Detailed explanation of branch and loop structures (analyzing if statements, switch statements, while loops, for loops, do-while loops)

introduce

C language is a structured programming language. The structure here refers to the sequential structure, selection structure, and loop structure. The C language can implement these three structures. In fact, if we analyze carefully, what we daily Everything you see can be broken down into these three structures or a combination of these three structures.
We can use if, switchto implement the branch structure, and use for , while, do while to implement the loop structure.

branch structure

if statement

if(表达式)
    语句1
else
    语句2

If the expression is established (true), the statement is executed; if the expression is not established (false), the statement is not executed.
In C language, 0 means false and non-0 means true., that is, if the result of the expression is 0, the statement will not be executed; if the result of the expression is not 0, the statement will be executed.
Let’s demonstrate by judging whether you are an adult:

#include<stdio.h>

int main()
{
    
    
    int age=0;
    scanf("%d",&age);
    if(age<18)
        printf("已成年\n");
    else
        printf("未成年\n");
    return 0;
}

Of course, this is just the most basic structure. There are three more complex ones:

A branch contains multiple statements

There is only one statement in the above, whether it is ifor elseafter, what if there are more? then you need to {}enclose the statement with

#include<stdio.h>

int main()
{
    
    
    int score=0;
    scanf("%d",&score);
    if(score>=80)
    {
    
    
        printf("您已达良好:>\n");
        printf("考得好，下次继续:>\n");
    }
    else
    {
    
    
        printf("您未达良好:<\n");
        peintf("下次加油:>\n");
    }
    return 0;    
}

Nested if

Multiple judgments can be achieved using nested ifs, the structure is as follows:

if(表达式1)
    语句1;
else if（）
    语句2;
else
    语句3;

Leave empty else

Let’s take a random example here:

it can be seen that nothing is printed. This is because although the expression after the first if is true, the expression after the second if is false.
Another important point is:When elseleft empty, elsethe closest ifmatch is always used.

Although you know the alignment rules, the logic of the code will be clearer with appropriate curly brackets. Therefore, you should pay attention to the use of brackets when writing code in the future to make the code more readable.

switch statement

General structure:

switch(expression)
{
    
    
    case value1:statement
    case value2:statement
    default:statement
}

If expression does not have a corresponding value, default will be executed.
The order of case and default in the switch statement
Is there any required order between clauses and sub-clauses in switcha statement ? Can it only be put at the end? In fact, there is no order requirement for the statements in the switch statement , as long as your order meets the actual needs. But we usually deal with the clause last.case defaultdefaultcasedefault
default
Note:
• The expression after switch must be an integer expression
• The value after case must be an integer constant expression

Why does the error in the picture appear when entering 4? This is because it case4does not end after entering, but continues to enter case5..., so switchyou should also pay attention when using statements:
• There must be a space between the case and the following number.
• After the code in each case statement is executed, break needs to be added to jump out of the switch statement.

---

Loop structure

while loop

while(表达式)
    语句;

whileThe specific execution process of the statement:

The first step is to execute the judgment expression. If the value of the expression is 0, the loop ends directly; if the value of the expression is not 0, the loop statement is executed. After the statement is executed, it will continue to judge whether to proceed to the next time. judge.
Let’s take an example to learn more about it. Input a positive integer and print each digit of this integer in reverse order
: input: 1234, output: 4 3 2 1
Question analysis:

1. To get the lowest digit of n, you can use the operation n%10, and the remainder is the lowest digit, for example: 1234%10 gets 4
2. If you want to remove the lowest digit of n and find the second to last digit, you can use the n=n/10 operation to remove the lowest digit. For example: n=1234/10 to get 123. Compared with 1234, 123 has the lowest digit removed. digit, 123%10 will get the second to last digit 3.
3. By looping through steps 1 and 2, all bits can be reached before n becomes 0.

#include<stdio.h>

int main()
{
    
    
    int num=0;
    scanf("%d",&num);
    while(n)
    {
    
    
        printf("%d ",n%10);
        n/=10;
    }
    return 0;
}

break and continue in while loop
Let's look at the following piece of code:

after printing 1, 2, 3, and 4, when i equals 5, the loop terminates at the break point, no longer prints, and no longer loops.
So breakits function is to permanently terminate the loop. As long as breakit is executed, breakthe outer first-level loop will be terminated. After that, if we are in the loop and want to terminate the loop under certain conditions, we can use it break to achieve the effect we want.
Then we are thinking about a question, what if we breakreplace continueit?

At this point we can analyze it, continuewhich can help us skip the code behind a certain loop continue and go directly to the judgment part of the loop to judge the next loop. If the loop adjustment is at the continue back, it may cause
an infinite loop. .

for loop

General form:

for(表达式1;表达式2;表达式3)
    语句;

Expression 1 is used to initialize the loop variable.
Expression 2 is used to judge the loop end condition.
Expression 3 is used to adjust the loop variable.
forThe loop execution process:

first execute expression 1 to initialize the loop variable, and then execute the judgment of expression 2. Part, if the result of expression 2 == 0, the loop ends; if the result of expression 2 ! = 0, the loop statement is executed. After the loop statement is executed, expression 3 is executed, the loop variable is adjusted, and then Judgment is performed where expression 2 is executed. Whether the result of expression 2 is 0 determines whether the loop continues.
During the entire loop process, the initialization part of expression 1 is only executed once, and the rest is expression 2, loop statement, and expression 3 in the loop.
Let’s do an exercise: Calculate the sum of numbers that are multiples of 3 between 1 and 100

#include <stdio.h>
int main()
{
    
    
    int i = 0;
    int sum = 0;
    for(i=1; i<=100; i++)
    {
    
    
        if(i % 3 == 0)
        sum += i;
    }
    printf("%d\n", sum);
    return 0;
}

break and continue in for loop
In fact, just like the ones while in the loop , the ones in the loop are also used to terminate the loop. No matter how many times the loop needs to be looped, as long as it is executed , the loop will be completely terminated. What about that ? Let's go directly to the code. We see that the debugging result is less than 5, so the function of the for loop is to skip the code after this loop and go directly to the adjustment part of the loop. In the future, when a certain condition occurs and this cycle does not need to perform certain subsequent operations, it can be used to achieve this.breakforbreakbreak
break

continuecontinue continue

do-while loop

General form:

do
   语句；
while(表达式)；

do-whileLoop execution process: first execute the "statement" on the diagram in the loop, and then execute the "judgment expression" after executing the statement. If the result of the judgment expression is !=0, then the loop will continue and the loop statement will be executed;

judgment do whileThe result of the expression == 0, the loop ends.
Therefore, in the do while statement, the loop body is executed at least once, which is do whilea special aspect of the loop.
Let's continue to practice with a question: input a positive integer, how many digits does this integer have?
eg: Input: 24534 Output: 5

#include <stdio.h>
int main()
{
    
    
    int num = 0;
    scanf("%d", &num);
    int cnt = 0;
    do
    {
    
    
        cnt++;
        num = num / 10;
    } while (num);
    printf("%d\n", cnt);
    return 0;
}

It is not necessary to use statements here do while , but this code is more suitable for using do whileloops, because even if num is 0, it is still a 1-digit number, and the number of digits needs to be counted.
break and continue in do-while loop
do-whilebreakThe sum in the loop foris whilevery similar. What about that continue? From the above code

, you can see that the program is in an infinite loop. This is because continuethe following code is skipped, so that it iis always 5, so the function in the loop is to skip the last code in this loop do-whileand go directly to the judgment part of the loop.continuecontinue