habits tips

• Else always matches its nearest if, so it's recommended that you write an if instead of an else
• Indentation does not imply a match for else!!! Indentation does not imply a match for else!!! Indentation does not imply a match for else!!!
• You can use curly brackets when only a single line statement follows if, but they are recommended for overall style coordination

Case study: expression of piecewise function (if-else if-else)

The following piecewise functions require a program to be designed with input x and output y

f(x) = -1 x<0

f(x) = 0 x=0

f(x) = 2x x>0

//Show only if section
if(x<0)
{
    f = -1;
}
else if(x=0)
{
    f = 0;
}
else
{
    f = 2*x;
}

Programming habits tips

• Note else alignment when indenting (cascade structure)
• Code as single export as possible, multiple encapsulation to reduce export (f above)

Common errors in if statement

• Forget to add braces
• if followed by a semicolon
• Misuse==
• else is not canonical

switch structure

If there are too many cascades, the execution efficiency will be very low, and a switch structure is available

switch(Control expression){
case Constant:

Sentence;

break;

case Constant:

Sentence;

break;

default:

Sentence

break;

}

A control expression can only be the result of an integer type

Constant expressions can be constants or expressions

A switch statement can be viewed as a calculation-based jump, with multiple values corresponding to one statement

Case: Performance Conversion

A score greater than or equal to 90

A score greater than or equal to 80 is B

A score greater than or equal to 70 is C

A score greater than or equal to 60 is D

E for results less than 60

int grade;
scanf("%d",&grade);
grade /= 10;
//We only abstract 10 digits for operation, so we can use the switch structure
switch(grade)
{
    case 10:
    case 9:
        printf("A\N");
        break;
    case 8:
        printf("B\N");
        break;
    case 7:
        printf("C\N");
        break;
    case 6:
        printf("D\N");
        break;
    default:
        printf("E\N");
        break;
}

Judge from top to bottom when cascading >

Judge from bottom to top when cascading <

Case: Number of digits (while cycle)

int x;
int n = 0;
scanf("%d",&x);
//This line of code guarantees that the number of output bits for input 0 is 1
n++;
x /= 10;
while(x>0)
{
    n++;
    x /= 10;
}
printf("%d\n",n);

tips

• The circulatory body must have the opportunity to change the circulating conditions
• When interrupting points test their own program, commonly used bounded data, valid ranges, special multiples of test data
• Adding comments and printf testing to your program can improve program feasibility and readability

optimization

Many of our situations require a cycle of judgment and execution

do-while executes a do-while and then determines the loop

int x;
int n = 0;
scanf("%d",&x);
do
{
    n++;
	x /= 10;
}
//Make sure to have a semicolon after this while
while(x>0);
printf("%d\n",n);

Extended Case: Count Cycle

int count = 100;
while(count>=0)
{
    //These two lines determine what the data for the first and last output is
	count--;
    printf("%d\n",count);
}
printf("launch\n");

Case: Guess Cycle

The computer thinks of a number and lets the user guess

Remind me if it's too big or too small to guess

Supplementary knowledge

• Each call to rand() yields a random integer
• The result of x%n is an integer of [0,n-1]
• x%100 means that x takes over 100, that is, there are only 10 bits left

Programming Thought

• Store the number of user inputs and set a loop that you can't go out without guessing
• Judging whether it's too big or too small
• How many times a variable was created to be stored
• optimization

srand(time(0));
int number = rand()%100+1;
int count = 0;
int a = 0;
printf("I've got one in mind-100 Integer");
do{
    printf("Please enter this 1-100 Integer");
    scanf("%d",&a);
    count++;
    if(a>number)
    {
		printf("Big");
    }
    else
    {
		printf("Small")；
    }
}
while(a!=number);
prinf("Used%d The number was guessed%d"，count,number);

Case: Average

Ask the user to enter a series of positive integers, end with -1, and the program calculates the average and number of these numbers

Programming Thought

• Use loops to keep users typing, and do not loop when -1 is equal
• Create a variable to store the number of digits
• Use 1.0 to force data types to float

int number;
int sum = 0;
int count = 0;
scanf("%d",&number);
while(number!=-1)
{
	sum +=number;
    count++;
    scanf("%d",&number);
}
printf("%f\n",1.0*sum/count);

