chapter 13 control structures in c. byu cs/ecen 124variables and operators2 topics to cover…...

Chapter 13 Control Structures in C

BYU CS/ECEn 124 Variables and Operators 2

Topics to Cover…

Control Structures if Statement if-else Statement switch Statement while Statement do-while Statement for Statement Loops Break and Continue GOTOs and Labels Optimizing Compilers Prime Examples


Control Structures

We looked at three constructs for systematic decomposition:

Test

Task 1 Task 2

True False

The conditional constructTest

Task 1

True

False

The iteration construct

Part I

Part II

The sequential construct

C has many conditional and iteration constructs: if, if-else switch for while, do-while

Control Structures


The if Statement

Performs an action if a condition is true. The condition, which is a C expression, evaluates to zero (false) or nonzero (true).

Form: if (expression) statement

Examples: if (x > 20) x = 20;

if (x <= 10){ y = x * x + 5; z = (2 * y) / 3;}

if (0 <= age && age <= 11) kids = kids + 1;

x>20

x=20;

True False

if Statement


The if-else Statement

Perform if-action if a condition is true. Otherwise, perform else-action.

Form: if (expression) statement1else statement2

Example: if (x){ y++; z++;

}else{

y--; z--;

}

x==0?

y++;z++;

y--;z--;

True False

if-else Statement


The if-else statement (continued…)

You can connect conditional constructs to form longer sequences of conditional tests:

if (expression1) statement1

else if (expression2) statement2

else if (expression3) statement3else statement4

if-else Statement


The if-else statement (continued…)

An else is associated with the closest unassociated if.

if (expression1) if (expression2) statement2 else

statement3

if (expression1) { if (expression2) statement2 else statement3}

if (expression1) { if (expression2) statement2}else statement3

Just as parentheses modify the order of evaluation of expressions...

braces modify how statements are executed.

Correct Interpretation

if-else Statement


The switch Statement

Performs actions based on a series of tests of the same variable.

Form: switch (expression)

{ case const-expr: statements case const-expr: statements case const-expr: statements default: statements }

The break statement causes an immediate exit from the switch.

Because cases serve only as labels, execution falls through to the next unless there is explicit action to escape.

c='a'

...;break;

True False

c='b'

...;break;

True False

c='c'

...;break;

True False

switch Statement


switch (c){

case '+':r = a + b;break;

case '-':r = a - b;break;

case '*':r = a * b;break;

default:printf("\nInvalid operation!");break;

}

c='+'

r=a+b;break;

True False

c='-'

r=a-b;break;

True False

c='*'

r=a*b;break;

True False

switch Statement

The switch Statement


main:0x9614: 8031 000A SUB.W #0x000a,SP0x9618: 3C1D JMP (C$L5) C$L1:0x961a: 411F 0006 MOV.W 0x0006(SP),R150x961e: 511F 0004 ADD.W 0x0004(SP),R150x9622: 4F81 0008 MOV.W R15,0x0008(SP)0x9626: 3C20 JMP (C$L6) C$L2:0x9628: 411F 0004 MOV.W 0x0004(SP),R150x962c: 811F 0006 SUB.W 0x0006(SP),R150x9630: 4F81 0008 MOV.W R15,0x0008(SP)0x9634: 3C19 JMP (C$L6) C$L3:0x9636: 411C 0004 MOV.W 0x0004(SP),R120x963a: 411D 0006 MOV.W 0x0006(SP),R130x963e: 12B0 9C98 CALL #__mpyi0x9642: 4C81 0008 MOV.W R12,0x0008(SP)0x9646: 3C10 JMP (C$L6) C$L4:0x9648: 40B1 A016 0000 MOV.W #0xa016,0x0000(SP)0x964e: 12B0 97C0 CALL #lcd_printf0x9652: 3C0A JMP (C$L6) C$L5:0x9654: 411F 0002 MOV.W 0x0002(SP),R150x9658: 803F 002A SUB.W #0x002a,R150x965c: 27EC JEQ (C$L3)0x965e: 831F DEC.W R150x9660: 27DC JEQ (C$L1)0x9662: 832F DECD.W R150x9664: 27E1 JEQ (C$L2)0x9666: 3FF0 JMP (C$L4) C$L6:0x9668: 5031 000A ADD.W #0x000a,SP0x966c: 4130 RET

