Repetition In C: Loops

Repetition in C: Loops

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Repetition: Loops • Loop statements repeat (jargon: ‘iterate on’) on’ a {block of statements;} for as many times as you wish, • Until a terminating condition tells a loop when and where to stop repeating those statements. • No terminating condition? Your program will never get finished! (that is usually a bug) 2

Loops: 3 x 2 Kinds 1) Pre-Test Loop: “Ask First” Keep going? YES if cond is true. Run statement(s), repeat…

start is(cond)true?

no

yes {

block of statements;

}

( see the loop? )

finish 3

Loops: 3 x 2 Kinds 1) Pre-Test Loop: “Ask First” Two kinds of condition: cond 1a) Event-driven Event 1b) Count-driven Count

start is(cond)true?

no

yes {

block of statements;

}

finish 4

Loops: 3 x 2 Kinds 2) Post-Test Loop: “Ask Last” Run statement(s). Keep going? YES if cond is true. repeat…

start {

block of statements;

} is(cond)true? yes

( see the loop? )

no

finish 5

Loops: 3 x 2 Kinds 2) Post-Test Loop: “Ask Last” Two kinds of condition: cond 2a) Event-driven Event 2b) Count-driven Count

start {

block of statements;

} is(cond)true? yes

no

finish 6

Loops: 3 x 2 Kinds 3) Interrupted Loop: “Ask Anytime”

start

{ block of statements; } Run some statement(s), Keep going? is(cond)true? no yes YES if cond is true. { block of statements; } Run some statement(s) repeat… ( see the loop? ) finish

BAD IDEA! AVOID THIS!!

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Loops: 3 x 2 Kinds 3) Interrupted Loop: “Ask Anytime” Two kinds of condition: cond 3a) Event-driven Event 3b) Count-driven Count

start { block of statements; } is(cond)true? no yes { block of statements; }

BAD IDEA! AVOID THIS!!

finish 8

C Looping Summary • Just 3 kinds of loops (for ANY language) pre-test (runs 0, 1 or more times) post-test (runs 1,2, or more times) interrupted (runs 0.7? 1.3? or more? Bad idea. )

• C: Just 3 kinds of loop statements, statements BUT (of course) they don’t match the 3 kinds of loops! for(init; cond; step){stmts;} while(cond) {stmts;} do{stmts;} while(cond)

pre-test pre-test post-test 9

Pre-test Loops in C: while : event-driven while (cond) {stmts;}

start is(cond)true?

/* Frog Feeding I*/ moveTo(air); open_eyes(); while(TRUE==see_fly()) { flick_tongue(); clamp_mouth(); }

/* /* /*

init init cond

*/ */ */

no

yes {

block of statements;

}

/*action*/ finish 10

Pre-test Loops in C: while : count-driven while (cond) {stmts;}

is(cond)true?

/* Frog lifetime*/ int days; days = 155; /* while(days > 0) /* { work_all_day(); sleep_all_night(); days--; /* } die_quietly();

start no

yes init */ cond */

step */

{

block of statements;

}

finish 11

Pre-test Loops in C: for : event-driven start

for(init; } init cond; cond step){stmts; ){ /* Frog lifetime*/ int days, wet; /*for( init; cond; step)*/ for(days=155,wet=1; 1==wet && days>0; days--;) { work_all_day(); sleep_all_night(); wet = jumpInWater(); } die_quietly(wet);

is(cond)true?

no

yes {

block of statements;

}

finish 12

Pre-test Loops in C: for : count-driven start

for(init; } init cond; cond step){stmts; ){ /* Frog lifetime*/ int days;

is(cond)true? yes {

/*for( init; cond; step)*/ for(days=155; days>0; days--) { work_all_day(); sleep_all_night(); } die_quietly();

no

block of statements;

}

finish 13

Post-Test Loops in C: do{}while() : event or count

do{stmts; }while(cond ) do{ }while(

/*Frog Feeding II*/ do { chew_and_mash(); swallow(); } while (mouth_empty()==FALSE;)

start {

block of statements;

} is(cond)true? yes

no

finish 14

Interrupted Loops in C: break keyword 3) Interrupted Loop: (avoid this!) start float energy; ... while(TRUE) { drink_water(); if(energy <= 2.384) break; jump_10_meters(); }

