control structures lone leth thomsen. february 2006basis-c-2/ll2 contents operators lots of control...

Control structures

Lone Leth Thomsen

February 2006 Basis-C-2/LL 2

Contents

• Operators

• Lots of control statements


Operators

• So – what are they really?– General expressions are formed by joining together

constants and variables (also called operands) via various operators

– In C, operators are split into categories• Arithmetic• Unary• Relational and logical• Assignment• Equality• Conditional


Operators 2

• The most common unary operator is minus– Make sure not to confuse this with the arithmetic minus

– +123, -123 (positive and negative numbers)

– ++i, --i (adds/subtracts one to/from i, then returns the new value of i

– i++, i-- (adds/subtracts one to/from i, then returns the old value of i

• Binary operators work on two operands• We’ll look at a few


Operators 3• Usual ones

+ addition- subtraction* multiplication/ division% remainder, also called modulo< less than> greater than<= less than or equal>= greater than or equal== equals!= does not equal? conditional operator


Operators 4• Bitwise

<< left-shift (i<<j means i is shifted to the left by j bits)

>> right-shift (i>>j means i is shifted to the right by j bits)

& bitwise AND

| bitwise OR

^ bitwise exclusive-OR

&& logical AND (returns 1 if both operands are non-zero, else 0)

|| logical OR (returns 1 if either operand is non-zero, else 0)


Operators 5

• In C the equals operator = is used to change the value of a variable, e.g temp = 22;

• Multiple assignments are allowed, e.g.– i = j = k = 4;– This causes simultaneous assignment

• Other examples– velocity = distance / time;– count = count + 1;


Operator precedence

• Operators are grouped hierarchically according to their order of evaluation (precedence)

• * and / have precedence over + and –• Multiplication and division is performed before

addition and subtraction• Rules of precedence can be bypassed by using

parentheses ()• E.g. a – b / c + d really means a – (b / c) + d


Conditional operators

• Relational, equality and logical operators operate on expressions, resulting in either int value 1 (true) or int value 0 (false)

• In C the value false is represented by any zero value and true is represented by any nonzero value

• General form: expr 1 op expr 2


Relational operators

• As an example, consider a < b. This brings us back to

false true

a < b


Equality operators

• We have two equality operators– == meaning equal to

– != meaning not equal to

• Be careful not to use = instead of == since it will still give a valid statement in C but with an unintended behaviour– If ( i = j) … instead of writing == means that the two

values will be assigned to the same value which is an always true statement!


Logical operators

• Relational and equality operators are used to form logical expressions

• In C there are three logical operators– Logical AND written &&– Logical OR written ||– Logical NOT written !

• Individual logical expressions are combined into more complex expressions


Control statements

• Specify the order in which computations are performed– Also known as branching statements

• Two principle control statements– if-else– switch


if-else

• The if-else statement is used to perform a logical test and then take one of two possible actions, depending on the test evaluating to true or false

• The else part is optional, i.e. there doesn’t have to be an else in connection with if


if

• The simplest form isif (expression)

statement;

• It is important to remember the () after if

• E.g.if (speed < 130)

printf(“Legal speed\n”);


Compound if• Compound statements can be used to group a

number of statements under the control of just one if statement

if (j < k)

min = j;if (j < k) max = k;

if (j < k) { min = j; max = k;}


if-else• if-else is an extension of if used where there are two

alternativesif (expression)

statement1;else statement2;

• E.g. if (x < y)

min = x; else min = y;


The dangling-else problem

• There is a problem when else is omitted/forgotten from a nested if statement

if (n > 0) if (a > b) c = a; else c = b;• Resolved by associating else with the closed

previous “else-less” if


The dangling-else problem 2

• To change this, use {}:

if (n > 0){

if (a > b)

c = a;

}

else

c = b;


Nested if

• Sometimes (often?) if-else is used to implement a multi-way decision

• In this case, the expressions are evaluated in order. If any expression is true, the statement associated with it is executed and the whole chain is terminated

• The final else can be used to deal with the default case where none of the other conditions evaluate to true


Nested if 2

if (expression_1)

statement_1;

else if (expression_2)

statement_2;

.