void main(void){ int c, a, b, r; switch (c) { case '+': r = a + b; break;

case '-': r = a - b; break;

case '*': r = a * b; break;

default: printf("\nInvalid!"); break; }}

The switch Statementswitch Statement


while loop

Check test (sentinel) at beginning of loop May (or may not) execute loop

Form:

while (expression) statement

// Print digits from 7 down to 1

int a = 7;while (a){ printf("%c\n", a + '0'); a--;}printf("All done...\n");

FalseTest

Body

True

while Statement


sub #2,sp ; int a = 7;mov #7,0(sp)

LOOP: cmp #0,0(sp) ; while (a) jeq DONE ; {mov 0(sp),r12 ; printf("%c\n", a + '0');add #'0',r12call #PUTCmov #10,r12call #PUTC sub #1,0(sp) ; a--;jmp LOOP ; }

DONE: mov #MSG,r12 call #PUTS ; printf("All done...\n");...

MSG: .string "All done...\n".byte 0

FalseTest

Body

True

while Statement

while loop


do-while loop

Check test (sentinel) at end of loop Always executes loop once

Form:

do statementwhile (expression);


int a = 7;do{ printf("%c\n", a + '0'); a--;} while (a);printf("All done...\n");

Test

Body

True

False

do-while Statement


sub #2,sp ; int a = 7;mov #7,0(sp)

; do {LOOP: mov 0(sp),r12 ; printf("%c\n", a + '0');

add #'0',r12call #PUTCmov #10,r12call #PUTC sub #1,0(sp) ; a--;jne LOOP ; } while (a);



Test

Body

True

False

do-while Statement

do-while loop


for loop

Check test at beginning of loop May (or may not) execute loop

Form:

for (expr1 ; expr2 ; expr3) statement

where expr1 executes at the beginning of loop (init)expr2 is a relational expression (test) expr3 executes at the end of loop (re-init)


for (a=7; a>0; a--){ printf("%c\n", a + '0');}printf("All done...\n");

FalseTest

Body

True

Re-init

Init

for Statement


sub #2,sp ; int a;

mov #0,0(sp) ; for (a=0; a<10; a++)

LOOP: bit #-1,0(sp) ; { jeq DONEmov 0(sp),r12 ; printf("%c\n", a + '0');add #'0',r12call #PUTCmov #10,r12call #PUTC add #1,0(sp)cmp #10,0(sp)jl LOOP ; }



FalseTest

Body

True

Re-init

Init

for Statement

for loop


One final C operator is the comma “,”, which most often finds use in the for statement.

A pair of expressions separated by a comma is evaluated left to right, and the type and value of the result are the type and value of the right operand.

Note: The commas that separate function arguments, variables in declarations, etc., are not comma operators, and do not guarantee left to right evaluation.

Example:for (i=0, j=10, k=-1; i < j; j--){ statement}

FalseTest

Body

True

Re-init

Init

for Statement

for loop


What do the following do?if (x = y) y = 10;for (;;) { body }

while (TRUE) { body }

Note that: for (expr1; expr2; expr3)statement

is equivalent to: expr1;while (expr2){statementexpr3;

}

for Statement

for loop


Nested Loops

A loop body may contain another loop, called a nested loop.

The first loop is called the outer loop and the second is called the inner loop.

for (a = 4; a > 0; a--){ for (b = a; b > 0; b--) { printf("%c %c\n", a+‘0’, b+‘0’); }}printf("All done...\n");

4 44 34 24 13 33 23 12 22 11 1All done ...

Loops


Loop Style

To a large degree, all iteration constructs can be used interchangeably.

Stylistically, different constructs make sense for different situations.

The type of loop you choose will convey information about your program to a reader.

Loops


Infinite Loops

The following loop will never terminate:x = 0;while (x < 10) printf("%d ", x);

Loop body does not change condition, so test never fails.

This is a common programming error that can be difficult to find.

