Finding sample frequency?

Alright so I have a text file of 600 samples from a ECG.

Using this code, I found the peaks:

%Find ECG peaks
close all, clear all, clc

sig=load('ecg.txt'); %Load
plot (sig) %plot
xlabel('Samples')
ylabel('Electrical Activity')
title('ECG Signal')
hold on, plot(sig,'ro')

Peak=0; %Peak Value
Beat_count=0 %Number of Heartbeat

for i=2:length(sig)-1
    if(sig(i)>sig(i-1) && sig(i)>sig(i+1) && sig(i)>1.7)
        disp ('Peak found')
        i, Peak=sig(i)
        Beat_count=Beat_count+1
    end

I also created a graph comparing samples to electrical activity.

The question says: "Plot a circle around each of the P wave peaks that you have found." So I'm guessing I need to take this a step further, but I'm stuck. It also says: (*hint: In the time vector, tmax can be determined by subtracting 1 from the sample size and dividing by the sample frequency.) But how can I find the sample frequency without time? Or am I overthinking this and the graph above is all I need?

[Chad Greene]

Sounds like your professor has given you a time vector. Is sig a vector or is it 2D? As an aside, I suspect you only need to plot red circles at the peaks, not at every sig data point. So instead of

plot(sig,'ro')

try

plot(sig(sig>1.8),'ro')

[Star Strider]

You must be provided the sampling frequency or sampling interval. There is no way to determine it from the data alone. To detect the P-waves, you have to use the findpeaks function and the intersect function to find the points between 0.5 and 0.7. Those will be the P-waves, at least in the EKG segment you posted.