Learning notes of C programming [compiled by Mr. Tan Haoqiang] (Chapter III sequence programming) 04 C sentence

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One 、C Sentence structure

1. Control statement ：

① if()…else…（ Conditional statements ）
② for()…（ Loop statement ）
③ while()…（ Loop statement ）
④ do…while ()（ Loop statement ）
⑤ continue（ End the loop ）
⑥ break（ To suspend execution switch Or loop statement ）
⑦ switch（ Multi branch select statements ）
⑧ return（ Return statement from function ）
⑨ goto（ Turn to statement , In structured programs, there is basically no need to goto sentence ）

() Indicates that there is a discriminant condition in brackets
… Represents an embedded statement

2. Function call statements

A function call statement consists of a function call plus a semicolon .

printf("I am ihan001. ");

 among printf("I am ihan001. "); It's a function call , Add a semicolon to make a statement .

3. Expression statement

An expression statement consists of an expression plus a semicolon , The most typical is that an assignment statement is composed of an assignment expression . for example :
a=3
Is an assignment expression , and
a=3;
It's an assignment statement .

4. Empty statement

;

A statement with only one semicolon is an empty statement .
Can be used as a turning point in the process ( The flow goes from other parts of the program to this sentence );
It can also be used as the loop body in a loop statement （ The loop body is an empty statement , Indicates that the loop does nothing ）.

5. Compound statement

It can be used {} Put some sentences and statements together to form a compound sentence ( Also known as statement block ).

{
    
	float pi=3.14159, r=2.5, area; // Defining variables 
	area=pi*r*r;
	printf("area=%f",area);
}

Compound statements are often used in if Statement or loop , At this time, the program needs to execute a set of statements continuously .
The semicolon at the end of the last statement in a compound statement cannot be ignored without writing .

Two 、 Assignment statement

give an example 1： Give the three sides of a triangle , Find triangle area .

Ideas ：area=√s(s−a)(s−b)(s−c), among s=(a+b+c)/2.

#include <stdio.h>
#include <math.h>
int main ()
 {
    
	double a,b,c,s,area;				// Define variables , Are all double type  
	a=3.00;							// To the side chief a assignment  
	b=5.00;							// To the side chief b assignment  
	c=4.00;								// To the side chief c assignment 
	s=(a+b+c)/2; 						// Calculation s 
	area=sqrt(s*(s-a)*(s-b)*(s-c));		// Calculation area 
	printf("a=%f\tb=%f\t%f\n",a,b,c); 	// Output trilateral a,b,c Value  
	printf("area=%f\n",area);			// Output area area Value 
	return 0;
 }

3、 ... and 、 Assignment operator

“=” The function of is to assign a data to a variable .
for example ：a=3 The function of is to perform an assignment operation （ Or assignment operation ）. Put constant 3 Assigned to a variable a.
You can also assign the value of an expression to a variable .

Four 、 Composite operators

In the assignment = Add the other operators before , Operators that can form compounds .

a+=3	 Equivalent to a=a+3

x*=y+8 	 Equivalent to x=x*(y+8)

x％=3	 Equivalent to x=x％3

Usually binary （ Two eyes ） Operator , Can be combined with the assignment character to form a composite assignment character .
The compound assignment operators related to arithmetic operations are +=,－=,=,／=,％=.

Be careful ：
If the right side of the assignment character is an expression containing several items , It has parentheses . for example ,
x%=y+3 Equivalent to x=x%(y+3), Don't write it wrong x=x%y+3.

5、 ... and 、 Assignment expression

The function of assignment expression is to assign the value of an expression to a variable , Therefore, the assignment expression has the dual functions of calculation and assignment .
The process of solving the assignment expression is :
① Find “ expression ” Value ,② Assign to the variable to the left of the assignment operator . Since it's an expression , There should be a value , The value of the expression is equal to the value of the left variable after assignment .
The left side of the assignment operator should be a modifiable value “ The left value ”(left value, Shorthand for lvalue).
The expression that can appear to the right of the assignment operator is called “ Right value ”(right value, Shorthand for rvalue).

Be careful ： Not all forms of data can be used as lvalues , Lvalue should be storage space and can be assigned . Variables can be used as lvalues , And arithmetic expression a+b Can't be used as an lvalue , Constants cannot be used as lvalues .

a=b=c=5 	 Expression value is 5,a,b,c Values are 5
a=5+(c=6) 	 Expression value is 11,a The value is 11,c The value is 6
a=(b=4)+(c=6) 	 Expression value is 10,a The value is 10,b be equal to 4,c be equal to 6
a=(b=10)/(c=2)	 Expression value is 5,a be equal to 5,b be equal to 10,c be equal to 2
a=(b=3*4)	 Expression value is 12,a,b Values are 12

The assignment expression makes the assignment operation not only appear in the assignment statement , And it can appear in other statements ( Such as output statement 、 Loop statement, etc )
Such as : printf("%d", a=b);
If b The value of is 3, The output a Value ( Also an expression a=b Value ) by 3. In a printf Function completes the dual functions of assignment and output .

6、 ... and 、 Type conversion during assignment

If the types on both sides of the assignment operator are the same , Then assign the value directly .
If the types on both sides of the assignment operator are inconsistent , But when they are all basic types , Type conversion is required during assignment . Type conversion is performed automatically by the system , The rule of transformation is ：
Convert floating point data （ Including single 、 Double precision ） When you assign an integer variable , First round the floating-point number , That is, the decimal part is discarded , Then give an integer variable .
Assign integer data to a single 、 Double precision variable , Constant value , But it's stored in a variable as a floating point number .
Will a double Type data is assigned to float variable , First, convert double precision numbers to single precision numbers , That is to say, only 6～7 Significant digits , Store in float Type variable 4 In bytes . It should be noted that the size of the double precision value cannot exceed float The range of values for a variable of type ; Will a float Type data is assigned to double Type variable , Constant value , In memory with 8 Byte store , The number of significant digits extends to 15 position .
When character data is assigned to an integer variable , The ASCII Code assigned to integer variables .
When an integer with more bytes is assigned to an integer or character variable with less bytes , Only send its low byte intact to the assigned variable （ That is to say “ truncation ”）.