Loops


Break and Continue

break and continue can be used with iteration construct or with a switch construct

break exits the innermost loop or switch continue restarts the innermost loop or switch They both generate an unconditional branch in

the assembly code

Break and Continue


Horrors! GOTOs and Labels

A goto statement is used to branch (transfer control) to another location in a program. The goto statement can only be used within the body

of a function definition. “The goto statement is never necessary; it can

always be elimitated (sic) by rearranging the code.” Use of the goto statement violates the rules of

structured programming. Label statement can be used anywhere in the

function, above or below the goto statement.

GOTOs and Labels


GOTOs and LabelsGOTOs and Labels

int found = 0;for (i=0; i<10; i++){ for (j=0; j<10; j++) { if (same…) { found = 1; break; } } if (found) break;}if (found){}else{}

for (i=0; i<10; i++){ for (j=0; j<10; j++) { if (same…) goto FOUND; }}NOT_FOUND:

FOUND:


Optimizing Compilers

Constant folding / subexpression elimination Eliminate useless code Intelligent switch statement processing Interprocedural optimization

inlining code – replace function call w/function body replacing code with function calls

Peephole optimization examine adjacent instructions Minimize number of times a variable loaded/stored

Optimize for speed or memory

Optimizing Compilers


#include "msp430x22x4.h"#include "eZ430X.h"#include "lcd.h"#include <math.h>#define FALSE 0#define TRUE 1#define MAX 100

int main(){ int maxdiv, divisor, prime, num;

int primeCnt = 0;eZ430X_init(CALDCO_8MHZ); // initialize boardlcd_init();

maxdiv = sqrt(MAX) + 1; // maximum divisor

for (num = 2; num <= MAX; num++){ prime = TRUE;

for (divisor = 2; divisor <= maxdiv; divisor++){ if (((num % divisor) == 0) && num != divisor)

{ prime = FALSE;}

}if (prime) primeCnt++;

}printf("\nTotal primes found: %d", primeCnt);return 0;

}

Program I - Compute PrimesPrime Examples



int main(){ int maxdiv, divisor, prime, num;

int primeCnt = 0;eZ430X_init(CALDCO_8MHZ); // initialize boardlcd_init();

maxdiv = sqrt(MAX) + 1; // maximum divisor

for (num = 2; num <= MAX; num++){ prime = TRUE;

for (divisor = 2; divisor <= maxdiv; divisor++){ if (((num % divisor) == 0) && num != divisor)

{ prime = FALSE;break;

}}if (prime) primeCnt++;

}printf("\nTotal primes found: %d", primeCnt);return 0;

}

Program II - Compute PrimesPrime Examples



int main(){ int maxdiv, i, num;

int primeCnt = 0;char list[MAX]; // list to markeZ430X_init(CALDCO_8MHZ); // initialize boardlcd_init();

for (i = 0; i < MAX; i++) list[i] = 0; // zero list (assume all primes)maxdiv = sqrt(MAX) + 1; // maximum divisor

for(num = 2; num <= maxdiv; num++){ // Find next prime number in the list

while (num < MAX && list[num] == 1) num = num + 1;if (num == MAX) break;// Mark all multiples of this in the listfor (i = num*2; i < MAX; i += num) list[i] = 1;

}// List has been marked, now print out resultsfor (i = 2; i < MAX; i++) if (list[i] == 0) primeCnt++;printf("\nTotal primes found: %d", primeCnt);return 0;

}

Sieve Program - Compute PrimesPrime Examples


Quiz…

Fill in the resulting values for x, y, and z after evaluating the construct. Assume for each row, x, y, and z are initialized to 10, 20, and 30 respectively.

x=10 y=20 z=301) if (x = y) y = 100;

2) if (x < 10) y = 1;else if (x < 20) y = 5;else if (x < 30) y = 10;

3) switch ('a') { case 'a': y++; z *= 5; case 'b': --y; z /= 10;}

4) for (x=1; x<y; x++, y--) z = x + y;

5) while (!z) { z %= y;}

6) do { x = --y; z = x++;}while (z);

chapter 13 control structures in c. byu cs/ecen 124variables and operators2 topics to cover…...

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