Eight common errors in programming

Everybody knows , Learning programming is a very boring thing , In particular, you can never get the desired results when running the program , After some debugging, it still cannot be solved , I believe every programmer has encountered this kind of situation , This is also a necessary stage for the birth of a great programmer .

Sadly , The same mistakes continue to happen . But luck followed , ad locum , I'll introduce the most common new programmers I've ever met 8 A collection of errors , Convenient for everyone to learn

1  Use sheet “=” Check for equality

char x='Y';
while(x='Y')
{  
    //...  
    cout<<" Whether or not to continue ? (Y/N)";  
    cin>>x;
}

The above code will be an endless loop ~~ As free as the wind ~~~

The code uses an equal sign to check whether the loop condition is equal , In fact, the program will assign the value on the right side of the expression to the variable on the left during execution , In fact, the assignment of variables is performed . In this case , The value is 'Y', It is regarded as true. therefore , The cycle will never end . Therefore, the following changes are required to solve the above problems ：

• Use == Check for equality ;
• To avoid accidental assignment , Put the variable to the right of the expression , If you accidentally use an equal sign , There will be compilation errors , Because you can't assign a value to something that isn't a variable .

char x='Y';
while('Y'==x)
{  
    //...  
    cout<<" Whether or not to continue ? (Y/N)";  
    cin>>x;
}

2  Variable assignment

#include <iostream>
using namespace std;
int main()
{    
    int a, b;
    int sum=a+b;    
    cout<<"input two numbers to add: ";    
    cin>>a;    
    cin>>b;    
    cout<<"The sum is: "<<sum;    
    return 0;
}

After the above code is run, the result is as follows ：

Running results

Give it to a and b It's all assigned , Why? sum Or will you get an outlier ？ Let's see what happens in the program . Usually beginners think that variables act like equations —— If you assign a variable to the result of several other variables , When these variables change , As in the code a and b, The value of the variable （sum） It's going to change . But this is not the case . stay C++ in , Once you assign a value to a variable , The value of this variable will not change , Unless you assign him a new value . therefore , In the program above , Because at first I gave sum An uninitialized variable is used for assignment ,sum You will also get a random value , Although the back is right a and b It's assigned , however sum It won't be changed .

3  Variable not declared

#include <iostream>
using namespace std;
int main()
{
    cin>>x;    
    cout<<x;    
    return 0;
}

Compile time , The compiler will report an error . Because the compiler doesn't know that we x The meaning of , So it must be declared when using . as follows ：

#include <iostream>
using namespace std;
int main()
{
    int x=0;
    cin>>x;    
    cout<<x;    
    return 0;
}

4  Variable not initialized

#include <iostream>
using namespace std;
int main()
{
    int count;    
    while(count<100)    
    {      
        cout<<count<<";";      
        count++;    
    }    
    return 0;
}

The code above is executed , Not as expected while loop , Because in C++ in , Integer variables are not assigned by default 0. In the code above count It can be int Any value in the range . for example , It may be 121, under these circumstances ,while The condition of a loop is never true . The output of the program may be output from -99 To 99 The number of .

Bear in mind ： Variables must be initialized ！！！

5  Function undefined

int main()
{ 
    add();
}
void add()
{  
    //...
}

Error will be reported at compile time , It is clearly defined later add function , Why is there no definition add~

When compiling code , The compiler doesn't know add() What is the , Unless you tell it in advance , If you tell it that there is a name after using it add Function of , It will be confused . So be sure to use the function for the first time , Just define the prototype of the function or the entire definition of the function . Such as ：

void add()
{  
    //...
}
int main()
{ 
    add();
}

6  Redundant semicolons

Most of these problems occur in for In circulation , Such as ：

#include <iostream>
using namespace std;
int main()
{    
    int x;    
    for(x=0; x<100; x++);      
    cout<<x;    
    return 0;
}

The above code is expected to output 0 To 99 The numerical , But after the actual operation, the output is ：100; The reason is that for There is an extra semicolon after the statement . This will change the original logic of the code . So remember when programming ： Semicolons cannot appear in if sentence 、 After loop or function definition . If you put one of these places , Your program will not work properly .

7  Array boundary overflow

#include <iostream>
using namespace std;
int main()
{    
    int array[10];    
    for(int i=1; i<=10; i++)      
        cout<<array[i];    
    return 0;
}

stay C++ The array index is derived from 0 Start . for example , If you have one 10 Array of elements , The first element is at position 0, The last element is in position 9.

 int array[10];
 for(int =1; i<10; i++)      
   cout<<array[i];

There are other problems in the above code , If the array is not initialized , So when outputting , The printed values are actually random values .

8  Misuse “&&” and “||”

#include <iostream>
using namespace std;
int main()
{
    int value;    
    do    
    {      
        //...      
        value=10;    
    }while(!(value==10) || !(value==20));    
    return 0;
} 

The original intention of the code implementation should be to value be equal to 10 perhaps value be equal to 20 When , Just close the loop . However, it is found that , Even if value be equal to 10, The loop statement will also be executed all the time . from while Expression analysis shows that . It can't exist at the same time value Equal to 10 It's equal to 20 The situation of . If you want to meet the end condition of the above cycle , You need to change the expression to ：!(value==10) && !(value==20) perhaps !((value==10) || (value==20)) To meet the exit conditions . If you have any questions about this condition, you can review discrete mathematics by yourself .