{ block of statements; } is(cond)true? no yes { block of statements; }

finish 15

Interrupted Loops in C: break keyword 3) Interrupted Loop: (sometimes MUST do it) (rare!) int days; float food,fat; for(days=155; days>0; days--) { work_all_day(); if( food+fat < 0.01) break; sleep_all_night(); } die_quietly();

start { block of statements; } is(cond)true? no yes { block of statements; }

finish 16

Summarize: Loops in C • Three kinds of loop-making statements in C, – – –

while(cond) {stmts;} do{stmts;} while(cond); for(init; cond; step){stmts;}

• Different kinds are best for different tasks, so choose loops wisely.  Don’t use ‘for’ loops for everything!  17

while(cond){stmts;}; • Pre-test, Event-driven -test the cond expression -if TRUE, run {stmts;} and start again. -else STOP. • Messy for count-driven loops; use ‘for’. 18

do{stmts;}while(cond;) • Post-Test, OK for Event- or Count-driven; -run {stmts;} statement, -test cond; -If TRUE, start again; else STOP. • Count-driven? Try to re-arrange problem into pre-test form, then use ‘for’ statement. 19

for(init; cond; step){stmts;} • Pre-Test, Count-driven run init; (statement); test cond; (statement); if true, run stmts; else STOP run step expression repeat… 20

Nested Loops Nesting: A loop is a complete statement. int i,j; ... i = 0; for (j=0; j<3; j++) { printf(“( %d,%d )”, i, j); }

Output: > 0,0

0,1

0,2

>

. 21

Nested Loops Nesting: Place one loop inside another inner loop is one statement within the outer loop printf(“\n”); for (i=0; i<4; i++) { for (j=0; j<3; j++) { printf(“( %d,%d )”, i, j); } printf(“\n”); }

Output: > 0,0 1,0 2,0 3,0

0,1 1,1 2,1 3,1

0,2 1,2 2,2 3,2

>

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Nested Loops: Notation • Inner loop is one statement within the outer loop • C notation permits very ‘compact’ notation, BUT • Hard to read, may hide errors printf(“\n”); for (i=0; i<4; i++){ for (j=0; j<3; j++) printf(“( %d,%d )”, i, j); printf(“\n”); }

BAD IDEA! Output: > 0,0 1,0 2,0 3,0 >

0,1 1,1 2,1 3,1

0,2 1,2 2,2 3,2 23

Nested Loops: Notation • Inner loop is one statement within the outer loop • C notation permits very ‘compact’ notation, BUT • Hard to read, may hide errors printf(“\n”); for (i=0; i<4; i++){ for (j=0; j<3; j++) printf(“( %d,%d )”, i, j); printf(“\n”); }

If you put semicolon here, what happens?

BAD IDEA! Output: > 0,0 1,0 2,0 3,0 >

0,1 1,1 2,1 3,1

0,2 1,2 2,2 3,2 24

Nested Loops: Notation • Inner loop is one statement within the outer loop • C notation permits very ‘compact’ notation, BUT • Hard to read, may hide errors printf(“\n”); for (i=0; i<4; i++){ for (j=0; j<3; j++); printf(“( %d,%d )”, i, j); printf(“\n”); }

The ‘j’ loop does NOTHING!

BAD IDEA! Output: > 0,3 1,3 2,3 3,3 >

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Nested Loops: Notation • Inner loop is one statement within the outer loop • C notation permits very ‘compact’ notation, BUT • Hard to read, may hide errors printf(“\n”); for (i=0; i<4; i++){ for (j=0; j<3; j++) printf(“( %d,%d )”, i, j); printf(“\n”); }

If you skip the curly braces here what happens?

BAD IDEA! Output: > 0,0 1,0 2,0 3,0 >

0,1 1,1 2,1 3,1

0,2 1,2 2,2 3,2 26

Nested Loops: Notation • Inner loop is one statement within the outer loop • C notation permits very ‘compact’ notation, BUT • Hard to read, may hide errors

BAD IDEA!

printf(“\n”); The 2nd printf(“\n”); is not for (i=0; i<4; i++) for (j=0; j<3; j++) inside the ‘i’ loop statement! printf(“( %d,%d )”, i, j); printf(“\n”); Output: > 0,0 0,1 0,2 1,0 1,1 1,2 2,0 2,1 2,2 3,0 3,1 3,2 27 >

Pre-Test Loops: while==for • Can you turn a for loop into a while loop? • Yes!

for(init; cond; step) { statements; }

≈

init; while(cond) { statements; step; } 28

Bug Warnings • Infinite loops if cond is never true • Floating-point data in a FOR statement

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