Lecture #11: Statements 2

Bob Pitha

bpitha@cs.ucsd.edu

http://www-cse.ucsd.edu/classes/fa99/cse5a

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

example

• printing powers of 2

$Text Box: #include <stdio.h> int main (void) { int i = 0; int power = 1; printf (“2 to the %d power is %d\n”, i, power); power *=2; i++; printf (“2 to the %d power is %d\n”, i, power); power *=2; i++; printf (“2 to the %d power is %d\n”, i, power); return 0; }$

what kinds of statements do we have?

• Expression statements

• Compound statements

• Selection statements

• Iteration statements

• Jump statements

compound statements

• Another example:

$Text Box: #include <stdio.h> int main (void) { int i; int power = 1; printf (“2 to the %d power is %d\n”, i, power); for (i = 0; i < 10; ++i) { power *=2; printf (“2 to the %d power is %d\n”, i, power); } return 0; }$

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

$Text Box: if (grade >= 90) printf (“Congratulations!\n”); printf (“Your grade is %d\n”, grade);$

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

return number;
}

if statement examples

$Text Box: int i, j; int min; … if (i < j) { min = i; printf (“i is less than j\n”); } if (j < i) { min = j; printf (“j is less than i\n”); }$

if-else statement

if ( expression ) statement₁ else statement₂

• If expression evaluates to true, statement₁ is executed.

• If expression evaluates to false (0), statement₂ is executed

• Both statements can be (and very often are) compound statements

if-else example

$Text Box: int min; int i, j; … if (i < j) { min = i; printf (“i is less than j\n”); } else { min = j; printf (“j is less than i\n”); }$

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

Text Box: 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

$Text Box: int min; int i, j; … if (i < j) { min = i; printf (“i is less than j\n”); } else { min = j; printf (“j is less than i\n”); }$

if statement style

• It’s also easier to read code when nested blocks are indented

$Text Box: int min; int i, j; … if (i < j) { min = i; printf (“i is less than j\n”); } else { min = j; printf (“j is less than i\n”); }$

nesting of if statements

• You can use any statement as the body of an if

– even another if statement

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

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

$Text Box: int i, j, k; … if (j != 0) k = i / j; else printf (“Error: division by 0!\n”);$

but what about 3 or more cases?

• We can simulate this using nested ifs:

$Text Box: if (grade >= 90) printf (“A\n”); else if (grade >= 80) printf (“B\n”); else if (grade >= 70) printf (“C\n”); else if (grade >= 60) printf (“D\n”); else printf (“F\n”);$

but we can write this more readably

• We’ll just break our new style rules

$Text Box: if (grade >= 90) printf (“A\n”); else if (grade >= 80) printf (“B\n”); else if (grade >= 70) printf (“C\n”); else if (grade >= 60) printf (“D\n”); else printf (“F\n”);$

– There’s a good example of this in the book in section 3.3

Lecture #11: Statements 2

Bob Pitha

bpitha@cs.ucsd.edu

http://www-cse.ucsd.edu/classes/fa99/cse5a

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

example

• printing powers of 2

what kinds of statements do we have?

• Expression statements

• Compound statements

• Selection statements

• Iteration statements

• Jump statements

compound statements

• Another example:

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 statement examples

if-else statement

if ( expression ) statement1 else statement2

• If expression evaluates to true, statement1 is executed.

• If expression evaluates to false (0), statement2 is executed

• Both statements can be (and very often are) compound statements

if-else example

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

if statement style

• It’s also easier to read code when nested blocks are indented

if statement style

• It’s also easier to read code when nested blocks are indented

if ( expression ) statement₁ else statement₂

• If expression evaluates to true, statement₁ is executed.

• If expression evaluates to false (0), statement₂ is executed

if ( expression ) statement₁ else statement₂

statement₁

statement₂