Case: Decomposition of Integers

Enter a positive integer (an integer in the int range) and output a number in reverse order

Programming Thought

• Get bits for an integer%10, 10 bits for an integer/10%10, and so on
• Reverse Sequence Processing
• Processing with 0 at the end

int x = 0;
int sum = 0;
int t = 0;
scanf("%d",&x);
int count = 0;
//Prevent x changes Operating with a variable instead of X
t = x;
//So let's first get how many digits x is, so in order to prevent the input from being zero, we need to do a calculation first
t /= 10;
count++;
while(t!=0)
{
    t /= 10;
    count++;
}
//Reverse Sequence Processing
//Operate several times with a few digits
for(int i=0;i<count;i++)
{
    //Abstract the last digit and leave x one less to the right
	int a = x % 10;
    x /= 10;
    //Using j=count-i-1 to abstract how many times a bit needs to change to the front by X 10, this method can also solve the problem that ends with 0 because 0*any number is 0
    for(int j=count-i-1;j>0;j--)
    {
		a *= 10;
    }
    //Calculate Sum
    sum += a;
}
printf("%d",sum);

for loop summary

Case 1: Calculating factorials

int n;
scanf("%d",&n);
int fact = 1;
int i = 1;
for(i=1;i<=n;i++)
{
	fact *= i;
}
printf("%d",fact);

Case 2: Is it a prime number

int x;
scanf("%d",x);
int i;
int isPrime = 1;
for(i=2;i<x;i++)
{
	if(x%i == 0)
    {
		isPrime = 0;
        break;
    }
    else
    {
        continue;
    }
}
if(isPrime = 0)
{
    printf("%d Not a prime number"，x);
}
else
{
    printf("%d prime number"，x);
}

About for Loop

• for loops are like counting loops: Initialization - Loop Conditions - Repeated Execution and Adjustment
• Each expression in for can be omitted
• for(; condition;)=while (condition)

tips

• If there is a fixed number of times use for
• do_while must be executed once
• Use while for the rest

Case study: Coins (how to get out of a nested loop)

How to use 1.2.5 cents to make up an amount less than 10 yuan

Option 1: Relay break

int x;
int one,two,five;
int exit = 0;
scanf("%d",&x);
for(one=1;one<x+10;one++)
{
	for(two=1;two<x*10/2;two++)
    {
        for(five=1;five<x*10/5;five++)
        {
			if(one+two*2+five*5==x*10)
            {
             	printf("%d1 horn%d2 horn%d5 horn"，one,two,five);
                //Set a variable as an exit condition to create a relay break
                exit = 1;
                break;
            }
            if(exit == 1)
            {
                break;
            }
        }
        if(exit == 1)
        {
                break;
        }
    }
}

Option 2:goto

int x;
int one,two,five;
int exit = 0;
scanf("%d",&x);
for(one=1;one<x+10;one++)
{
	for(two=1;two<x*10/2;two++)
    {
        for(five=1;five<x*10/5;five++)
        {
			if(one+two*2+five*5==x*10)
            {
             	printf("%d1 horn%d2 horn%d5 horn"，one,two,five);
                goto FLAG;
            }
        }
    }
}
FLAG:
return 0;

Case: Positive Ordinal Decomposition Integer

Enter a non-negative integer, output each bit in positive order, with a space between each bit, and no space at the end

Programming Thought

• Judge how many digits there are first
• Abstract positive ordinals per bit with space intervals
• Handle last non-space

int x;
scanf("%d",&x);
int mask = 1;
while(t>9)
{
    t /= 10;
    mask *= 10;
}
do
{
	int d = x / mask;
    printf("%d",d);
    if(mask>9)
    {
        printf(" ");
    }
    x %= mask;
    mask /= 10;
}
while(mask>0);
printf("\n");

Case: Maximum common divisor (enumeration divided by rolling)

