Today's lab will continue our focus on designing program and introduces pseudocode. It also briefly introduces decisions.
For example, say you need to display text messages for your company, and you are designing the basic template. To do so, you would like to the average length of a line in the file, as well as the maximum line length. Here's a simple algorithm:
1. total, maximum = 0 2. n = number of lines in the file 3. For each line in the file: 4. l = length of the line 5. total = total + l 6. if l > max 7. maximum = l 8. average = total / nThe algorithm finds the length of each line in the file. Then, following the accumulator design pattern (introduced at the end of Chapter 3), it adds the length of the current line to the variable total. The variable total `accumulates' the line lengths. Next (lines 6 and 7), the pseudocode uses a new programming construct (defined formally in the next chapter) called an if statement. An if-statement allows you to make decisions in a program. We will use it to test if our current line is longer than max:
6. if l > maximum 7. maximum = lIf it is, we update the maximum variable to be l. When the algorithm starts, max is set to 0, and each time a longer line is seen, maximum is updated to that length.
A simple way to translate pseudocode into a program is to cut-and-paste it into a python file as comments:
# CIS 166 Lab 9
# A program to calculate average and maximum line lengths for a file
def main():
print("This program will calculate the average and maximum line lengths for a file")
#1. total, maximum = 0
#2. n = number of lines in the file
#3. For each line in the file:
#4. l = length of the line
#5. total = total + l
#6. if l > maximum
#7. maximum = l
#8. average = total / n
main()
Save this and try running it. Note that the only thing it does is print an
introductory message.
Now, let's fill in the pieces of suggested in the pseudocode. Let's start with setting up our variables and opening the file:
# CIS 166 Lab 9
# A program to calculate average and maximum line lengths for a file
def main():
print("This program will calculate the average and maximum line lengths for a file")
fname = input("Enter file name: ")
infile = open(fname, "r")
#1. total, maximum = 0
#2. n = number of lines in the file
#3. For each line in the file:
#4. l = length of the line
#5. total = total + l
#6. if l > maximum
#7. maximum = l
#8. average = total / n
main()
Next, we can fill in the pseudocode. Line #1 is straightforward: it says to set up
two variables total and maximum to have value 0:
# CIS 166 Lab 9
# A program to calculate average and maximum line lengths for a file
def main():
print("This program will calculate the average and maximum line lengths for a file")
fname = input("Enter file name: ")
infile = open(fname, "r")
total = 0
maximum = 0
#2. n = number of lines in the file
#3. For each line in the file:
#4. l = length of the line
#5. total = total + l
#6. if l > maximum
#7. maximum = l
#8. average = total / n
main()
We have several choices for reading data from a file.
Since we are going through the file line-by-line, we will use readlines()
since it will return a list of the lines. Since each line is stored as a separate
entry in the list, the length of the the list is the number of lines in the file:
# CIS 166 Lab 9
# A program to calculate average and maximum line lengths for a file
def main():
print("This program will calculate the average and maximum line lengths for a file")
fname = input("Enter file name: ")
infile = open(fname, "r")
total = 0
maximum = 0
lines = infile.readlines()
n = len(lines)
#3. For each line in the file:
#4. l = length of the line
#5. total = total + l
#6. if l > maximum
#7. maximum = l
#8. average = total / n
main()
The next three lines of the pseudocode (lines #3-5) we have seen before. We
also added in two print statements to check that things are working correctly (they
are just
temporary and will be removed for the final version):
# CIS 166 Lab 9
# A program to calculate average and maximum line lengths for a file
def main():
print("This program will calculate the average and maximum line lengths for a file")
fname = input("Enter file name: ")
infile = open(fname, "r")
total = 0
maximum = 0
lines = infile.readlines()
n = len(lines)
for line in lines:
l = len(line)
print(l, line[:-1]) #Temporary, to check that things are working
total = total + l
#6. if l > maximum
#7. maximum = l
#8. average = total / n
print("num lines =", n,"total =", total)
main()
If statements are formatted very similarly to for loops (we will cover them in detail
in a later chapter). Here is the translation into python (with a bit more also added to
the file print statement):
# CIS 166 Lab 9
# A program to calculate average and maximum line lengths for a file
def main():
print("This program will calculate the average and maximum line lengths for a file")
fname = input("Enter file name: ")
infile = open(fname, "r")
total = 0
maximum = 0
lines = infile.readlines()
n = len(lines)
for line in lines:
l = len(line)
print(l, line[:-1]) #Temporary, to check that things are working
total = total + l
if l > maximum:
maximum = l
#8. average = total / n
print("num lines =", n,"total =", total, "max =", maximum)
main()
Lastly, let's calculate the average and print it (and remove the extra prints that we
were using for testing):
# CIS 166 Lab 9
# A program to calculate average and maximum line lengths for a file
def main():
print("This program will calculate the average and maximum line lengths for a file")
fname = input("Enter file name: ")
infile = open(fname, "r")
total = 0
maximum = 0
lines = infile.readlines()
n = len(lines)
for line in lines:
l = len(line)
total = total + l
if l > maximum:
maximum = l
average = total / n
print("max =", maximum, "\taverage line length =", average)
main()
Since pseudocode is a standard way to convey algorithms, we will work more with pseudocode as the semester progresses.
If you finish early, you may work on the programming problems.