Finding the value of x for a corresponding known y value for a curve

I have a code for temperature against time step. The time step is dt and temperature is ucn. I used the plot function to plot ucn against dt. I have to find the first value of dt where the temperature crossess the y=143 or y=142 line. i need the value of dt wherever it crosses first

Answer 1

[Matt J]

find(diff(sign(ucn-143))~=0,1) + 1

[Atheeb Amjard]

thank you very much Matt J, would you mind explaining the code if possible

[Matt J]

You're quite welcome. Please Accept-click the answer to indicate that a solution to your question was reached.

The code just looks for the first occurence where ucn-143 changes sign, i.e., when diff(sign(ucn-143))~=0.