Lecture Notes on For Loops and Range Function in CPSC 121 (Fall 2012), Study notes of Computer Science

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• For loops
• Range function

HW

• HW 6 out
• Proj 1 out

The for statement

Iteration (looping) allows us to repeat blocks of code

For example, instead of writing:

the list = ["spam", "ham", "eggs"]

print(the list[0])

print(the list[1])

print(the list[2])

We can use a for statement:

for v in the list: print(v)

• Note we don’t need to know the length of the list here!

The “accumulator pattern”

acc = initial value # "accumulate" results in this variable for x in the list: acc = acc operation x

do something with acc

Generalizes many computations over lists

• For example, sum, product, difference, etc.

Example

the_list = [3, 4, 5, 6] diff = 50 for x in the_list: diff = diff - x print(diff)

Q: What is the value of diff after every iteration?

Writing functions that take lists as parameters

Works the same as other parameters

def print_list(the_list): for x in the_list: print(x)

my_list = [1, 2, 3] print_list(my_list) 1 2 3 print_list(["spam", "ham", "eggs"]) spam ham eggs

• Here the list is a regular old parameter
• When the function is called, the list is assigned to the list object

range allows us to iterate a fixed number of times

for i in range(3): print("spam")

Q: What is printed?

range also gives access to list indexes

the list = [10, 20, 30] the length = len(the list) for i in range(the length): the list[i] = the list[i] * 2

Q: What does this do to the list?

• Doubles each value!