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有关不调用库函数的字符串操作!

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【推荐】2019 Java 开发者跳槽指南.pdf(吐血整理) >>> 有关不调用库函数的字符串操作! char *strstr(char *str1,char *str2);在字符串str1中，寻找字串str2，若找到返回找到的位置，否则返回NULL。 #include <iostream> char *strstr(char *str1,char *str2) { char *s1,*s2; assert(( str1 != (char *)0 && (str2 != (char *)0)); /* 空字符串是任何字符串的子字符串 */ if('/0' == *str2) { return ((char *)str1); } while(*str1) { s1 = (char *)str1; s2 = (char *)str2; while((*s1 == *s2) && *s1 && *s2) { s1++; s2++; } if('/0' == *s2) { return ((char *)str1); } str1++; } /* 查找不成功，返回NULL */ return((char *)0); } void *memcpy(void *pvTo, const void *pvFrom, size_t size) { assert((pvTo != NULL) &&

期末复习——头插和尾插法的再次复习

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之前一直习惯用尾插法，因为尾插法更符合我自己的逻辑，每次看到别人使用头插法的时候也不是不能理解，就是感觉不舒服，今天再次来复习一下头插法和尾插法， 1.头插法故名思议，从链表头部不断插入，然后将头指针不断后移，这样插入导致的结果就是输出的时候和输入的顺序相反。头指针被只是被声明，开辟了一个四个字节的指针变量用来存地址，在头插法中，这个是头指针，而在尾插法中，我更愿意认识是头结点。因为在尾插法中我们用头结点的指针域不断的去存下一个节点的地址。下面给出头插法的代码 PNODE creat() { PNODE Head=NULL; int n,i,val; cout<<"请输入链表长度:"; cin>>n; for(i=0;i<n;i++) { PNODE pNew=new NODE;//开辟新节点 cout<<"请输入第"<<i+1<<"个节点的值："; cin>>val; pNew->data=val;//将值存入数据域 //头插 pNew->pNext=Head; Head=pNew; } return Head; } 在尾插法中，会多出来一个节点，这个节点就是头节点，它是一个结构体指针变量，用指针域存下一个节点的起始位置，在逻辑思维上，我更习惯这个尾插法。 //尾插 PNODE creat()//尾插法 { PNODE Head=new NODE;//造一个头节点

Postgres NOT IN (null) gives no result

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问题 I'm using Postgres with this query select * from Entity this_ where (this_.ID not in (null)) Why does this give me no results? I would expect to get all rows where id is not null with (this_.ID not in (1)) i get the expected results 回答1: The result of [not] in (null) will always be null. To compare to null you need is [not] null or is [not] distinct from null select * from Entity this_ where this_.ID is not null If you want where (ID not in (1,null)) as in your comment you can do where ID is

How to check for NULL in MySqlDataReader by the column's name?

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问题 How can I check for a NULL value in an open MySqlDataReader ? The following doesn't work; it's always hitting the else : if (rdr.GetString("timeOut") == null) { queryResult.Egresstime = "Logged in"; } else { queryResult.Egresstime = rdr.GetString("timeOut"); } rdr.IsDbNull(int i) only accepts a column number, not name. 回答1: var ordinal = rdr.GetOrdinal("timeOut"); if(rdr.IsDBNull(ordinal)) { queryResult.Egresstime = "Logged in"; } else { queryResult.Egresstime = rdr.GetString(ordinal); }//if

How does WPF handle binding to the property of a null object?

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问题 I have a listBox using an itemTemplate that contains the following line: <Image Source="{Binding MyProperty.PossiblyNullObject.UrlProperty}"/> Bound to this listBox is a model view collection that loads components of the items in the collection on a separate thread. The 'PossiblyNullObject' may not be set to a value when the xaml code is first rendered by the composition engine. How does WPF handle this? Does it use a default value(no image source so no image) and continue on? Does it wait?

When does a java object become non-null during construction?

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问题 Say you are creating a java object like so: SomeClass someObject = null; someObject = new SomeClass(); At what point does the someObject become non-null? Is it before the SomeClass() constructor runs or after? To clarify a little, say if another thread was to check if someObject was null while the SomeClass() constructor was halfway through completion, would it be null or non-null? Also, what would be the difference if someObject was created like so: SomeClass someObject = new SomeClass();

Rails where condition with nil value

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问题 I found out that using where with symbol :my_id => nil and using old school one with ? is different. Could anyone explain me why? MyTable.where("my_id = ? ", nil).first SELECT `my_tables`.* FROM `my_tables` WHERE (my_id = NULL ) LIMIT 1 Does not get any data MyTable.where(:my_id => nil).first SELECT `my_tables`.* FROM `my_tables` WHERE (`my_tables`.`my_id` IS NULL) LIMIT 1 Get data which has my_id is null. What is the best practise to use in rails? I think I didn't make clear about my

Don't update column if update value is null

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问题 I have a query like this (in a function): UPDATE some_table SET column_1 = param_1, column_2 = param_2, column_3 = param_3, column_4 = param_4, column_5 = param_5 WHERE id = some_id; Where param_x is a parameter of my function. Is there a way to NOT update those columns, for which the param is NULL ? For example - if param_4 and param_5 are NULL , then update only the first three columns and leave old values for column_4 and column_5 . The way I am doing it now is: SELECT * INTO temp_row FROM

Don't update column if update value is null

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Why does an empty dataframe fail an is.null() test?

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问题 please excuse me if my question is quite basic. I created an empty data frame by df <- data.frame() and obviously the data frame is NULL (empty). when I try to check if the data frame is empty by is.null(df) , the result comes FALSE. Is there any difference between NULL and empty in R. In this case if the data frame is not NULL , then what is in the empty data frame and when it will be NULL . Thanks 回答1: df is not NULL because it is a data frame and thus has some defined properties. For