Operators and Operands¶

Operators are special tokens that represent computations like addition, multiplication and division.

The following are all legal Python expressions whose meaning is more or less clear:

20+32   hour-1   hour*60+minute   minute/60   5**2   (5+9)*(15-7)

The tokens +, -, and *, and the use of parenthesis for grouping, mean in Python what they mean in mathematics.
The asterisk (*) is the token for multiplication, and
** is the token for exponentiation.

print(2 ** 3)
print(3 ** 2)

(exponentiation)

When a variable name appears in the place of an operand, it is replaced with its value before the operation is performed.

Example: so let us convert 645 minutes into hours:

(conversion)

In Python 3, the division operator / always yields a floating point result.
The floor division uses the token //.
- if it has to adjust the number it always moves it to the left on the number line.
- So 6 // 4 yields 1, but -6 // 4 yields -2!

minutes = 90
hours = minutes // 60
print(hours)

(conversion2)

Type converter functions¶

Here we’ll look at three more Python functions, int, float and str, which will (attempt to) convert their arguments into types int, float and str respectively.
We call these type converter functions.

The int function can take a floating point number or a string, and turn it into an int. For floating point numbers, it discards the decimal portion of the number — a process we call truncation towards zero on the number line. Let us see this in action:

print("int(3.14) returns")
print(int(3.14))
print("int(3.9999) doesn't round to the closest int!")
print(int(3.9999))
print("int(3.0) returns the same of int(3.9999)")
print(int(3.0))
print("int(-3.999) returns the result closer to zero")
print(int(-3.999))
print("int(minutes / 60) replaces minutes with its value")
minutes=120
print(int(minutes / 60))
print("Parsing a string to produce an int")
print(int("2345"))
print("It even works if arg is already an int")
print(int(17))

#print(int("23 bottles"))

(intconverter)

The type converter float can turn an integer, a float, or a syntactically legal string into a float:

print(float(17))
print(float("123.45"))

(floatconverter)

The type converter str turns its argument into a string:

print(type(str(17)))
print(type(str(123.45)))

(strconverter)

Operations on strings¶

In general, you cannot perform mathematical operations on strings, even if the strings look like numbers "15"+2 is an error

Interestingly, the + operator does work with strings, but for strings, the + operator represents concatenation.

Concatenation means joining the two operands by linking them end-to-end. For example:

fruit = "banana"
baked_good = " nut bread" #The spaces is part of the string.
print(fruit + baked_good)

(concat)

The * operator also works on strings; it performs repetition. For example,

fruit = "banana"
print(fruit*3)
#This is the same as
print(fruit+fruit+fruit)

(repetition)

The modulus operator¶

The modulus operator works on integers (and integer expressions) and gives the remainder when the first number is divided by the second.
In Python, the modulus operator is a percent sign (%).
It has the same precedence as the multiplication operator.

q = 7 // 3     # This is integer division operator
print(q)
r  = 7 % 3
print(r)

(modulus)

You can check whether one number is divisible by another
x % 10 yields the right-most digit of x (in base 10).
x % 100 yields the last two digits.

It is also extremely useful for doing conversions, say from seconds, to hours, minutes and seconds. So let’s write a program to ask the user to enter some seconds, and we’ll convert them into hours, minutes, and remaining seconds.

total_secs = int(input("How many seconds, in total?"))
hours = total_secs // 3600
secs_still_remaining = total_secs % 3600
minutes =  secs_still_remaining // 60
secs_finally_remaining = secs_still_remaining  % 60

print("Hrs=", hours, "  mins=", minutes,  "secs=", secs_finally_remaining)

Next Section - Input