list<String>copy到list<User>
时间: 2024-10-12 15:12:11 浏览: 6
将`List<String>`转换成`List<User>`通常在编程中涉及到泛型和类型转换。由于`String`和`User`是完全不同的数据类型,这一步需要明确设计:
1. 如果`User`有一个属性直接对应于`String`,比如`username`,那么可以创建一个新的`List<User>`,然后遍历`List<String>`,对于每个字符串,构造一个新的`User`对象并将其添加到新列表中。例如,在Java中:
```java
List<String> stringList = ...;
List<User> userList = new ArrayList<>();
for (String str : stringList) {
User user = new User();
user.setUsername(str);
userList.add(user);
}
```
2. 如果`User`可以从`String`生成,比如是一个用户ID映射到用户对象的查找操作,那可能需要额外的数据结构或服务来进行实际的转换。
请注意,这种类型的转换可能需要检查和处理错误情况,例如如果无法从字符串创建有效的`User`实例。
相关问题
请问以下代码有何问题? try: db = pymysql.Connect(host='10.32.158.162', port=3306, user='dscuser', passwd='dscuser_123', db='COST') if y == "岗位费用": df_upload = pd.melt(df_upload, id_vars=['project', 'role'], value_vars=["2023", "2024", "2025", "2026", "2027", "2028", "2029", "上一版"], var_name='year', value_name='value') elif y == "年度项目月份工时": df_upload = pd.melt(df_upload, id_vars=['Project', 'Function'], value_vars=temp, var_name='year', value_name='value') df_upload = df_upload.dropna() df_upload = df_upload.drop_duplicates() df_upload = df_upload[(df_upload['value'] != 0) & (df_upload['value'].notna())] else: sql = "delete from COST.{} where project = '{}';".format(upload_name, project_name) cursor = db.cursor() cursor.execute(sql) # db.commit() # db.close() col_names = list(df_upload) for col in col_names: df_upload[col] = df_upload[col].astype('string', copy=False) df_upload.to_sql(upload_name, engine, if_exists="append", index=False) else: # db.commit() # db.close() error = error msg = "" except Exception as e: db.rollback() msg = str(e)[0:100] else: db.commit() finally: db.close()
这段代码存在一些问题:
1. 引入的库`pandas`和`pymysql`没有看到在哪里导入,需要在代码开头进行导入。
2. 在执行SQL语句时,没有使用参数化查询,容易出现SQL注入漏洞。应该使用参数化查询,例如使用`cursor.execute(sql, params)`方法。其中`params`是一个元组或字典类型,用于替换SQL语句中的参数。
3. 在`try`语句块中,如果`y`不等于"岗位费用"和"年度项目月份工时",则会执行`else`语句块,但是该语句块中没有对`df_upload`进行处理,会导致后面的代码出现错误。
4. 在`else`语句块中,如果使用`cursor`对象进行数据库操作,则应该在操作完成后调用`cursor.close()`方法释放资源。
5. 在`except`语句块中,`error = error`这行代码没有实际作用,应该删除。
下面是修改后的代码:
```
import pandas as pd
import pymysql
try:
db = pymysql.Connect(host='10.32.158.162', port=3306, user='dscuser', passwd='dscuser_123', db='COST')
if y == "岗位费用":
df_upload = pd.melt(df_upload, id_vars=['project', 'role'], value_vars=["2023", "2024", "2025", "2026", "2027", "2028", "2029", "上一版"], var_name='year', value_name='value')
elif y == "年度项目月份工时":
df_upload = pd.melt(df_upload, id_vars=['Project', 'Function'], value_vars=temp, var_name='year', value_name='value')
df_upload = df_upload.dropna()
df_upload = df_upload.drop_duplicates()
df_upload = df_upload[(df_upload['value'] != 0) & (df_upload['value'].notna())]
else:
sql = "delete from COST.{} where project = %s;".format(upload_name)
cursor = db.cursor()
cursor.execute(sql, (project_name,))
cursor.close()
col_names = list(df_upload)
for col in col_names:
df_upload[col] = df_upload[col].astype('string', copy=False)
df_upload.to_sql(upload_name, engine, if_exists="append", index=False)
except Exception as e:
db.rollback()
msg = str(e)[0:100]
else:
db.commit()
finally:
db.close()
```
在上面的代码中,我们首先导入了`pandas`和`pymysql`库。在`try`语句块中,我们首先创建了一个数据库连接`db`,然后根据不同的条件对`df_upload`进行处理。如果`y`等于"岗位费用",则使用`pd.melt()`方法进行数据重构。如果`y`等于"年度项目月份工时",则使用`pd.melt()`方法和`temp`变量进行数据重构。然后对数据进行清洗,去除空值和重复值,以及`value`列中的0值和空值。
如果`y`不等于"岗位费用"和"年度项目月份工时",则执行`else`语句块。在该语句块中,我们使用参数化查询的方式执行SQL语句,避免了SQL注入漏洞。然后将`df_upload`中的数据类型转换为字符串类型,并使用`df_upload.to_sql()`方法将数据写入数据库。
在`except`语句块中,如果出现异常,则会回滚事务并输出错误信息。在`else`语句块中,如果没有出现异常,则会提交事务。最后在`finally`语句块中,关闭数据库连接。
c++list add
To add an element to a list in the C programming language, you can use the `push_back()` function, which adds the element at the end of the list. Here is an example:
```
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#define MAX_SIZE 100
int main()
{
int i, n;
char str[MAX_SIZE];
bool cont = true;
typedef struct Node {
char data[MAX_SIZE];
struct Node *next;
} Node;
Node *head = NULL;
Node *tail = NULL;
while (cont) {
printf("Enter a string to add to the list (type 'quit' to exit): ");
scanf("%s", str);
if (strcmp(str, "quit") == 0) {
cont = false;
} else {
Node *newNode = (Node*)malloc(sizeof(Node));
strcpy(newNode->data, str);
newNode->next = NULL;
if (head == NULL) {
head = newNode;
tail = newNode;
} else {
tail->next = newNode;
tail = newNode;
}
}
}
printf("\nThe list is: \n");
Node *current = head;
while (current != NULL) {
printf("%s\n", current->data);
current = current->next;
}
return 0;
}
```
In this example, the user is prompted to enter a string to add to the list. The program uses the `malloc()` function to allocate memory for a new node, and then uses the `strcpy()` function to copy the string into the `data` member of the new node. If the list is empty, the new node becomes the head and tail of the list. Otherwise, the new node is added to the end of the list by setting its `next` member to `NULL` and updating the `next` member of the tail node to point to the new node. Finally, the program traverses the list and prints out each string in the list.
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