英文:

Why i'm facing Regex detection problem inside a loop in Python

问题

我在使用正则表达式时遇到了非常奇怪的问题。

```python
weight = 'abcd'
description = 'ml'
for symbol in Syntax['symbol']:
    print(symbol)
    weight = re.findall(symbol, description)
print(weight)
输出 --> []

Syntax是一个数据框，其中包含不同的单位，还包含在symbol列中的***"ml"，我已经手动打印了symbol变量，它打印出所需的单位"ml"***，它将在循环中设置为模式，但仍然返回[]或None，使用re.match()时。但是，当我尝试下面的代码时

description = 'ml'
pattern = 'ml'
print(re.findall(pattern, description))

它打印出***"ml"***，为什么？上面和上面的代码在逻辑上是相同的。

[![在此输入图片描述][1]][1]

<details>
<summary>英文:</summary>

I&#39;m facing very weird problem while using regex.

weight='abcd'
description='ml'
for symbol in Syntax['symbol']:
print(symbol)
weight=re.findall(symbol,description)
print(weight)
output --> []

***Syntax is a data frame*** that contains different units, also contains ***" ml "*** inside symbol column, i have manually printed the symbol variable it prints required unit that is ***"ml"*** which will be set as pattern in loop but still it returns [] OR None while using re.match().
But when i try code below

description='ml'
pattern='ml'
print(re.findall(pattern,description)

it prints ***"ml"***, Why ??? Both above and Top code are logically same.

[![enter image description here][1]][1]


  [1]: https://i.stack.imgur.com/yN84A.png

</details>


# 答案1
**得分**: 1

在顶部的代码中，由于`print(weight)`位于循环外部，所以您只打印了最终正则表达式搜索的结果。如果“ml”在您的数据框中的某个地方，那就很好，但如果`symbol`的*最后一个*值与`description`中的任何内容都不匹配，那么正则表达式将找不到任何匹配项，您将不会得到任何输出。

尝试在`for`循环内部打印`weight`，看看您会得到什么输出。

<details>
<summary>英文:</summary>

In the top code, you&#39;re only printing the result of the final regex search, since `print(weight)` is outside your loop. It&#39;s all well and good if &quot;ml&quot; is somewhere in your data frame, but if the *last* value of `symbol` doesn&#39;t match anything in `description`, the regex won&#39;t find any matches and you won&#39;t get any output. 

Try printing `weight` inside the `for` loop and see what output you get.

</details>



# 答案2
**得分**: 0

    description='ml'
    weight=0
    for symbol in self.units['symbol']:
        print("units in Syntax dataframe : ", symbol)
        weight=re.findall(symbol, description)
        if weight!=[]:
            break
    print(weight)

<details>
<summary>英文:</summary>

    description='ml'
    weight=0
    for symbol in self.units['symbol']:
        print("units in Syntax dataframe : ",symbol)
        weight=re.findall(symbol,description)
        if weight!=[]:
            break
    print(weight)

I have understood the problem, i was not stopping the loop when the 'ml' is found, that's why it was printing [] or None

</details>