Chapter 13 Control
Structures
13-2
Control Structures
Conditional
• making a decision about which code to execute, based on evaluated expression
• if
• if-else
• switch
Iteration
• executing code multiple times,
ending based on evaluated expression
• while
• for
• do-while
13-3
If
if (condition)
action; condition
action T
F
Condition is a C expression,
which evaluates to TRUE (non-zero) or FALSE (zero).
Action is a C statement,
which may be simple or compound (a block).
13-4
Example If Statements
if (x <= 10)
y = x * x + 5;
if (x <= 10) { y = x * x + 5;
z = (2 * y) / 3;
}
if (x <= 10)
y = x * x + 5;
z = (2 * y) / 3;
compound statement;
both executed if x <= 10
only first statement is conditional;
second statement is
always executed
13-5
More If Examples
if (0 <= age && age <= 11) kids += 1;
if (month == 4 || month == 6 ||
month == 9 || month == 11)
printf(“The month has 30 days.\n”);
if (x = 2) y = 5;
This is a common programming error (= instead of ==), not caught by compiler because it’s syntactically correct.
always true,
so action is always executed!
13-6
If’s Can Be Nested
if (x == 3)
if (y != 6) { z = z + 1;
w = w + 2;
}
if ((x == 3) && (y != 6)) { z = z + 1;
w = w + 2;
}
is the same as...
13-7
Generating Code for If Statement
; if (x == 2) y = 5;
LDR R0, R5, #0
; load x into R0ADD R0, R0, #-2
; subtract 2BRnp NOT_TRUE
; if non-zero, x is not 2AND R1, R1, #0
; store 5 to yADD R1, R1, #5 STR R1, R5, #-1
NOT_TRUE ...
; next statement13-8
If-else
if (condition) action_if;
else
action_else;
condition
action_if action_else
T F
Else allows choice between
two mutually exclusive actions without re-testing condition.
13-9
Generating Code for If-Else
if (x) { y++;
z--;
}
else { y--;
z++;
}
LDR R0, R5, #0 BRz ELSE
; x is not zero
LDR R1, R5, #-1 ; incr y ADD R1, R1, #1
STR R1, R5, #-1
LDR R1, R5, #02 ; decr z ADD R1, R1, #1
STR R1, R5, #-2
JMP DONE ; skip else code
; x is zero
ELSE LDR R1, R5, #-1 ; decr y ADD R1, R1, #-1
STR R1, R5, #-1
LDR R1, R5, #-2 ; incr z ADD R1, R1, #1
STR R1, R5, #-2 DONE ... ; next statement
13-10
Matching Else with If
Else is always associated with closest unassociated if.
if (x != 10) if (y > 3) z = z / 2;
else
z = z * 2;
if (x != 10) { if (y > 3) z = z / 2;
else
z = z * 2;
}
is the same as...
if (x != 10) { if (y > 3) z = z / 2;
}
else
z = z * 2;
is NOT the same as...
13-11
Chaining If’s and Else’s
if (month == 4 || month == 6 || month == 9 ||
month == 11)
printf(“Month has 30 days.\n”);
else if (month == 1 || month == 3 ||
month == 5 || month == 7 ||
month == 8 || month == 10 ||
month == 12)
printf(“Month has 31 days.\n”);
else if (month == 2)
printf(“Month has 28 or 29 days.\n”);
else
printf(“Don’t know that month.\n”);
13-12
While
while (test)
loop_body; test
loop_body T
F
Executes loop body as long as
test evaluates to TRUE (non-zero).
Note: Test is evaluated before executing loop body.
13-13
Generating Code for While
x = 0;
while (x < 10) {
printf(“%d ”, x);
x = x + 1;
}
AND R0, R0, #0
STR R0, R5, #0 ; x = 0 ; test
LOOP LDR R0, R5, #0 ; load x ADD R0, R0, #-10
BRzp DONE ; loop body
LDR R0, R5, #0 ; load x ...
<printf>
...
ADD R0, R0, #1 ; incr x STR R0, R5, #0
JMP LOOP ; test again
DONE ; next statement
13-14
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.
