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Python Loop Addition

MatthewDriscoll

I am using a search cursor to get some values. After I would like to add the next total to the total of all the results before it in order. Not sure what to do, iterate, range, re-loop?

Example:

[a,b,c] would result in [a, a+b, a+b+c]

[3,5,2,3] would result in [3, 8, 10, 13]

value <SPAN class="operator token">=</SPAN> <SPAN class="string token">""</SPAN>
<SPAN class="keyword token">with</SPAN> arcpy<SPAN class="punctuation token">.</SPAN>da<SPAN class="punctuation token">.</SPAN>SearchCursor<SPAN class="punctuation token">(</SPAN><SPAN class="string token">"Feature"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"Field"</SPAN><SPAN class="punctuation token">)</SPAN> <SPAN class="keyword token">as</SPAN> curs<SPAN class="punctuation token">:</SPAN>
 <SPAN class="keyword token">for</SPAN> row <SPAN class="keyword token">in</SPAN> curs<SPAN class="punctuation token">:</SPAN>
 value <SPAN class="operator token">=</SPAN> row<SPAN class="punctuation token">[</SPAN><SPAN class="number token">0</SPAN><SPAN class="punctuation token">]</SPAN><SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

Use the values in "value" to do what I want.

The script will run in ArcGIS Pro.

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Accepted answers

DarrenWiens2

Use the += operator to accumulate values:

<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> input_list <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">5</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">2</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN> output_list <SPAN class="operator token">=</SPAN> <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> value <SPAN class="operator token">=</SPAN> <SPAN class="number token">0</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN> <SPAN class="keyword token">for</SPAN> i <SPAN class="keyword token">in</SPAN> my_list<SPAN class="punctuation token">:</SPAN> <SPAN class="comment token"># do similar inside your cursor</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN>&nbsp;&nbsp;&nbsp;&nbsp; value <SPAN class="operator token">+=</SPAN> i <SPAN class="comment token"># same as value = value + i</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN>&nbsp;&nbsp;&nbsp;&nbsp; output_list<SPAN class="punctuation token">.</SPAN>append<SPAN class="punctuation token">(</SPAN>value<SPAN class="punctuation token">)</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN> <SPAN class="keyword token">print</SPAN> output_list
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN> 
<SPAN class="punctuation token">[</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="number token">8</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="number token">10</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="number token">13</SPAN><SPAN class="punctuation 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>

All comments

MatthewDriscoll

Thank you! I was getting tripped up by my simple mistake of using value = " " instead of value = 0.

DanPatterson_Retired

Then someone will want the cumulative sum returned so they can calculate proportions besides numpy and scipy are builtins in Pro distributions anyway

<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> a <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">5</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">2</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> s <SPAN class="operator token">=</SPAN> np<SPAN class="punctuation token">.</SPAN>sum<SPAN class="punctuation token">(</SPAN>a<SPAN class="punctuation token">)</SPAN>
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> cs <SPAN class="operator token">=</SPAN> np<SPAN class="punctuation token">.</SPAN>cumsum<SPAN class="punctuation token">(</SPAN>a<SPAN class="punctuation token">)</SPAN>
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> cstand <SPAN class="operator token">=</SPAN> cs<SPAN class="operator token">/</SPAN>s
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> cstand
array<SPAN class="punctuation token">(</SPAN><SPAN class="punctuation token">[</SPAN> <SPAN class="number token">0.23076923</SPAN><SPAN class="punctuation token">,</SPAN>&nbsp; <SPAN class="number token">0.61538462</SPAN><SPAN class="punctuation token">,</SPAN>&nbsp; <SPAN class="number token">0.76923077</SPAN><SPAN class="punctuation token">,</SPAN>&nbsp; <SPAN class="number token">1</SPAN><SPAN class="punctuation token">.</SPAN>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <SPAN class="punctuation token">]</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>

or just plain old slicing... but you will need to use a def if you really want to fill values into a table one record at a time.... enough for code golf

<SPAN class="punctuation token">[</SPAN>sum<SPAN class="punctuation token">(</SPAN>a<SPAN class="punctuation token">[</SPAN><SPAN class="punctuation token">:</SPAN>i<SPAN class="punctuation token">]</SPAN> <SPAN class="punctuation token">)</SPAN> <SPAN class="keyword token">for</SPAN> i <SPAN class="keyword token">in</SPAN> range<SPAN class="punctuation token">(</SPAN> <SPAN class="number token">1</SPAN><SPAN class="punctuation token">,</SPAN> len<SPAN class="punctuation token">(</SPAN>a<SPAN class="punctuation token">)</SPAN><SPAN class="operator token">+</SPAN><SPAN class="number token">1</SPAN> <SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">]</SPAN><SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

bixb0012

Where are you storing the results, in the table the search cursor is iterating over? If so, you could use an update cursor and do it in place.

Looking just at cumulative totals, the previous responses are all good. One other option that splits the difference between rolling your own loop and using a 3rd-party library is the accumulate function in itertools (only work in Pro).

<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> <SPAN class="keyword token">from</SPAN> itertools <SPAN class="keyword token">import</SPAN> accumulate
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> input_list <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">5</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">2</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> list<SPAN class="punctuation token">(</SPAN>accumulate<SPAN class="punctuation token">(</SPAN>input_list<SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN>
<SPAN class="punctuation token">[</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="number token">8</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="number token">10</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="number token">13</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>

DanPatterson_Retired

If you are scripting an doing a lot of math, invest some time in learning numpy, a lot of these calculations are builtin

<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> <SPAN class="keyword token">import</SPAN> numpy <SPAN class="keyword token">as</SPAN> np
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> input_list <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">5</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">2</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> np<SPAN class="punctuation token">.</SPAN>cumsum<SPAN class="punctuation token">(</SPAN>input_list<SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">.</SPAN>tolist<SPAN class="punctuation token">(</SPAN><SPAN class="punctuation token">)</SPAN>
<SPAN class="punctuation token">[</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="number token">8</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="number token">10</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="number token">13</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>

DarrenWiens2

Use the += operator to accumulate values:

<SPAN class="operator token">>></SPAN><SPAN class="operator token">></SPAN> input_list <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">5</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">2</SPAN><SPAN class="punctuation token">,</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN> output_list <SPAN class="operator token">=</SPAN> <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> value <SPAN class="operator token">=</SPAN> <SPAN class="number token">0</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN> <SPAN class="keyword token">for</SPAN> i <SPAN class="keyword token">in</SPAN> my_list<SPAN class="punctuation token">:</SPAN> <SPAN class="comment token"># do similar inside your cursor</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN>&nbsp;&nbsp;&nbsp;&nbsp; value <SPAN class="operator token">+=</SPAN> i <SPAN class="comment token"># same as value = value + i</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN>&nbsp;&nbsp;&nbsp;&nbsp; output_list<SPAN class="punctuation token">.</SPAN>append<SPAN class="punctuation token">(</SPAN>value<SPAN class="punctuation token">)</SPAN>
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN> <SPAN class="keyword token">print</SPAN> output_list
<SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN><SPAN class="punctuation token">.</SPAN> 
<SPAN class="punctuation token">[</SPAN><SPAN class="number token">3</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="number token">8</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="number token">10</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="number token">13</SPAN><SPAN class="punctuation 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>

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