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Using .replace to change words in a list

AhmadSALEH1

Hi,

I am trying to with a script that replaces the word "feet" to "meter" in a list:

AreaList = ["1000 feet", "23908 feet", "1200000000 feet"]
for area in list(AreaList):
area.replace("feet", "meters")

print AreaList

but I still see the results as the following:

the word feet still in the list! what might be my mistake ?

Thanks

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Accepted answers

MitchHolley1

try this:

AreaList <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">"1000 feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"23908 feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"1200000000 feet"</SPAN><SPAN class="punctuation token">]</SPAN>

MeterList <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN>a<SPAN class="punctuation token">.</SPAN>replace<SPAN class="punctuation token">(</SPAN><SPAN class="string token">"feet"</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="string token">"meters"</SPAN><SPAN class="punctuation token">)</SPAN> <SPAN class="keyword token">for</SPAN> a <SPAN class="keyword token">in</SPAN> AreaList<SPAN class="punctuation token">]</SPAN>

<SPAN class="keyword token">print</SPAN> MeterList<SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

All comments

bixb0012

Ahmad SALEH, it is possible in Python to update list values in place, as opposed to constructing a new list, which is what a list comprehension does. Looking at your original code, incorporating enumerate() allows you to keep your basic code structure and do the updates you want:

<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> AreaList <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">"1000 feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"23908 feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"1200000000 feet"</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> <SPAN class="keyword token">for</SPAN> i<SPAN class="punctuation token">,</SPAN> area <SPAN class="keyword token">in</SPAN> enumerate<SPAN class="punctuation token">(</SPAN>AreaList<SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">:</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN>&nbsp;&nbsp;&nbsp;&nbsp; AreaList<SPAN class="punctuation token">[</SPAN>i<SPAN class="punctuation token">]</SPAN> <SPAN class="operator token">=</SPAN> area<SPAN class="punctuation token">.</SPAN>replace<SPAN class="punctuation token">(</SPAN><SPAN class="string token">"feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"meters"</SPAN><SPAN class="punctuation token">)</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN> 
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> <SPAN class="keyword token">print</SPAN> AreaList
<SPAN class="punctuation token">[</SPAN><SPAN class="string token">'1000 meters'</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">'23908 meters'</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">'1200000000 meters'</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> <SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

For your specific situation, I think using a list comprehension makes more sense because you will be updating every list item. In cases where the number of elements updated in a list is high, especially when nearly all elements are updated, constructing a new list and reassigning it back to your variable of choice often makes more sense then doing in-place updates.

curtvprice

I'm a moderator so I converted this thread to a question and marked Mitch's answer correct.

DanPatterson_Retired

Ahmad... that is because you often open up 'Discussions' rather than 'Questions' ... Discussions are just that, something that you want to talk about, the question option is the only one that allows you to mark a 'Correct Answer' or words to that effect.

AhmadSALEH1

Thanks Mitch this worked fine with me, also I figure out where the error was in my code, it should be written as this:

AreaList = ["1000 feet", "23908 feet", "1200000000 feet"]
for area in list(AreaList):
area=area.replace("feet", "meters")
print area

by the way I cannot see "mark as correct answer" button next to your name, this is the second time that happens with me!!!

ClintonDow1

The string.replace() function does not change the string in place, it returns a new string with the replaced words.

Instead you could do:

feet_list <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">"1000 feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"23908 feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"1200000000 feet"</SPAN><SPAN class="punctuation token">]</SPAN>
meter_list <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="keyword token">for</SPAN> area <SPAN class="keyword token">in</SPAN> feet_list<SPAN class="punctuation token">:</SPAN>
&nbsp;&nbsp;&nbsp; s <SPAN class="operator token">=</SPAN> area<SPAN class="punctuation token">.</SPAN>replace<SPAN class="punctuation token">(</SPAN><SPAN class="string token">"feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"meters"</SPAN><SPAN class="punctuation token">)</SPAN>
&nbsp;&nbsp;&nbsp; meter_list<SPAN class="punctuation token">.</SPAN>append<SPAN class="punctuation token">(</SPAN>s<SPAN class="punctuation token">)</SPAN>
<SPAN class="keyword token">print</SPAN><SPAN class="punctuation token">(</SPAN>meter_list<SPAN class="punctuation token">)</SPAN>
<SPAN class="comment token"># ['1000 meters', '23908 meters', '1200000000 meters']</SPAN>
<SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

