# Finding Coordinates of a Peak on a Plot

I have plotted a graph from a CSV file which consists of two columns of data Time and Intensity. I simply want to find the time where the intensity is largest.

This code:

MaxIntensity = Max[Data[[All, 2]]]


outputs the correct value for the peak y axis value.

However, this code:

Position[Data[[All, 1]], MaxIntensity]


just outputs {}.

Also, this code:

Position[Data, MaxIntensity]


outputs {{429, 2}}.

Looking at the CSV file in a text editor the 429 is the line number where the peak value occurs, but I can't figure out what that 2 means and I can't use that output to pick out the corresponding time for index 429.

Any ideas?

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• The "2" means it comes from the second column, i.e. you're finding the position of the intensity in a 2D list. – dr.blochwave Mar 18 '15 at 22:29
• How can I extract the Row number only then so I can use that further? – Q.P. Mar 18 '15 at 22:31

data = {{0, 98}, {1, 100}, {2, 110}, {3, 99}, {4, 100}}

(* ==> {{0, 98}, {1, 100}, {2, 110}, {3, 99}, {4, 100}} *)

maxIntensity = Max[data[[All, 2]]]

(* ==> 110 *)

First@Flatten[Position[data, {_, maxIntensity}]]

(* ==> 3 *)


Here, I used Position on data directly, but with a pattern in which the first entry of the pair {_, maxIntensity} means that it is irrelevant for the position search. Have a look at Blank. The First@Flatten is needed because Position could return more than one match, and I assume you only want the first match. If you want all of them, just omit First.

There are also other approaches. For example, you could use Ordering:

Ordering[data, 1, #1[] > #2[] &]

(* ==> {3} *)

• Beautiful. Thanks! – Q.P. Mar 18 '15 at 22:39