plot ErrorBar

<pre name="code" class="python">#quote from MIT 'introduction to computation and programming using python, Revised'import randomimport pylabdef stdDev(X):    mean = sum(X)/len(X)    tot = 0.0    for x in X:        tot += (x-mean)**2    return (tot/len(X))**0.5    def flipSim(flipsPerTrial, numTrials):    fracHeads = []    for n in range(numTrials):        heads = 0        for f in range(flipsPerTrial):            if random.random() < 0.5:                heads += 1        fracHeads.append(heads/float(flipsPerTrial))    mean = sum(fracHeads)/len(fracHeads)    sd = stdDev(fracHeads)    return (fracHeads, mean, sd)    def showErrorBars(minExp, maxExp, numTrials):    """Assumes minExp and maxExp positive ints; minExp < maxExp         numTrials a positive integer       Plots mean fraction of heads with error bars"""    means, sds = [], []    xVals = []    for exp in range(minExp, maxExp + 1):        xVals.append(2**exp)        fracHeads, mean, sd = flipSim(2**exp, numTrials)        means.append(mean)        sds.append(sd)    pylab.errorbar(xVals, means, yerr=2*pylab.array(sds))    pylab.semilogx()    pylab.title('Mean Fraction of Heads (' + str(numTrials) + ' trials)')    pylab.xlabel('Number of flips per trial')    pylab.ylabel('Fraction of heads & 95% confidence')showErrorBars(3, 10, 100)pylab.show()