Expressions and Operators in C

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Expressions in C

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• We use math formulae all the time in physics, chem, math
• a = b / 5
• x = y * y + z * z
• x = (int)(pow((double)y, 2.0) + pow((double)z, 2.0))

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• these formulae expressions
• x = y * y + z * z is an expression for C
• y * y + z * z is also an expression for C
• y * y is also an expression for C
• z * z is also an expression for C

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Expressions and Operators

• Expressions in C consist of one or more variables/constants
• An expression contains one or more operators, such as

c = a + b - 2;

• Operators in C can be one of the following type
• Arithmetic
• Unary
• Relational and logical
• Assignment
• Conditional

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Yes.

But I will tell you some other interesting things about them and other operators

I think I have already seen/used Arithmetic and Assignment operators in previous lectures/labs!

Arithmetic operators

• Already seen. Operate on int, float, double (and char)

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Unary operators

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Unary Operators - Negative

• Operators that take only one argument (or operand)
• -5
• -b
• Observe that – is both an arithmetic and unary operator
• Meaning depends on context
• This is called overloading

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Unary operators – increment and decrement

• Increment (++) increases a variable by 1
• Decrement (--) decreases a variable by 1
• ++variable is the pre-increment operator
• Means increment, then use
• variable++ is the post-increment operator
• Means use, then increment
• Likewise, the -- can be pre/post decrement

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int main(){

char a = ‘A’; float b = 3.31;

printf("%c\t%f\n",++a,b++);

printf("%c\t%f",--a,b--);

return 0;

}

B 3.31

A 4.31

Work with all data types

Note: The variable’s value will change for the rest of the program

Unary operators - sizeof

• Syntax
• sizeof var
• sizeof(type)
• Returns size of the operand in bytes
• sizeof(char) will return 1
• sizeof(float) will (mostly) return 4
• Very useful when you are porting programs across computers

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Unary operators - typecast

• Syntax
• (type) var, for example – (int) a, (float) a, etc
• We have already seen this
• What will be the output of this program?

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int main(){

double a = 67.2;

printf("size is %d\n", sizeof a);

printf("size is %d\n", sizeof((char) a));

printf("%c", (char) a);

return 0;

}

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Size is 8

Size is 1

C

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Precedence Rules for Unary Operators

• Precedence rules tell us the order in which the operators will be applied in any C expression
• Unary ops are above arithmetic ops, only below brackets
• If a is 1 and b is 2, what will a + -b be evaluated as?

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• What about this program?

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int main(){

int a = 1; int b = 2;

printf("%d", a + - + - b);

return 0;

}

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3

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-1

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Bracket has the highest precedence

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Note +x is x

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Associativity Rules for Unary Operators

•  Associativity rules tell us how the operators of same precedence are grouped (e.g., a+b+c will be evaluated as (a+b)+c, not a+(b+c))
• For unary operators, the associativity is from right to left
• Important to remember this
• Most other operators’ associativity is left to right (e.g., + operator)
• What will this program print?

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int main(){

int a = 1;

printf("%d", - ++a);

return 0;

}

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-2

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Relational Operators

• Compare two quantities

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• Work on int, char, float, double…

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Result is 0 or 1

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1 means condition true, 0 means false

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Relational Operators: Some Examples

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Avoid mixing int and float values while comparing. Comparison with floats is not exact!

Assignment Operator

• Basic assignment (variable = expression)

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Precedence of Assign Operators

• Always the last to be evaluated
• x *= -2 *(y+z)/3
• x = x*(-2*(y+z)/3)
• Seldom need to worry about it

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Operator Precedence

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Example: a + b – c * d % e /f

LOW

HIGH

(a+b) - (((c *d ) % e) / f)

Earlier the ASCII table. Now this table? Have to memorize this??

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No.

Write it in your notebook

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Logical Operators

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• Remember
• value 0 represents false.
• any other value represents true. Compiler returns 1 by default

Logical Operators: Truth Table

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“E” for expression

Logical Operators: Some Examples

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Logical Operators: Precedence and Associativity

• NOT has same precedence as equality operator
• AND and OR are lower than relational operators
• OR has lower precedence than AND
• Associativity goes left to right

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1 && 0 || 1 || 0

0 || 1 || 0

1 || 0

1

2 == 2 && 3 == 1 || 1==1 || 5==4

Operator Precedence for various operators

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INCREASING

LOW

HIGH

Note: Precedence of brackets () are above every other operator

Note: This list doesn’t include some other operators that we have not yet seen

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