Or alternatively:

meter_list <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN>f<SPAN class="punctuation token">.</SPAN>replace<SPAN class="punctuation token">(</SPAN><SPAN class="string token">"feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"meters"</SPAN><SPAN class="punctuation token">)</SPAN> <SPAN class="keyword token">for</SPAN> f <SPAN class="keyword token">in</SPAN> feet_list<SPAN class="punctuation token">]</SPAN>
<SPAN class="keyword token">print</SPAN><SPAN class="punctuation token">(</SPAN>meter_list<SPAN class="punctuation token">)</SPAN>
<SPAN class="comment token">#&nbsp; ['1000 meters', '23908 meters', '1200000000 meters']</SPAN>
<SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

DanPatterson_Retired

You printed AreaList, which doesn't get changed even though you would be printing and changing the values as you went.

<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> AreaList <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">"1000 feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"23908 feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"1200000000 feet"</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> a <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN>i<SPAN class="punctuation token">.</SPAN>replace<SPAN class="punctuation token">(</SPAN><SPAN class="string token">'feet'</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="string token">'meters'</SPAN><SPAN class="punctuation token">)</SPAN> <SPAN class="keyword token">for</SPAN> i <SPAN class="keyword token">in</SPAN> AreaList<SPAN class="punctuation token">]</SPAN>
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> a
<SPAN class="punctuation token">[</SPAN><SPAN class="string token">'1000 meters'</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">'23908 meters'</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">'1200000000 meters'</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> <SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

Now see what went wrong.

<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> <SPAN class="keyword token">for</SPAN> i <SPAN class="keyword token">in</SPAN> AreaList<SPAN class="punctuation token">:</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;i <SPAN class="operator token">=</SPAN> i<SPAN class="punctuation token">.</SPAN>replace<SPAN class="punctuation token">(</SPAN><SPAN class="string token">'feet'</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">'meters'</SPAN><SPAN class="punctuation token">)</SPAN>&nbsp; <SPAN class="comment token"># note the reassignment</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<SPAN class="keyword token">print</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="string token">"i value {}"</SPAN><SPAN class="punctuation token">.</SPAN>format<SPAN class="punctuation token">(</SPAN>i<SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
i value <SPAN class="number token">1000</SPAN> meters
i value <SPAN class="number token">23908</SPAN> meters
i value <SPAN class="number token">1200000000</SPAN> meters

<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> <SPAN class="keyword token">print</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="string token">"Area list at the end \n{}"</SPAN><SPAN class="punctuation token">.</SPAN>format<SPAN class="punctuation token">(</SPAN>AreaList<SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN>
Area list at the end 
<SPAN class="punctuation token">[</SPAN><SPAN class="string token">'1000 feet'</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">'23908 feet'</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">'1200000000 feet'</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> <SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

Got to watch what you think is going on... just because a value is pulled from and list and changed, doesn't mean the value in the original list gets changed. You have to recollect the changes as I did in the original example, or replace them in the input list.

RandyBurton

See List Comprehensions for more information on Mitch's answer.

MitchHolley1

try this:

AreaList <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">"1000 feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"23908 feet"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"1200000000 feet"</SPAN><SPAN class="punctuation token">]</SPAN>

MeterList <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN>a<SPAN class="punctuation token">.</SPAN>replace<SPAN class="punctuation token">(</SPAN><SPAN class="string token">"feet"</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="string token">"meters"</SPAN><SPAN class="punctuation token">)</SPAN> <SPAN class="keyword token">for</SPAN> a <SPAN class="keyword token">in</SPAN> AreaList<SPAN class="punctuation token">]</SPAN>

<SPAN class="keyword token">print</SPAN> MeterList<SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

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