Loop Structures


Exercise: Prompt the user to enter a sequence of positive integers, one at a time. The user enters zero or a negative number to quit. Print the sum of the positive integers.


This is an example of a loop called a sentinel loop. The loop gets and processes values until seeing a special value called the sentinel, that indicates that processing should stop. In the above exercise, the sentinel is either 0 or a negative number.

In general, the psuedo-code for a sentinel loop is like this:

read the first value
while value is not the sentinel:
     process the value
     read the next value


Example: Prompt the user to enter exam scores, and print the average score. The user enters a negative value to quit.

Psuedo-code:
Set sum to 0
Set count to 0
Read the first score
while score is non-negative:
     add score to sum
     add 1 to count
     read next score
print average

Sample Output:
Enter a number (negative to quit): 67
Enter a number (negative to quit): 88
Enter a number (negative to quit): 99
Enter a number (negative to quit): -1

The average of the grades is 84.67.


Example (Using a loop to check for valid input)
Write a program that asks the user for a positive integer, and continues prompting the user until the value entered is positive. Then compute the sum of the integer's digits.
Note: Suppose the user enters 526. Then 526/10 = 52, and
526%10 = 6. So 52/10 = 5, and 52%10 = 2. And 5/10 = 0, and
5%10 = 5.
 
For Loops

for loops are sometimes called definite loops, since for loops execute the body of the loop a specified, or definite, number of times.

General form:
for variable in sequence:
   statement
   statement
   etc.

The variable after the keyword "for" is called the loop index - it takes on the values in the sequence, one at a time, and the loop body is executed once for each value.

Example:
for i in [1, 2, 3, 4]:
   print i

Output:
1
2
3
4




The range() function

Example:
>>> range(5)
[0, 1, 2, 3, 4]
>>> range(3, 8)
[3, 4, 5, 6, 7]
>>> range(2, 10, 3)
[2, 5, 8]


Example: Write a for loop that prints the even numbers from 2 to 20. The output should appear on one line, with the numbers separated by a blank space.


Example: What is the output of the following loop? Can you re-write it so it looks nicer?
for i in range(6):
   print i, 2**i


Example: What is the output of this loop?
for num in [3, 2, 1, 5]:
   print num,
Simple Functions

Recall the form of a function definition:
def functionName(parameter1, parameter2, parameter3):
    statement 
    statement
    etc.

The statements which are indented under the function's header are called the function body, and these statements are executed when the function is called.




To call this function:
functionName(value1, value2, value3)

When the function is called, the statements in the body of the function are executed, with parameter1 set to value1, parameter2 set to value2, and parameter3 set to value3.


Example:
# This program contains a function that takes an integer and
#  prints 2 raised to the specified power.

def main():
    # for each i from 1 to 10, print 2**i
    for i in range(1, 11):
        printPower(i)  # call (execute) the printPower function

def printPower(theExponent):
    value = 2**theExponent
    print value


main()  # call the main function



Output:
2
4
8
16
32
64
128
256
512
1024


Example:
Write a function called printLineTwice() that takes a string as its parameter, and prints that string twice on a single line.
Write a main function that  calls printLineTwice() 10 times.