Introduction to Python

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Project Warmup Exercise Solutions, Session 1



PLEASE NOTE: EXERCISES ARE NOT TO BE TURNED IN. FOR HOMEWORK, PLEASE SEE 'PROJECTS'
 
Ex. 1.1 Assign two new integer objects and one float object to three variable names. Sum up the three variables and assign the resulting object to a new variable name. Indicate (by printing) the value and type of this resulting object.
Suggested Solution: 
a = 5              # int, 5
b = 10             # int, 10
c = 3.0            # float, 3.0

d = a + b + c      # float, 18.0

print(d)
print(type(d))     # <class 'float'>

This demonstrates that an integer and float together in a math expression result in a float. This is a "behavior" that should be internalized. This will help in the "tip calculator" when calculating math expressions where a float value is expected.
 
Ex. 1.2 Assign two new integer objects to two variable names. Multiply the two variables and assign the resulting object to a new variable name. Indicate (by printing) the value and type of this resulting variable's object.
Suggested Solution: 
aa = 5             # int, 5
bb = 10            # int, 10

cc = aa * bb       # int, 50

print(cc)
print(type(cc))    # <class 'int'>

Demonstrates that two integers in a math expression result in an integer. Again, this "behavior" should be internalized.
 
Ex. 1.3 Starting with the sample code (make sure it is included exactly as written) sum up the values to produce the integer value 15. (Hint: apply a conversion function to each string so it can be used as a number.)
Suggested Solution: 
var = '5'               # str, '5'
var2 = '10'             # str, '10'

ivar = int(var)         # int, 5
ivar2 = int(var2)       # int, 10

var3 = ivar + ivar2     # int, 15

print(var3)

Numeric values often come into our program as strings (for example, text files or input()). In order to use them as numbers, we must convert. Use int() to produce a new integer value based on the string object, or float() to produce a float. int() will be essential in the homework assignments, since each assignment will rely on input() to take numeric values from the user.
 
Ex. 1.4 Take user input for an integer and print that value doubled.
Suggested Solution: 
# input() returns a str
xx = input('Please enter an integer:  ')  # str, '5' (sample value)

yy = int(xx)      # int, 5
zz = yy * 2       # int, 10

# we use + to concatenate strs
# return val of str() can be
# used directly in + expression
print(xx + ' doubled is ' + str(zz) + '.')

This exercise takes the previous exercise a step further by having the string numeric value come from input(). Another technique demonstrated here is the use of str() passed to print() that concatenates strings -- this is a very basic way to produce a single string made up of string and non-string (i.e. the integer in yy) objects.
 
Ex. 1.5 Take user input for a 'place' and then greet the place enthusiastically.
Suggested Solution: 
pp = input('Please enter a place name:  ')

print('Hello, ' + pp + '!')

This also shows how a string stored in a variable pp can be used in a string passed to print() by concatenating the strings.
 
Ex. 1.6 Take user input for an integer and apply that many exclamation points to the phrase, "Hello, world!" (Hint: use the "string repetition" operator)
Suggested Solution: 
# input() returns a str
aa = input('Please enter an integer:  ')

bb = int(aa)
cc = '!' * bb          # str * int repeats the str that many times
                       # and returns a new str

print('Hello, World' + cc)  # string concatenation

This demonstrates the use of the string repetition operator (*, which repeats a string when used with a string and int operand), which will be useful in the "Exponientation with tidy border" assignment.
 
Ex. 1.7 Assign the float value 35.30 to a variable, then round the value to 35.
Suggested Solution: 
var = 35.30           # a float value

var2 = round(var)     # round takes a float and with no 2nd
                      # argument, returns an int with the rounded
                      # value.

print(var2)

Rounding will be useful in the "tip calculator". With only one float argument, round() returns an int, in this case 35. The second argument (used in the next exercise) will allow for a rounding to a set number of decimal places.
 
Ex. 1.8 Assign the float value 35.359958 to a variable, then round the value to two decimal places.
Suggested Solution: 
var = 35.359958     # this many-places value is the kind of number
                    # one might expect from float division

# round the float value to 2 places (another float)
var2 = round(var, 2)

print(var2)

This use of round() will be useful in the "tip calculator" assignment, because we will want to round a float calculation to its nearest dollars and cents value (i.e., 2 decimal places).
 
Ex. 1.9 Starting with the following code (make sure it is included exactly as written), divide the first number by the second number.
Suggested Solution: 
var = "5"
var2 = "4"

var3 = int(var) / int(var2)  # return values from int() are
                             # used in a math expression

print(var3)

This exercise combines our technique of converting string values to numeric types. Note how we are using calls to int() in a mathmatical expression. We could have assigned the int() calls to individual variables, but instead we chose to simply put them into the math expression.
 
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