[C language] branch and loop statements (Part 1)

Preface

This article mainly talks about branch statements in branch and loop statements , The explanation of circular sentences will be explained in the next blog .

1. What is a sentence ？

• C Language is 【 structured 】 Control statement of

• C Sentences can be divided into the following five categories ：

1. Expression statement
2. Function call statements
3. Control statement
4. Compound statement
5. Empty statement

• Control statement Used to control the execution flow of the program , In order to realize the various structures of the program , They are made up of specific statement definers ,C There are nine control statements in language .
  It can be divided into three categories ：

1. Conditional judgment statements are also called branch statements ：if sentence 、switch sentence ;
2. Loop statement ：do while sentence 、while sentence 、for sentence ;
3. Turn to statement ：break sentence 、goto sentence 、continue sentence 、return sentence .

2. Branch statement （ Selection structure ）

2.1 if sentence

if Sentence grammar structure

If the result of the expression is true , Then the statement executes .
stay C In language 0 Said the false , Not 0 Said really .

（1）

if( expression ）
    sentence ;

(2)

if( expression ）
    sentence 1;
else
    sentence 2;

（3） Multiple branches

if( expression 1）
    sentence 1;
else if( expression 2)
     sentence 2;
else
     sentence 3;

• Sample code 1

#include <stdio.h>
int main()
{
    
	int age = 0;
	scanf("%d", &age);
	if (age < 18)
	{
    
		printf(" A minor \n");
	}
	return 0;
}

• Sample code 2

#include <stdio.h>
int main()
{
    
	int age = 0;
	scanf("%d", &age);
	if (age < 18)
	{
    
		printf(" A minor \n");
	}
	else
	{
    
		printf(" adult \n");
	}
	return 0;
}

• Sample code 3

#include <stdio.h>
int main()
{
    
	int age = 0;
	scanf("%d", &age);
	if (age < 18)
	{
    
		printf(" juvenile \n");
	}
	else if (age >= 18 && age < 30)
	{
    
		printf(" youth \n");
	}
	else if (age >= 30 && age < 50)
	{
    
		printf(" middle-aged \n");
	}
	else if (age >= 50 && age < 80)
	{
    
		printf(" The elderly \n");
	}
	else
	{
    
		printf(" Old birthday star \n");
	}
	return 0;
}

If the condition holds , To execute multiple statements , How to use code blocks ？
if and else Can only control one statement , There are multiple statements that must use { }. A couple here { } It's just a code block

#include <stdio.h>
int main()
{
    
	if ( expression )
	{
    
		 Statement list 1;
	}
	else
	{
    
		 Statement list 2;
	}
	return 0;
}

Code example

#include <stdio.h>
int main()
{
    
	int age = 10;
	if (age < 18)
	{
    
		printf(" A minor \n");
		printf(" Don't drink \n");
	}
	else
	{
    
		printf(" adult \n");
		printf(" Play the game \n");
	}
	return 0;
}

2.1.1 In the air else

Look at the code below , Guess the printed result is hehe still haha

#include <stdio.h>
int main()
{
    
	int a = 0;
	int b = 0;
	if (a == 1)
		if (b == 2)
			printf("hehe\n");
	else
		printf("haha\n");
	return 0;
}

analysis :
Looking at the print results, we will find that the print results are not hehe Neither haha, It's empty , Looking back at the format of this code will make us think else Yes, and the first if matching , however else It's the one closest to him if Matching , therefore if(a==1) Conditions not established , The result is false , Do not execute statements , The result is empty. . One if…else Statement is a complete statement .

correct ：

// Use properly {} It can make the logic of the code clearer .
#include <stdio.h>
int main()
{
    
	int a = 0;
	int b = 0;
	if (a == 1)
	{
    
		if (b == 2)
		{
    
			printf("hehe\n");
		}
	}
	else
	{
    
		printf("haha\n");
	}
	return 0;
}

2.1.2 if Contrast of writing forms

Code 1

if (condition) {
    
    return x;
}
return y;

Code 2

if(condition)
{
    
    return x;
}
else
{
    
   return y;
}

Code 3

int num = 1;
if(num == 5)// If you don't write an equal sign here, the compiler won't report an error , When there is only one equal sign, it means assignment , because num Is a variable that can be re assigned , So the compiler will not warn .
{
    
    printf("hehe\n");
}

Code 4

int num = 1;
if(5 == num)//if(num == 5), If there is only one equal sign here , The compiler will report an error , Let's put constants on the right , It is not possible to assign a variable to a constant , Even if we write the wrong compiler, it will quickly help us find problems .
{
    
    printf("hehe\n");
}

Code 2 And code 4 Better , Logic is clearer , Not easy to make mistakes .

