Computer Programming (5) - Operators, Expressions, and Statements

administrivia

Web Page - http://www.cs.tau.ac.il/~efif/courses/ComputerProgramming

equality operators

==, !=
test whether operands are equal or not equal
return 1 or 0

precedence and associativity

Precedence	Associativity	Operator
15	left	++, -- (post)
14	right	++, -- (pre)
13	left	*, /, %
12	left	+, -
10	left	<, >, <=, >=
9	left	==, !=
2	right	=, +=, -=, *=, /=, %=

equality operator issues

don't count on testing floating-point types with ==
- why?
  - real numbers in the computer can differ by an insignificant amount
  - ==tests for exactly equal
- so instead, test the difference between them
  if (abs (a - b) < 0.01) { … }

important!

Don't confuse == with =
if (i = 0) …
versus
if (i == 0) …
the worst part is, this is legal C code; it just doesn’t do what you (probably) want
- the expression i = 0
  - sets the value of i to 0
  - its value is 0
- the expression i == 0
  - doesn’t set the value of anything
  - its value is 1 or 0 depending on whether i is 0
this can cause bugs that are very hard to find!

logical operators

&&, ||
logical AND and OR
- (a && b) is true if both a and b are true
- (a || b) is true if either a or b are true, or both.

evaluation stops when truth value is known

int a = 1, b = 2, c = 3, d = 4;

if ((a == b) && (c != d)) {
 …
}

precedence and associativity

Precedence	Associativity	Operator
15	left	++, -- (post)
14	right	++, -- (pre)
13	left	*, /, %
12	left	+, -
10	left	<, >, <=, >=
9	left	==, !=
5	left	&&
4	left	\|\|
2	right	=, +=, -=, *=, /=, %=

consider
expr₁ && expr₂ || expr₃ && expr₄
- first expr₁ is evaluated
- if true, expr₂ is evaluated, and if true evaluation stops
- if false, expr₃ is evaluated, and so on

consider

expr₁ || expr₂ && expr₃ || expr₄

this is interpreted as
(expr₁ || (expr₂ && expr₃)) || expr₄

what is an expression?

nobody has a good, clear definition.
a sequence of operators and operands that has a value and/or does something interesting
there are a bunch of types

how are expressions used?

as operands in larger expressions

i = j++
5 * (fahrenheit - 32) / 9
(a < b) || (c > d)

as parts of statements
while (i > 0) for (i = 0; i < 10; ++i)

side effects in expressions

some operators have “side effects”
- change the value of their operands
side effects can be combined, but probably shouldn't
- modifying the same object twice?
  i = i++; i = (j = 5) + j++;

we've finally finished chapter 2!

2.1 Variable Names
2.2 data types
2.3 constants
2.4 declarations
2.5 arithmetic operators
2.6 relational operators
2.7 type conversions
2.8 increment/decrement operators
2.9 bitwise operators
2.10 assignment operators
2.12 precedence

what's a statement?

a specification of some action to be performed
executed sequentially

what kinds of statements do we have?

expression statements
compound statements
selection statements
iteration statements
jump statements

what kinds of statements do we have?

expression statements
compound statements
selection statements
iteration statements
jump statements

expression statements

any expression can be a statement
- just stick a semicolon (;) on the end
- but they're only evaluated for their side effects
  - the expression's value is ignored
    i = 5; i++; ++i; celsius = 5 * (fahrenheit – 32) / 9;

expression statements

since the value is ignored, expressions that only have a value are not meaningful statements
- they're not illegal, just useless
- some compilers will warn you about them
  - the assumption is that you forgot something
    a + b; 32;

special case: the null statement

an empty statement – just a semicolon
used when the syntax requires a statement, but you don't want to do anything.
- noting to initialize in a for loop
  for(; i < n; i++) ...

what kinds of statements do we have?

expression statements
compound statements
selection statements
iteration statements
jump statements

compound statements

also called statement blocks
groups one or more statements into a unit
- syntactically equivalent to a single statement
- that means that wherever it's legal to have a single statement, you can have a compound statement.
can have declarations and initializations
- but only at the beginning of a block

compound statement syntax

statements surrounded by braces { }
- no semicolon at the end
- although the statements and declarations in the block use semicolons as normal

use of compound statements

impossible to use C without using compound statements

we've already seen them a lot

/*
 * HelloWorld: A simple C program
 */

#include <stdio.h>

int main (void)
{
  printf ("Hello, world!\n");
  return 0;
}

compound statements

example

#include <stdio.h>

int main (void)
{
  int i;
  int power = 1;

  printf ("2 to the %d power is %d\n", i, power);

  for (i = 0; i < 10; ++i) {
    power *=2;
    printf ("2 to the %d power is %d\n", i, power);
  }
  return 0;
}

what kinds of statements do we have?

