Lab 7: Python Loops and Images 1

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1 Lab 7: Python Loops and Images 1 Objectives to use while loops and for loops to further explore image and pixel features to further understand and practice using parameters in functions Copy the Lab 7 folder into your M:/CS150/Labs folder. Rename the folder to yournamelab07_python_loops_images_1 (e.g. MillerJohnLab07_Python_Loops_Images_1). Now write Python programs for each of the following exercises. As you complete this lab, show your lab instructor or TA the result of each part before continuing and when you are finished, hand in your program in the N:\HandIns\CS150 folder. Exercise 1: Using a while-loop The human population of the world has been rising over the years. In recent years the annual growth rate has been around 1.1%, but was at its peak of around 2% in the 1960 s. Create a Python function called yearstodouble() that takes two parameters, an integer representing the world s current population, and a decimal value representing the annual growth rate of the population and returns the number of years it would take for the population to double. Your function should use a while loop that will continue to repeat the calculation for the future population of each succeeding year until the population doubles. Once you have created the yearstodouble() function, write a main() function to test it. How many years will it take for the population to double if the annual growth rate is 0.011? 0.02? 0.05? (1.1%, 2%, 5%)

2 Exercise 2: Using a while-loop and a for-loop Create a Python function called yearstodoublegraph that graphically represents the doubling of the earth s human population using a horizontal bar chart. Each bar of the chart will represent the population for a particular year. Your function will take two parameters: an integer representing the world s current population, and a decimal value representing the annual growth rate of the population and will return a picture of the chart. This function is a modification of the function from the first exercise. (Start with a copy of the first program and rename it.) We know that the current population is around seven and a half billion, so twice that will be less than 16 billion. We can use a graph scale of 50 pixels per billion. See: to get a glimpse into how fast it is growing with the visual of a live counter. To write this function, let s break this task into parts (use an algorithm). You may first want to look at the graph shown below to get a quick sense of what the algorithm will produce when coded into a solution. 1. Create a blank canvas using the makeemptypicture() function with a size of Use the addtext() function to add a title to the canvas. 3. Add a filled rectangle to the canvas representing the current population. Use a scale of 50 pixels per one billion people to compute the rectangle width, rounding the value to the nearest integer, and use 10 pixels for the height of the bar. 4. Add text to the rectangle, indicating the current population. 5. Use a while-loop to add additional bars, each one 10 pixels lower, until the population has at least doubled. Compute the new population by multiplying the old population value by the growth rate. Again, multiply the new population by the scaling factor 50 pixels per one billion people, rounded to the nearest integer, to get the bar width. Include a counter in your loop that keeps track of how many years have elapsed, and add text to the graph at the end of each bar with the year number. 6. Add text to report how many years it took to double and the population at that point 7. Add a line below the graph to be used as the x-axis 8. Use a for-loop to put number labels on the x-axis for every one billion in population, spacing the numbers every 50 pixels to match the scale of the bars. 9. Add a label for the x-axis below the numbered line. 10. Return the canvas; the main() function will display the graph. On the next page, there is an example of what a solution might look like. Your solution should look similar.

3 As usual, write main() to test your function. Also, add the code to main() to write your name on the bar chart image on the middle-right side, (use addtext to add your name.) Finally, add the code to save your bar chart image in your lab folder. You will need to use the writepictureto() function. Details for saving an image are on the next page.

4 To save your image as a file: 1. Include a call to the setmediapath() function as the first line in main().when you run your program for the first time you will be prompted to browse to the folder where you will store and retrieve any files accessed during the run of your program. (This is the path where your files are located for this exercise.) This function sets the path used to find and or save your files and allows you to use the getmediapath() function instead of typing in the complete path to your files. 2. Ask the user for a name for the file: filename = requeststring("type in the name of the file to save: ") 3. Use the filename in the writepictureto() function as shown below. We use the getmediapath() function to determine where to store the file. writepictureto(picname, getmediapath(filename))

5 Exercise 3. Optional challenge exercise. Create a new Python program with a function called greentoblueandblacktored() that takes one picture object parameter. Your function should change the color of all green pixels to blue and all black pixels should be set to the color red. Use the getpixels() function to create a list of all of the pixels in the picture and then use a for-loop to examine each one and change the color if it is green or black. for apixel in listofallpixels: When you write the code in main() to test your function, use the file named greeneyes.bmp as your original image. Once you have that working as desired, add two more lines of code to main() to save the new image. (Ask the user for the name of the file and then use the writepictureto() function to save it in your Lab folder. For this exercise name your file blueeyes.bmp) Show your TA or Lab Instructor your work when you are finished and remember to hand in your program to the N:\HandIns\CS150 folder.