Expressions
Richard Gesick
Figures from Lewis, “C# Software Solutions”, Addison Wesley
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• Expressions
• Data Conversion
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• Combination of one or more operators and operands
• Has both data type & value
• Operands may be
– literals
– constants
– variables
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Operator Operation
/
*
-
+
% addition subtraction multiplication division modulus
(remainder after division)

• When dividing two integers:
– the quotient is an integer
– the remainder is truncated (discarded)
• To get the remainder, use the modulus operator with the same operands
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• 25 / 3
• 25 % 3
• 3 / 25
• 3 % 25
• 25.0 / 5
• 10 / 3.0
8
1
0
3
5.0
3.33333

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Operator Precedence
What is the order of evaluation in the following expressions?
a + b + c + d + e a + b * c - d / e
1 2 3 4 3 1 4 2 a / (b + c) - d % e
2 1 4 3 a / (b * (c + (d - e)))
4 3 2 1
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• The assignment operator has a lower precedence than the arithmetic operators
First the expression on the right hand side of the = operator is evaluated answer = sum / 4 + MAX * lowest;
4 1 3 2
Then the result is stored in the variable on the left hand side

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First the expression on the right hand side of the = operator is evaluated
Expression has data type and value variable = expression;
Then the result is stored in the variable on the left hand side (has if types are compatible data type ),

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• The right and left hand sides of an assignment statement can contain the same variable
First, one is added to the original value of count count = count + 1;
Then the result is stored back into count
(overwriting the original value)

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++ increment by 1
-decrement by 1
Example: count++; // count = count + 1; count--; // count = count - 1;

• The increment and decrement operators can be applied in postfix form: count++
• or prefix form:
++count
• When used as part of a larger expression, the two forms can have different effects
• Because of their subtleties, the increment and decrement operators should be used with care
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• when the increment (or decrement) operator is used in a “stand alone” statement solely to add one (or subtract one) from a variable’s value, it can be used in either prefix or postfix form
USE EITHER dogs -; -dogs;
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• when the increment (or decrement) operator is used in a statement with other operators, the prefix and postfix forms can yield different results
LET’S SEE HOW. . .
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PREFIX FORM
“First increment, then use ” int alpha ; int num ;
13 num alpha num = 13;
14 num alpha = ++num * 3;
14 42 num alpha

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POSTFIX FORM
“Use, then increment ” int alpha ; int num ;
13 num alpha num = 13;
13 39 alpha alpha = num++ * 3; num
14 num

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Operator Example Equivalent
+=
-=
*=
/=
%= a += 3; a = a + 3; a -= 10; a = a - 10; a *= 4; a /= 7; a = a * 4; a = a / 7; a %= 10; a = a % 10;

• The right hand side of an assignment operator can be a complex expression
• The entire right-hand expression is evaluated first, then the result is combined with the original variable
• Therefore result /= (total-MIN) % num; is equivalent to result = result / ((total-MIN) % num);
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• No spaces are allowed between the arithmetic operator and the equals sign
• Note that the correct sequence is +=, not =+
Example: add 2 to a
// incorrect a =+ 2; // a = +2; assigns 2 to 2
// correct a += 2; // a = a + 2;

Assigning the Values of Other Variables
• Syntax: dataType variable2 = variable1 ;
• Rules:
1. variable1 needs to be defined before this statement appears in the source code
2. variable1 and variable2 need to be compatible data types; in other words, the precision of variable1 must be lower than or equal to that of variable2.
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• When performing calculations with operands of different data types:
– Lower-precision operands are promoted to higher-precision data types, then the operation is performed
– Promotion is effective only for expression evaluation; not a permanent change
– Called "implicit type casting"
• Bottom line: any expression involving a floating-point operand will have a floatingpoint result.

• Widening conversion (promotion)
– safe, do not lose data
– goes to a “wider” (more bits) data type
– may lose precision (long to float)
• Narrowing conversion (demotion)
– may lose data
– may lose precision
– should be avoided
– compiler error unless specific cast done
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• assignment conversion
– assign an int to a long
• promotion
– divide an int by a double
• casting
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• Syntax:
(dataType)( expression )
Note: parentheses around expression are optional if expression consists of 1 variable
• Useful for calculating averages double result = (double) 25 / 3; double result = (double) total / count;
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