2.1.3 practice

1. Judge whether a number is odd

We learned here if sentence , We try to use if Statement write code .

#include <stdio.h>
int main()
{
    
	int num = 0;
	scanf("%d", &num);
	if (num % 2 == 1)
		printf(" Odd number \n");
	else
		printf("NO\n");
	return 0;
}

2. Output 1-100 Between the odd numbers

We need to use loop statements here , We can use while Loops can also be used for loop .

//while  loop 
#include <stdio.h>
int main()
{
    
	int i = 0;
	while (i <= 100)
	{
    
		if (i % 2 == 1)
			printf("%d ", i);
		i++;
	}
	return 0;
}

//for  loop 
#include <stdio.h>
int main()
{
    
	int i = 0;
	for (i = 1; i <= 100; i++)
	{
    
		if (i % 2 == 1)
			printf("%d ", i);
	}
	return 0;
}

2.2 switch sentence

switch Statement is also a branch statement .
It is often used in the case of multiple branches .

such as ：
Input 1, Output Monday
Input 2, Output Tuesday
Input 3, Output Wednesday
Input 4, Output Thursday
Input 5, Output Friday
Input 6, Output Saturday
Input 7, Output Sunday

I didn't write it if…else if …else if The form of is too complicated , Then we have to have different grammatical forms . This is it. switch sentence .

switch//( Shaping expression )
{
Statement item
}

What is? Statement item ？
// It's some case sentence ：
// as follows ：
case Integer constant expression :
sentence ;

2.2.1 stay switch Statement break

stay switch In the sentence , We can't branch directly , collocation break Use to achieve true branching .
such as ：

#include <stdio.h>
int main()
{
    
	int day = 0;
	scanf("%d", &day);
	switch (day)
	{
    
	case 1:
		printf(" Monday \n");
		break;
	case 2:
		printf(" Tuesday \n");
		break;
	case 3:
		printf(" Wednesday \n");
		break;
	case 4:
		printf(" Thursday \n");
		break;
	case 5:
		printf(" Friday \n");
		break;
	case 6:
		printf(" Saturday \n");
		break;
	case 7:
		printf(" Sunday \n");
		break;
	}
	return 0;
}

Sometimes our needs change ：

1. Input 1-5, The output is “weekday”;
2. Input 6-7, Output “weekend”
   So our code should be implemented like this ：

#include <stdio.h>
int main()
{
    
	int day = 0;
	scanf("%d", &day);
	switch (day)
	{
    
	case 1:
	case 2:
	case 3:
	case 4:
	case 5:
		printf("weekday\n");
		break;
	case 6:
	case 7:
		printf("weekend\n");
		break;
	}
	return 0;
}

It's explained here case There is no need to add break , No, break Will continue to go down .
break sentence The actual effect of is to divide the statement list into different branches .

Programming good habits
In the last case Add a clause after the statement break sentence .
（ The reason for writing this is to avoid appearing in the last one before case I forgot to add... After the statement break sentence ）.

2.2.2 default Clause

default：
Write in any one case Where labels can appear .
When switch The value of the expression does not match all case The value of the tag , This default The statement following the clause will execute .
therefore , Every switch Only one... Can appear in the statement default Clause .
But it can appear anywhere in the statement list , And the statement flow will execute a case Execute as tag default Clause .

Programming good habits
At every switch Put one in every statement default Clause is a good habit , You can even add another one in the back break .

2.2.3 practice

Here we do an exercise , See if we are right switch Master , Read the following code , To calculate the m,n What's the value of ？

#include <stdio.h>
int main()
{
    
    int n = 1;
    int m = 2;
    switch (n)
    {
    
    case 1:
        m++;
    case 2:
        n++;
    case 3:
        switch (n)
        {
    //switch Allow nesting 
        case 1:
            n++;
        case 2:
            m++;
            n++;
            break;
        }
    case 4:
        m++;
        break;
    default:
        break;
    }
    printf("m = %d, n = %d\n", m, n);
    return 0;
}

Be careful ：

• switch Statements can be nested .
• switch In the sentence break Will only jump out of their own switch, Not a break Jump out of all switch sentence .

This chapter ends here , If there is anything bad written , Please correct me. .
If you think it's good and helpful to you, please give support to the third company ！
Fighting!!!