Im new to Matlab and was having trouble doing this question. Any help would be much appreciated.

An engineer has recorded a sound signal from a microphone, and sampled it; these values are stored in a vector. The units of each sample is volts. The microphone was not on at all times, and, therefore, some data samples below a certain threshold are considered to not valid data samples; samples greater than or equal to the threshold are valid. The en- gineer would like to know the average voltage of the sound signal, given by, sum of valid samples/number of valid samples and the fraction of valid data samples, given by, number of valid samples/total number of samples . If the number of valid samples is 0, then the function must return 0 for both the average voltage and the fraction of valid data points.

Write a function, computeAverageVoltage with the following signature.

function [average_voltage, frac_valid] =
computeAverageVoltage(v, voltage_threshold)

Inputs:

v - a vector that contains the values of all the voltage samples (in volts) voltage_threshold - threshold voltage (in volts)

Output:

average_voltage - average voltage computed over valid samples frac_valid - fraction of valid samples

Note: You are expected to use an appropriate loop to iterate through the vector and compute the required values, rather than using MATLAB built-in functions such as, sum, average and vector computations

[Stephan]

Please show us what you have tried so far - provide your code and ask for the specific problem you have doing this homework

[Stephen23]

@Sadullah Tasbas: please show us what you have tried so far.

[Saad]

I'm having trouble getting started. I'm not sure how i can pick certain values from within a matrix which satisfy v<voltage_threshold.

 function [average_voltage, frac_valid]=computeAverageVoltage(v,voltage_threshold)
for i=1:length(v)
    if (v<voltage_threshold)
    average_voltage=(sum(v(i)))/(length(v(i)));
    frac_valid=(length(v(i)))/(size(v));
    i=i+1;
    else
        average_voltage=0;
        frac_valid=0;
    end
end

[Walter Roberson]

In your if you should be testing v(i) rather than v

[Saad]

I tried that but it still doesn't work. I know im doing something wrong but i don't know what it is.

[Steven Lord]

Hint: the problem statement said you weren't allowed to use sum. However, it did not say that the plus operator + was forbidden.

Rather than using length (which will give the number of elements of a vector but would give you something different if you called it on a matrix) or size to determine the number of elements, there is a function that gives you exactly that information for vectors, matrices, 17-dimensional arrays, etc. Search the documentation for "number of elements" to find it. [IMO that's a good habit to develop: if you're not sure if there's a function to do what you need, think of a couple of keywords related to your task and search the documentation.]

[Saad]

im assuming that you are referring to the numel operation. How would i select certain elements from a given matrix that satisfy a particular property and also count the number of elements that satisfy the property?

Answer 1

[Image Analyst]

Hint:

meanOfGoodValues = mean(v > voltage_threshold);

No loop needed.

[Saad]

The question specifically asks for a loop. is there a way i can do it whilst still using a loop?

[Image Analyst]

Yes but you'd need to check each value one at a time, not the whole vector, and keep track of the sum and count separately, then compute the mean after the loop is done:

if (v(i) > voltage_threshold)
    count = count + 1;
    theSum = theSum................

etc.

[Saad]

so am I on the right track so far?

[Image Analyst]

No, but if you follow my guidance you will be.

[Saad]

"you'd need to check each value one at a time, not the whole vector, and keep track of the sum and count separately, then compute the mean after the loop is done". How would i do this exactly?

[Image Analyst]

if (v(i) > voltage_threshold)

checked one element at a time, v(i), which is the i'th element.

if (v > voltage_threshold)

like you had, checks the whole vector and gives not one true or false value, but a whole array of them - a true or false value for every element in the vector so you get a binary/logical/boolean vector, not a single value.

Answer 2

[Robert Allison]

lol ESD2