State the value and type of each expressions.
Answers
Answer:
An expression is a combination of values, variables, and operators. If you type an expression on the command line, the interpreter evaluates it and displays the result:
>>> 1 + 1
2
The evaluation of an expression produces a value, which is why expressions can appear on the right hand side of assignment statements. A value all by itself is a simple expression, and so is a variable.
>>> 17
17
>>> x
2
Confusingly, evaluating an expression is not quite the same thing as printing a value.
>>> message = "What's up, Doc?"
>>> message
"What's up, Doc?"
>>> print message
What's up, Doc?
When the Python shell displays the value of an expression, it uses the same format you would use to enter a value. In the case of strings, that means that it includes the quotation marks. But the print statement prints the value of the expression, which in this case is the contents of the string.
In a script, an expression all by itself is a legal statement, but it doesn’t do anything. The script
17
3.2
"Hello, World!"
1 + 1
produces no output at all. How would you change the script to display the values of these four expressions?
2.6. Operators and operands
Operators are special symbols that represent computations like addition and multiplication. The values the operator uses are called operands.
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 symbols +, -, and /, and the use of parenthesis for grouping, mean in Python what they mean in mathematics. The asterisk (*) is the symbol for multiplication, and ** is the symbol for exponentiation.
When a variable name appears in the place of an operand, it is replaced with its value before the operation is performed.
Addition, subtraction, multiplication, and exponentiation all do what you expect, but you might be surprised by division. The following operation has an unexpected result:
>>> minute = 59
>>> minute/60
0
The value of minute is 59, and 59 divided by 60 is 0.98333, not 0. The reason for the discrepancy is that Python is performing integer division.
When both of the operands are integers, the result must also be an integer, and by convention, integer division always rounds down, even in cases like this where the next integer is very close.
A possible solution to this problem is to calculate a percentage rather than a fraction:
>>> minute*100/60
98
Again the result is rounded down, but at least now the answer is approximately correct. Another alternative is to use floating-point division. We’ll see in the chapter 4 how to convert integer values and variables to floating-point values.
2.7. Order of operations
When more than one operator appears in an expression, the order of evaluation depends on the rules of precedence. Python follows the same precedence rules for its mathematical operators that mathematics does. The acronym PEMDAS is a useful way to remember the order of operations:
Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluated first, 2 * (3-1) is 4, and (1+1)**(5-2) is 8. You can also use parentheses to make an expression easier to read, as in (minute * 100) / 60, even though it doesn’t change the result.
Exponentiation has the next highest precedence, so 2**1+1 is 3 and not 4, and 3*1**3 is 3 and not 27.
Multiplication and Division have the same precedence, which is higher than Addition and Subtraction, which also have the same precedence. So 2*3-1 yields 5 rather than 4, and 2/3-1 is -1, not 1 (remember that in integer division, 2/3=0).
Operators with the same precedence are evaluated from left to right. So in the expression minute*100/60, the multiplication happens first, yielding 5900/60, which in turn yields 98. If the operations had been evaluated from right to left, the result would have been 59*1, which is 59, which is wrong
Explanation: