Lecture #6: Statements

Computer Programming (6) - Statements

administrivia

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

precedence and associativity

Precedence	Associativity	Operator
15	left	expr>++, expr>-- (post)
14	right	++expr>, --expr> (pre) !<, ~, +, -, *, & (unary)
13	left	*, /, %
12	left	+, -
11	left	<<, >>
10	left	<, >, <=, >=
9	left	==, !=
8	left	& (binary)
7	left	^ (binary)
6	left	\| (binary)
5	left	&&
4	left	\|\|
3	left	expr ? true_expr : false_expr
2	right	=, +=, -=, =, /=, %=, <<=, >>=*, <=, ^=, \|=
1	left	,

the if statement

only allows 2 choices: true and false
can be extended using if/else if/…/else

there is a conditional expression, too

a ternary expression (3 operands)
allows you to embed a simple decision inside an expression
no side-effects

a convenient shorthand for simple if statements.

/*
 * z = max (a, b);
 */
if (a > b)
  z = a;
else
  z = b;

conditional expression

expr₁ ? expr₂ : expr₃
expr₁ is evaluated first
if expr₁ is non-zero (true), then the value of the overall expression is the value of expr₂
if expr₂ is zero (false), then the value of the overall expression is the value of expr₃
/* * z = max (a, b); */ z = (a > b) ? a : b;

conditional expression details

expr₁ ? expr₂ : expr₃
only one of the two expressions after the ? is evaluated.
- this means if one of those expressions has a side-effect, it only happens if that's the selected expression
  z = (a > b) ? a++ : b--;

the switch statement

a multiway conditional statement
- similar to the if-else if-else "statement"
- allows the selection of an arbitrary number of choices based on an integer value
  switch (expression) {
    case const-expr: statements
    case const-expr: statements
    …
    default: statements
  }

the switch statement

expression must have an integral value
when the switch statement is executed:
- the expression is evaluated
- if a case matches the value of the expression, the program jumps to the first statement after that case label
- otherwise, the default case is selected

switch example

switch (grade/10) {
  case 10:
  case 9:
    printf ("A\n");
    break;
  case 8:
    printf ("B\n");
     break;
  case 7:
    printf ("C\n");
    break;
  case 6:
    printf ("D\n");
    break;
  default:
    printf ("F\n");
    break;
}

what's that "break;"?

case 8:
  printf ("B\n");
  break;

when the switch transfers to the chosen case, it starts executing statements at that point
it will keep going into the next case unless you do something
- break; is that something
  - it causes the program to immediately jump to the next statement after the switch statement
  - "breaking" out of the switch statement

falling through

without the break statement, execution would keep going into the next case

we call this "falling through"

switch (grade/10) {
  case 10:
  case 9:
    printf ("A\n");
  case 8:
    printf ("B\n");
  case 7:
    printf ("C\n");
  case 6:
    printf ("D\n");
  default:
    printf ("F\n");
}

falling through

we can sometimes make use of this:

switch (c) {
  case 'a':
  case 'A':
    a_count++;
    break;
  case 'b':
  case 'B':
    b_count++;
    break;
}

switch vs. if

in many cases, you can rewrite one using the other
- but often one is easier and more readable

iteration statements

also called "loops"
we have three "flavors":
- while
- for
- do-while
all do similar things
- repeatedly execute a statement until some condition is not met

iteration statements

like if, all iteration statements have one or more control expressions
- these control the execution of the loop
…and a loop body
- this is the statement (or block) that gets repeatedly executed

while loops

while ( expression ) statement
as with the if statement, expression is evaluated as true (non-zero) or false (zero)
when this loop is executed:
- the expression is evaluated
- if the expression is true the statement is executed
- then the expression is evaluated again
- if the expression is true the statement is executed again
- then the expression is evaluated again
- etc.

while loops

while ( expression ) statement
If the expression evaluates to false, execution transfers to the next statement after the while
the statement may, of course, be a compound statement

while loop examples

/* compute n factorial */
int i = 0;
int factorial = 1;
while (i < n) {
  i++;
  factorial *= i;
}

/* print powers of 2 that are less than 1000 */
int i = 1;
while (i < 1000) {
  printf ("%d\n", i);
  i *= 2;
}

while loop examples

while ((c = getchar()) != EOF) {
  if (c == ' ')
    blank_count++;
  else if (c >= '0' && c <= '9')
    digit_count++;
  else if (c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z')
    letter_count++;
  else if (c == '\n')
    newline_count++;
}

while loop examples

while (fahr <= upper) {
  celsius = 5 * (fahr - 32) / 9;
  printf ("%d\t%d\n", fahr, celsius);
  fahr = fahr + step;
}

for loops

for ( expr₁; expr₂; expr₃ ) statement
this is (almost) equivalent to writing:
expr₁;
while (expr₂) {
statement
expr₃;
}
this is easiest to see by example.

for loops

for ( expr₁; expr₂; expr₃ ) statement

in an early lecture, we saw a program with this for loop:

for (i = 0; i < 10; i = i + 1)
  printf ("i is %d\n", i);

in this case:
- expr₁ is i = 0
- expr₂ is i < 10
- expr₃ is i = i + 1
- statement is printf ("i is %d\n", i);

for loops

and we could have written this for loop

for (i = 0; i < 10; i = i + 1)
  printf ("i is %d\n", i);

using while instead

int i = 0;
while (i < 10) {
  printf ("i is %d\n", i);
  i = i + 1;
}

for loop examples

/* compute n factorial */
int i = 0;
int factorial = 1;
while (i < n) {
  i++;
  factorial *= i;
}

/* compute n factorial */
int factorial = 1;
for (i = 1; i <= n; ++i) {
  factorial *= i;
}

for loop examples

/* print powers of 2 that are less than 1000 */
int i = 1;
while (i < 1000) {
  printf ("%d\n", i);
  i *= 2;
}

/* print powers of 2 that are less than 1000 */
int i;
for (i = 1; i < 1000, i *= 2) {
  printf ("%d\n", i);
}

review

we've talked about:
- expression statements
  - any valid expression with a semicolon stuck on the end
- compound statements
  - one or more statements (and optional declarations) surrounded by braces
- selection statements
  - if-else
  - switch
- iteration statements
  - while
  - for

one more iteration statement: do-while

do statement while ( expression );
a lot like the while statement
- except the expression is tested after statement, not before
- the difference is that statement is always executed at least once, even if expression is never true
  - with the while statement, the body would not be executed even once in that case

do-while example

do {
  printf ("Input a positive integer: ");
  scanf ("%d", &n);
  if (error = (n <= 0))
    printf ("\nERROR: Try again!\n");
} while (error);

jump statements: break and continue

break

break exits a loop or a switch statement immediately
- but only from the innermost loop if it's a nested loop
this example code repeatedly reads a number from the keyboard and prints its square root. If the number is negative, it stops.
while (1) { scanf ("%f", &x); if (x < 0.0) break; printf ("%f\n", sqrt(x)); }

break example

#include <stdio.h>
#include <math.h>

int main (void)
{
  int i, j;

  printf ("%d\n", 2);
  for (i = 3; i <= 100; ++i) {
    for (j = 2; j < i; ++j) {
      if (i % j == 0)
        break;
      if (j > sqrt(i)) {
        printf (%d\n", i);
        break;
      }
    }
  }
  return 0;
}

continue

continue ends the current iteration of a loop
- the control expression is evauated
- depending on the value, the next iteration starts
unlike break, continue doesn't end the loop, it just skips to the end of the current cycle
- it's not used a whole lot, but sometimes it comes in handy

goto

there's another jump statement, called goto
- it is almost universally deplored, and rarely necessary
- we will not use it in this class

how do we write programs?

we’ve looked at a number of example programs
- we’ve even talked about how they work
- but we haven’t talked about how to create one
what we do know:
- how to create variables (declarations and data types)
- most common/useful operators and expressions
- all of the possible statements
with these building blocks, we can begin writing simple programs that actually do something interesting

but… but…

it’s a big step from knowing these thing to:
- being able to interpret a program
- being able to write a program
let’s learn how to do both of these

interpreting a program

say we’re given a program like this one:

#include <stdio.h>

int main (void)
{
  int i, j;

  printf ("  ");
  for (j = 1; j <= 10; ++j)
    printf (" %3d", j);
  printf ("\n");

  for (i = 1; i <= 10; ++i) {
    printf ("%2d", i);
    for (j = 1; j <= 10; ++j)
      printf (" %3d", i*j);
    printf ("\n");
  }
  return 0;
}

what do we see right away?

we see a declaration
- 2 variables, i and j
we see a loop
- he variable j goes from 1 to 10, and we just print it out
we see another loop
- this time, i goes from 1 to 10
  - and a nested loop, where j goes from 1 to 10

the first loop

anyone want to guess what this first loop does?

printf ("  ");
  for (j = 1; j <= 10; ++j)
    printf (" %3d", j);
printf ("\n");

it loops over the numbers from 1 to 10
- and prints each one out
- remember that ‘\n’ is a newline
- so all the numbers are printed on the same line
1 2 3 4 5 6 7 8 9 10

the second loop

for (i = 1; i <= 10; ++i) {
  printf ("%2d", i);
  for (j = 1; j <= 10; ++j)
    printf (" %3d", i*j);
    printf ("\n");
}

this is a bit more involved
- we have a nested loop here
- that means that each time we go through the outer loop, we run the whole inner loop completely.
  - we run the body of the outer loop 10 times
  - each time, the inner loop runs 10 iterations
  - so the body of the inner loop is run 100 times

the second loop

So first, i is set to 1
- and we print it (no newline)
- then the inner loop takes j from 1 to 10
  - and each time we print out i * j with no newline
  - 1 2 3 4 5 6 7 8 9 10
the next time, i is 2
- and we print it (no newline)
- then the inner loop takes j from 1 to 10
  - and each time we print out i * j with no newline
  - 2 2 4 6 8 10 12 14 16 18 20

so put the output together…

This program prints a multiplication table

1 2 3 4 5 6 7 8 9 10
1 1 2 3 4 5 6 7 8 9 10
2 2 4 6 8 10 12 14 16 18 20
3 3 6 9 12 15 18 21 24 27 30
…
- would you have been able to figure that out?
- you should
- this is a very simple program. Most real programs are much more complicated.

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

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