.

else if (expression_N)

statement_N;


Nested if 3

if (expression_1)statement_1;

else if (expression_2)statement_2;

.

.else if (expression_N)

statement_N;else

default_statement;


Example

if (x>0)

num_pos = num_pos + 1;

else if (x < 0)

num_neg = num_neg + 1;

else /* x equals 0 */

num_zero = num_zero + 1;


switch

• You’ve seen the multi-way decision. The switch statement is a multi-way conditional generalising if-else

• switch evaluates the value of an expression and branches to one of the case labels. Note that– Duplicate labels are disallowed, only one case will be

selected– The expression must evaluate an integer, character or

enumeration– Case labels can be in any order and must be constants


switch (expression) {

case constant_1: statement_1;

case constant_2: statement_2;

…

case constant_N: statement_N;

default: default_statement;

}

switch


The effect of switch

• Evaluate a switch expression• Then go to the case label having a constant value

that matches the value of the expression found in the first step. If no match is found, go to the default label. If there is no default label, terminate the switch

• Finally, terminate the switch when a break is encountered, or by “falling off the end”


default• The default label can be placed anywhere in

the switch– When C sees a switch statement it evaluates the

expression and then looks for a matching case label

– If none is found, the default label is used– There cannot be more than one default label in a

switch if any– Typically default will occur last, but there is no

rule


Example

switch (colour) {

case ‘B’: printf(“ the colour is blue\n”);

case ‘R’: printf(“ the colour is red\n”);

case ‘G’: printf(“ the colour is green\n”);

default: printf(“ the colour is yellow\n”);


break

• Typically the last statement before the next case or default label is a break statement

• A break statement inside a switch means that execution will continue after the switch statement

• If no break is included, execution falls through to the next statement


switch (expression) {case constant_1: statement_1;

break;case constant_2: statement_2;

break;…case constant_N: statement_N;

break;default: default_statement;

}

break 2


• So – a break statement interrupts the normal flow of control

switch (x) {case 1: x = x + 1; case 2: x = x + 2;

break; /* exit switch */case 3: x = x + 3;

default: x = 0;}

// break jumps to here

break 3


Falling throughswitch (x) {

case 1: x = x + 1; case 2: x = x + 2;

break; /* exit switch */case 3: x = x + 3;

default: x = 0;}

• After case 1 there is no break, so the program falls through.This means that when x = 1 the following will be executed: x = x + 1;

x = x + 2;

• Now consider the default case. This does not need a break because it is placed as the final statement


Advice

• Good practice means that you should end every case in a switch with either break or a fall through comment

switch (x) {case 1: x = x + 1; /* fall through */case 2: x = x + 2; break; // exit switchcase 3: x = x + 3; /* fall through */default: x = 0;

}


switch w. compound statements

switch (expression) {case constant_1: {

compound_statement_1;}case constant_2: statement_2;…case constant_N: statement_N;default: default_statement;

}


From if to switch 1char operator;double a ,b, result;

if (operator == ‘+’)result = a + b;

else if (operator == ‘-’)result = a - b;

else if (operator == ‘*’)result = a * b;

else if (operator == ‘/’) {if ( b == 0)

printf(“error – division by 0\n”);else result = a / b;

}else printf(“unknown operator\n”);


From if to switch 2char operator;double a ,b, result;

switch (operator ) {case ‘+’: result = a + b; break;case ‘-’: result = a - b; break; case ‘*’: result = a * b; break;case ‘/’: if ( b == 0)

printf(“error – division by 0\n”); else result = a / b; break;default: printf(“unknown operator\n”);

}


Loops

• A loop allows a program to repeat a group of statements, either any number of times or until some loop condition occurs

• The statements to be repeated are in the loop body

• Common loops (aka iterative or repetitive statements) are for, while and do-while


Loops 2

• Repetition occurs as long as a condition is true

false

true


Loops 3

• Counting loops are used when it can be determined before loop execution how many loop repetitions will be needed– while– for

• Conditional loops are used when a loop should be repeated until a desired condition is met– while– do-while– for


for

• When the number of passes through a loop is known in advance, a for statement is often used

for (expr1; expr2; expr3)

statement;