Method 1: Set t to 2. If u and V are all divisible, write down t. Repeat the second step after t++ until it is equal to the minimum of u and v, then the largest t ever written down is goc

int a,b;
int min=0;
scanf("%d %d",&a,&b);
if(a>b)
{
	max = a;
}
else
{
	max = b;
}
int ret = 0;
for(int i=0;i<=min;i++)
{
	if(a%i==0)
    {
		if(b%i==0)
        {
            ret == i;
        }
    }
}
printf("%d",ret);

Method 2: Rolling-Rotating Division

int u = 32;
int v = 32;
//If v=0, at the end of the calculation, u is the greatest common divisor
//v!=0 calculates the remainder of u/v so that u=v=the remainder
while(v!+0)
{
	int temp = u%v;
    u = v;
    v = temp;
}
printf("%d",u);

Data type and language characteristics of C

C is a typed language and must be defined and typed before use

C Language development in two directions in the future

• C++/Java emphasizes types more and checks types more strictly
• Javascrip,Python,PHP don't care about types, and don't even need to define them

Types of C Language

1. integer
   • char
   • short
   • int
   • long
   • long long
2. Floating point number
   • float
   • double
   • long double
3. logic
   • bool
4. Pointer
5. Custom Type

sizeof

sizeof is an operator that calculates how many bytes an expression takes in memory

Be careful

• sizeof is a static operator whose operators are predefined at compile time
• Do not use parentheses in sizeof

Internal representation of integers

Everything inside the computer is binary

So how do we express negative numbers?

In decimal, our minus sign actually operates independently of numbers

Then there are three options for binary negative numbers

1. Imitate decimal with a special flag
2. Take the middle number 0, greater than it is an integer less than it is a negative number
3. Complement Code

But the first two are understandable for humans, but they are complex in computer design.

Design with upper limit of digits

0->00000000

1->00000001

-1->11111111

Whole is 255 when treated as pure binary and -1 when treated as complement

The meaning of a complement is that taking the complement and the code adds up an overflow of 0

Range of integers

char c = 255;
int i = 255;
printf("c=%d,i=%d",c,i);

Output result c has a value of -1 and i 255

So different types have different ranges

A negative number range is one more than an integer range

unsigned

If we don't want to use complements, we can use the unsigned keyword to make variables pure binary

The range of positive numbers is expanded, negative numbers are no longer expressed

unsigned is not meant to extend the range that numbers can express, but to do pure binary operations, to shift

Case: Calculate the maximum number of int types

Utilization Principle: Exceeding the upper limit will overflow into a negative maximum value, minus just after returning to the positive maximum value

int a = 0;
int b = 0;
while(++a>0);
printf("int The maximum number of data types is:%d\n",a-1);
b++;
while((a=a/10)!=0)
{
    b++;
}
printf("int The maximum number of digits in a data type group is:%d",b);

Formatting of Integers

Input and output of integers can only take two forms: int or long

A number starts with 0 and is 8-digit

A number starts with 0x and is hexadecimal

%0 for octal

%x for hexadecimal

There will always be binary in your computer, and the octets you write will be converted when you compile them

8 and 16 are just how numbers are expressed as strings, not how numbers are expressed internally

A hexadecimal bit is exactly a 4-digit, 2-digit number

Choose Integer Type

Why do integers have so many kinds, in order to accurately express memory, do the needs of underlying programs

But in general, choose int without special needs

• Now the word length of the cpu is generally 32 or 64 bits. Each memory read-write and calculation is an int. Choosing a shorter type may not be faster, but may be slower.
• Now the compiler designs memory alignment, so shorter types actually occupy an int size, although sizeof says it's smaller than an int

Unigned differs from whether it is just output, internal calculations are the same

Floating Point Type

double and float

float significant number is 7 bits

double Significant Number is 15 Bits

There is always a number close to 0 but not equal to 0 that we cannot express, that is, infinity close to 0 cannot express

Scientific Counting in E

Output Precision

Add.n between%and f to specify the number of decimal places after the output, so that the output can be rounded

printf("%.3f\n",-0.0049);
printf("%.30f\n",-0.0049);
printf("%.3f\n",-0.00049);