Loop assistance

1. check your while() condition. If you want always to exit the loop via the break at the start, you can use while(1), otherwise while(index_value<=0)

2. your code does not assign a new value to index_value after the curvefit. Perhaps you need another BinarySearch to prevent the loop from running forever.

3. enable the debugger, set a breakpoint and step through your code to see what is happening.

	variable index_value	
	do
		CurveFit/M=2/W=0 dblexp_XOffset, selection [50,stop_point]/X=Time__seconds_[50,stop_point]/D
		index_value = BinarySearch(selection_fit, y_half)
		if (index_value > 0)
			break
		endif
		DoAlert 1 /T="ERROR: STOP POINT OUT OF RANGE", 0, "The stop point entered is out of range for the half-life. Do you want to select a stop point within the range?"
		if (v_flag == 2)
			break
		endif
		Prompt stop_point, "Enter desired stop point"       //  prompt user to give new stop value
		DoPrompt "Enter_Stop_Point", stop_point		
	while (1)

you might also want to consider what happens if curvefit fails, or if the user enters a problematic value for stop_point. programming to catch fitting errors is a bit more tricky.

Thank you for such a quick response! I tried using the code you sent but it was affecting the values for my other variables. I'm not sure why. I took elements of your code and tried it in my program to see if that could resolve the issue. However, when I did, I encountered the same issue of index_value being read as -2. Using debugger, I discovered that the values for the new graph were really the old wave values from the first curve-fitting. I corrected this by using KillWaves to get rid of the values from the first wave, allowing the new wave with the curve-fitting values to be used. The old curve wave and the new curve wave have the same name. That's why I was having so many issues. Thanks for your help!

do
					wave /z	w_coef						// reference w_coef wave
					wave /z w_fitconstants				// reference w_fit_constants wave
		
					variable y0 = W_coef[0]						// define variables from w_coef wave
					variable A1 = W_coef[1]
					variable tau1 = W_coef[2]
					variable A2 = W_coef[3]
					variable tau2 = W_coef[4]
		
					variable x0 =  W_fitConstants[0]		// constant 
		
					variable y_half = ( ( y0 - ( y0 + A1 + A2 ) ) / 2 ) + ( y0 + A1 +A2 )		// y-half calculation is rough estimate, not exact y-half value
				
					variable index_value		// index value is number point on curve fit graph		
					index_value = BinarySearch(selection_fit, y_half)
				
				if (index_value > 0)		
					break
				endif
					KillWaves /Z w_coef 					// kill previous waves to avoid 'cross-talk' between old wave and new wave
					KillWaves /Z w_fit_constants
					DoAlert /T="ERROR: STOP POINT OUT OF RANGE", 0, "The stop point entered is out of range for the half-life. Click 'OK' and select a stop point within the range"		// alert user their selection is out of range
	
					Prompt stop_point, "Enter desired stop point"		//	prompt user to give new stop value	
					DoPrompt "Enter_Stop_Point", stop_point
   					Hideinfo
					CurveFit/M=2/W=0 dblexp_XOffset, selection [50,stop_point]/X=Time__seconds_[50,stop_point]/D
					AutoPositionWindow/E/M=1
	
				while (1)		// continue from break #1
					print "	index value = ", index_value