• expr1 controls the looping action, expr2 represents a condition that ensures loop continuation, expr3 modifies the value of the control variable initially assigned by expr1


• When a for statement is executed, expr2 is evaluated and tested at the beginning of each pass through the loop. expr3 is evaluated at the end of each pass

• If the loop continuation condition is initially false, the body part of the loop is not performed

• Any of the three parts can be omitted, but the semicolons must be kept

for 2


for 3

for (i=1; i <= n; i = i + 1)

keyword

control variable i

increment of control variable

loop continuation condition

initial valueof controlvariable

final value of control variablefor which the condition is true


Examples

• Vary the control variable from 1 to 100 in increments of 1for (i = 1; i <= 100; i++)

• Vary the control variable from 100 to 1 in increments of -1for (i = 100; i >= 1; i--)

• Vary the control variable from 5 to 55 in increments of 5for (i = 5; i <= 55; i+=5)


Examples 2

#include <stdio.h>

int main void{

/* a program to produce a Celsius to Fahrenheit conversion chart for the numbers 1 to 100 */int celsius;

for (celsius = 0; celsius <= 100; celsius++) printf(“Celsius: %d Fahrenheit: %d\n,

celsius, (celsius * 9) / 5 + 32); return 0;}


Nested for loops

• Nested means there is a loop within a loop

• Executed from the inside out– Each loop is like a layer and has its own

counter variable, its own loop expression and its own loop body

– In a nested loop, for each value of the outermost counter variable, the complete inner loop will be executed once


• General formfor (loop1_exprs) {

loop_body_1a

for (loop2_exprs) { loop_body_2 } loop_body_1b }

Nested for loops 2


• Most compilers allow 15 nesting levels – DON’T DO IT!!

Nested for loops 3


while

• When the number of passes through a loop is not known in advance, a while statement is often used

while (expression)

statement;

• Expression represents a condition that must be true for the loop to continue execution


• The statement is executed repeatedly as long as the expression is true (non zero)

• The cycle continues until expression becomes zero, at which point execution resumes after statement

while 2


• The statement must include some feature that eventually alters the value of the expression, providing an escape mechanism from the loop– This is not done automatically

• When a loop is constructed using while the test for the continuation of the loop is carried out at the beginning of each pass

while 3


while 4

• If the test expression in a while loop is false initially, the while loop will never be executed

int i = 1, sum = 0;while (i <= 10){

sum = sum + i; i= i + 1;

}printf(“Sum = %d\n”, sum);


for and whilefor(expr1; expr2; expr3)

statement

Is equivalent to

expr1;while(expr2){

statement;expr3;

}


break and continue

• These interrupt normal flow of control

• break causes an exit from the innermost enclosing loop

• continue causes the current iteration of a loop to stop and the next iteration to begin immediately


while (expression){

statementsbreak;more_statements

}

while (expression){

statementscontinue;more_statements

}

break and continue 2


do-while

• When a loop is constructed using while, the test for continuation is carried out at the beginning of each pass

• With do-while the test for continuation takes place at the end of each pass

dostatement

while (expression);


do vs. do-while

• while -- the expression is tested first, if the result is false, the loop body is never executed

• do-while -- the loop body is always executed once. After that, the expression is tested, if the result is false, the loop body is not executed again

• This often leads to logical mistakes!!


How to leave a program• More than one possibility, all in stdio.h• The function abort() causes abnormal program

terminationabort();

• The function exit(status) causes normal program termination

exit(status);• If the value of status is 0 (zero), the host environment

assumes successful program execution. All other values indicate unsuccessful execution


Tip - debugging

• Sometimes you will need to use printf to help debug a program. But how do we separate these printouts from the rest?

• One hint is to begin all debug printouts with “##”– printf(“## state = %d\n”, state);

• This makes it easy to identify debugging info. This also makes it easier to find and remove these statements later on


"In My Egotistical Opinion, most people's C programs should be indented six feet downward and covered with dirt."

-- Blair P. Houghton (On the subject of C program indentation)

"C makes it easy to shoot yourself in the foot. C++ makes itharder, but when you do, it blows away your whole leg."

-- Bjarne Stroustrup

control structures lone leth thomsen. february 2006basis-c-2/ll2 contents operators lots of control...

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