# How to stop Replace duplicating a list when using Table?

I have a 3 dimensional list of data pairs in the form:

{{{{x11,y11},{x12,y12}},{x1...,y1...}},{{x21,y21},{x22,y22}},{x2...,y2...}}... etc


Incidentally the data originates from a Gas Chromatograph, and is ordered by: [[Channel (1-4), Experiment Number (1 - some number), Peak Number (1 - some number)]], and the pairs represent {Retentiom time, Peak area}.

I want to use the replace function to search the data under the condition that the retention time falls within some range, when this condition is satisfied for the pair: {x_,y_}, x is replaced by the name of the gas that elutes e.g. CH4, C2H4 etc., and y is replaced by the peak area multiplied by a response factor.

The retention time window condition is defined for each channel and for each species. Therefore, I made a list of species and channels containing the time windows, the species name and response factor: e.g.

species={CH4,C2H4,etc...}
CH4={CH4ch1,CH4ch2,CH4ch3,CH4ch4}
CH4ch1={{retentiontimeMin,retentiontimeMax},{"speciesname",responsefactor}}


The code that I am using is as follows:

Table[gcData[[channel, All, All]] /. {x_, y_} /;
species[[l, channel, 1, 1]] < x <
species[[l, channel, 1, 2]] -> {species[[l, channel, 2, 2]],
y*species[[l, channel, 2, 1]]}, {channel, 1, 4}, {l, 1, 7}]


Where gcData is the list of data, and l is the species number.

My problem is that when I use replace I get my original list of data, duplicated 7 times (the number of species I am searching for), where in each iteration the species I am searching for is correctly identified and replace, but on the subsequent iteration I do not see the originally replaced species.

A sample section from the resultant data is shown below:

{{{{"CH4", 34423708},
{1.581, 240631},
{1.741, 63594},
{2.192, 5568}}}
{{{1.406, 34423708},
{"C2H6", 0.292896},
{1.741, 63594},
{2.192, 5568},}}},
{{{1.406, 34423708},
{1.581, 240631},
{"C2H4", 0.0772031},
{2.192, 5568},}}}}


How do I stop it duplicating the list?

• this is because you are clearly not storing the results of the replacements. I would recommend you to use Do to iterate and store the results after the replacement. ReplaceAll will not modify the original list and thus the need to be stored. – Ali Hashmi May 14 '17 at 14:39
• Thank you for your response Michael E2! I had never used the Fold function, or the Slot function very effectively before. The code you suggested didn't do quite what I wanted, but with some manipulation I got there. – Butters May 14 '17 at 18:52

Maybe this use of Fold?

Table[
Fold[
# /. {x_, y_} /; #2[[channel, 1, 1]] <
x < #2[[channel, 1, 2]] :> {#[[channel, 2, 2]],
y*#[[channel, 2, 1]]} &,
gcData[[channel, All, All]],
species],
{channel, 1, 4}]


See Applying Functions Repeatedly for discussion of FoldList and Fold.

• +1 ! i was thinking of Fold too but since i did not have a thorough understanding of the question so did not propose. With your answer i have a better understanding now. – Ali Hashmi May 14 '17 at 15:51