Control flow in c

Decisions – if then else

More decisions – switch

Loops- while, do while, for

Keyword break Keyword continue

If I’m bigger than him and I’m hungry…. then it’s mealtimeelse, if he’s bigger than me… hope he doesn’t look hungryelse, if we’re the same else sizeWait to grow bigger

Parentheses surround the best One statement becomes the “then

part’ If more are required, braces must be

readscanf(“%i”, &i);

If(i > 0)printf(“a positive number was entered\n”);

If(i < 0) {printf(“a negative number was entered\n”);i = -1;

}

A semicolon after the condition, form as “do nothing’ statement

printf(“input an integer: “);scanf(“%i”, &j);

If(j > 0);

printf(“a positive number was entered\n”);

input an integer: -6a positive number was entered

An optional else may be added One statement by default, if more are

required, braces must be usedif(i > 0)

printf(“i is positive\n”);else

printf(“i is negative\n”);

if(i > 0)printf(“i is positive\n”);

else {printf(“i is negative\n”);i = -1;

}

Else associated with the nearest ifint i = 100;if(i > 0) if (i > 1000)

printf(“i is big\n”); else

printf(“i is reasonable\n”);

int i = 100;if(i > 0) { if (i > 1000)

printf(“i is big\n”); } else

printf(“i is negative\n”);

i is reasonable

i is negative

C supports a switch for multi-way decision making

switch (c) { case ’a’ ; case ‘A’:

printf(“area = %.2f\n”, r * r * r pi);break;

case ‘c’: case ‘C’:printf(“circumference = %.2f\n” , 2 * r * pi);

case ‘q’:printf(“quit option chosen\n”);

default:printf(“unknown option chosen\n”);break;

}

Only integral constants may be tested If no condition matches, the default is

executed If no default, nothing is done (not an

error) The break is important

float f;switch (f) {

case 2:…..

switch (i) {

case 2 * j:…..

i = 3;

switch (i) { case 3: printf(“i = 3\n”); case 2: printf(“i = 2\n”); case 1: printf(“i= 1\n”);}

i = 3i = 2i = 1

printf (“on the “) ;switch (i) { case 1: printf(“1st”); break; case 2: printf(“2nd”); break; case 3: printf(“3rd”); break; default: printf(“%ith”,i); break;}printf (“day of Christmas my true love sent to me”);switch (i) { case 12: printf(“twelve lords a leaping,”); case 11: printf(“eleven ladies dancing,”); case 10: printf(“ten pipers piping,”); case 9: printf(“nine drummers drumming,”); case 8: printf(“eight maids a milking,”); case 7: printf(“seven swams a swimming,”); case 6: printf(“six geese a laying,”); case 5: printf(“five gold rings,”); case 4: printf(“four calling birds,”); case 3: printf(“three french hens,”); case 2: printf(“two turtle doves and ”); case 1: printf(“a patridge in a pear tree\n”);}

• The simplest C loop is the while• Parentheses must surround the condition• One statement forms the body of the loop• Braces must be added if more statements

are to be executed

int j = 5;

while (j > 0)printf (“j = %i\n”, j--);

while (j > 0) {printf(“j = %i\n”, j);j --;

}

j = 5j = 4j = 3j = 2j = 1

A semicolon placed after the condition forms a body that does nothing

Sometimes an empty loop body is required

int j = 5;while (j > 0 ); printf (“j = %i\n”, j --);

program disappears into an infinite loop

int c, j;while (scanf( “%i”, &j) ! = 1 while ( (c = getchar() )! = ‘\n’)

;

placing semicolon on the line below makes the intention obvious

Remember to get the condition the right way around!

int j = 5;printf (‘start\n”);while (j == 0) printf (“j = %i\n”, j--);printf (“end\n”);

startend

user probably intends “until j is equal to zero”, however this is NOT the way to write it

do while guarantees execution at least once

int j = 5 ;printf (“start\n”);do printf (“j = %i\n”, j--);while (j > 0);printf (“stop\n”);

startj = 5j = 4j = 3j = 2j = 1Stop

int j = -10 ;printf (“start\n”);do { printf (“j = %i\n”, j); j--;} while (j > 0);printf (“stop\n”);

startj = -10

stop

for encapsulates the essential elements of a loop into one statementfor (initial – part; while- condition; update-part) body;

int j;

for (j = 5; j > 0; j--) printf(“j = %i\n”, j);

j = 5 oddj = 4 evenj = 3 oddj = 2 even j = 1 odd

for (j = 5; j > 0; j--) { printf(“j = %i”, j); printf(“%s\n”, ((j%2)==0) ? “even” : “odd”);}

j = 5

j = 4

j = 3

j = 2

j = 1

Remember to get the for condition the right way around (it is really a while condition)

int j;printf (“start\n”);for (j = 5; j == 0; j--) printf(“j = %i\n”, j);printf (“end\n”);

startend

user probably intends “until j is equal

to zero”, however this is NOT the

way to write it either!

Unlike some languages, the for loop is not restricted to stepping up or down by 1

#include <math.h.

int main(void){

double angle;for(angle – 0.0; angle < 3.14159; angle += 0.2) printf(“sine of %.11f is %.21f\n”, angle, sin(angle));return 0;

}

The initial and update parts may contain multiple comma separated statements

the initial, condition and update parts may contain no statements at all!

int i, j, k;for (i = 0, j = 5, k = -1; i < 10; i++, j++, k--)

for (; i < 10; i++, j++, k--)

for (; i < 10;)

for (; ;)

use of a while loop would be clearer here!

creates an infinite loop

The break keyword forces immediate exit from the nearest enclosing loop

use in moderation!for (; ;) { printf (“type an int: “); if (scanf (“%i”, &j) ==1)

break; while ( (c = getchar() ) != ‘\n’)

;}printf (“j = %\n”, j);

type an int: an inttype an int: notype an int: 16j = 16

if scanf returns 1, jump out of the loop

The continue keyword forces the next iteration of the nearest enclosing loop

use in moderation!

for (j = 1; j <=10; j++) { if (j % 3 ==0 )

continue; printf (“ j = %i\n”, j);}

j= 1j= 2j =4j= 5j= 7j= 8j= 10

if j is exactly divisible by 3, skip

If(then) else – watch the semicolons switch can test integer values while, do while, for – watch the

semicolons again break continue

else…….I want to be a

tomato

!