expression statements
compound statements
selection statements
iteration statements
jump statements

what's a selection statement?

selects statements to execute based on the value of an expression
- the expression is sometimes called the controlling expression
these are more interesting statements
selection statements:
- if statement (§3.2-3.3)
- switch statement (§3.4)

selection statements: if

used to execute conditionally a statement or block of code
critical to writing interesting programs - otherwise, the code always follows the same path.

simple if statement

if( expression) statement
expression must have a scalar type
- can't be an array or complex data structure
  - but we don't know about those anyway
if the expression evaluates to true (i.e. non-zero) then statement is executed
Remember what we said about compound statements
- they are valid anywhere a single statement is valid
- so you can use a block here

if statement examples

if (grade >= 90)
  printf ("Congratulations!\n");
printf ("Your grade is %d\n", grade);

/* * calculate the absolute value of a number */ int abs (int number) { if (number < 0) number = -number; return number; }

if statement examples

int i, j;
int min;

if (i < j) {
   min = i;
   printf ("i is less than j\n");
}
if (j < i) {
   min = j;
   printf ("j is less than or equal to i\n");
}

if-else statement

if (expression) statement₁ else statement₂
if expression evaluates to true, statement₁is executed.
if expression evaluates to false (0), statement₂ is executed
both statements can be (and very often are) compound statements

if-else example

int min;
int i, j;

if (i < j) {
   min = i;
   printf ("i is less than j\n");
} else {
   min = j;
   printf ("j is less than i\n");
}

a comment on style

I wrote the generic syntax as:
if (expression) statement₁ else statement₂
and your book says:
  if (expression)
    statement₁
  else
    statement₂
what's the difference?
- there isn't any, to the compiler

if statement style

wherever a space is valid, you can also use a newline
- so the two statements are identical
- but it's easier to read the second one
  int min; int i, j; if (i < j) min = i; else min = j;

if statement style

it's also easier to read code when nested blocks are indented

int min;
int i, j;

if (i < j) {
min = i;
printf ("i is less than j\n");
} else {
min = j;
printf ("j is less than i\n");
}

versus

int min;
int i, j;

if (i < j) {
   min = i;
   printf ("i is less than j\n");
} else {
   min = j;
   printf ("j is less than i\n");
}

nesting of if statements

you can use any statement as the body of an if

even another if statement

if (month == FEB) {
  if (is_leap_year (year)) {
    if (day > 29)
      legal = FALSE;
  } else
    if (day > 28)
      legal = FALSE;
}

discussion of if statements

if statements let you distinguish between exactly 2 cases, and do something different for each.
they are very powerful
- many computer programs need to react to changing circumstances or different input
- computer programs need to check for bad data or error conditions
  - before the error or the bad data cause the program to crash
  int i, j, k; if (j != 0) k = i / j; else printf ("Error: division by 0!\n");

but what about 3 or more cases?

we can simulate this using nested ifs:

if (grade >= 90)
  printf ("A\n");
else
  if (grade >= 80)
    printf ("B\n");
  else
    if (grade >= 70)
      printf ("C\n");
    else
      if (grade >= 60)
        printf ("D\n");
      else
        printf ("F\n");

but we can write this more readably, breaking our new style rules

if (grade >= 90)
  printf ("A\n");
else if (grade >= 80)
  printf ("B\n");
else if (grade >= 70)
  printf ("C\n");
else if (grade >= 60)
  printf ("D\n");
else
  printf ("F\n");

there's a good example of this in the book in section 3.3

a glance at arrays

a variable that can hold several elements (values)
not limited to any particular type
- can have arrays of int, char, double, etc.
int a[10]; float temperatures[10];
You specify which of the several values you're referring to by using a numeric subscript
the subscript is an integer expression in square brackets

arrays are zero-based array

a[0] = 10;
a[1] = 20;
a[2] = a[0] + a[1];

cannot set all elements of an array at once
a = 0; /* WRONG */
cannot assign one array to another
int b[10]; b = a;/* WRONG */

one by one

int b[10];

for(i = 0; i < 10; i++)
  b[i] = a[i];

examples

computing powers of 2 - power2.c
computing Fibonacci series - fibonacci.c
rolling dices - dice.c
english to decimal - eng2dec.c

Maintained by Efi Fogel. Last modified: January 25 2003.