13-15
For
for (init; end-test; re-init)
statement init
test
loop_body re-init F
T Executes loop body as long as
test evaluates to TRUE (non-zero).
Initialization and re-initialization code includedin loop statement.
Note: Test is evaluated before executing loop body.
13-16
Generating Code for For
for (i = 0; i < 10; i++) printf(“%d ”, i);
; init
AND R0, R0, #0
STR R0, R5, #0 ; i = 0 ; test
LOOP LDR R0, R5, #0 ; load i ADD R0, R0, #-10
BRzp DONE ; loop body
LDR R0, R5, #0 ; load i ...
<printf>
...
; re-init
ADD R0, R0, #1 ; incr i STR R0, R5, #0
JMP LOOP ; test again
DONE ; next statement
This is the same
as the while example!
13-17
Example For Loops
/* -- what is the output of this loop? -- */
for (i = 0; i <= 10; i ++) printf("%d ", i);
/* -- what does this one output? -- */
letter = 'a';
for (c = 0; c < 26; c++) printf("%c ", letter+c);
/* -- what does this loop do? -- */
numberOfOnes = 0;
for (bitNum = 0; bitNum < 16; bitNum++) { if (inputValue & (1 << bitNum))
numberOfOnes++;
}
13-18
Nested Loops
Loop body can (of course) be another loop.
/* print a multiplication table */
for (mp1 = 0; mp1 < 10; mp1++) { for (mp2 = 0; mp2 < 10; mp2++) { printf(“%d\t”, mp1*mp2);
}
printf(“\n”);
} Braces aren’t necessary,
but they make the code easier to read.
13-19
Another Nested Loop
The test for the inner loop depends on the counter variable of the outer loop.
for (outer = 1; outer <= input; outer++) { for (inner = 0; inner < outer; inner++) { sum += inner;
}
}
13-20
For vs. While
In general:
For loop is preferred for counter-based loops.
• Explicit counter variable
• Easy to see how counter is modified each loop
While loop is preferred for sentinel-based loops.
• Test checks for sentinel value.
Either kind of loop can be expressed as the other,
so it’s really a matter of style and readability.
13-21
Do-While
do
loop_body;
while (test);
loop_body
T test
F Executes loop body as long as
test evaluates to TRUE (non-zero).
Note: Test is evaluated after executing loop body.
13-22
Problem Solving in C
Stepwise Refinement
• as covered in Chapter 6
...but can stop refining at a higher level of abstraction.
Same basic constructs
• Sequential -- C statements
• Conditional -- if-else, switch
• Iterative -- while, for, do-while
13-23
Problem 1: Calculating Pi
Calculate using its series expansion.
User inputs number of terms.
1 2
) 4 1 7 (
4 5
4 3
4 4
1n
nStart Initialize
Get Input
Evaluate Series Output Results
Stop
13-24
Pi: 1st refinement
Start Initialize Get Input
Evaluate Series Output Results
Stop
Initialize iteration count
count<terms
Evaluate next term
count = count+1
for loop
F T
13-25
Pi: 2nd refinement
Initialize iteration count
count<terms
Evaluate next term
count = count+1 F T
count is odd
subtract term add term
add term
if-else
F T
13-26
Pi: Code for Evaluate Terms
for (count=0; count < numOfTerms; count++) { if (count % 2) {
/* odd term -- subtract */
pi -= 4.0 / (2 * count + 1);
}
else {
/* even term -- add */
pi += 4.0 / (2 * count + 1);
}
Note: Code in text is slightly different, but this code corresponds to equation.
13-27
Pi: Complete Code
#include <stdio.h>
main() {
double pi = 0.0;
int numOfTerms, count;
printf("Number of terms (must be 1 or larger) : ");
scanf("%d", &numOfTerms);
for (count=0; count < numOfTerms; count++) { if (count % 2) {
pi -= 4.0 / (2 * count + 1); /* odd term -- subtract */
}
else {
pi += 4.0 / (2 * count + 1); /* even term -- add */
}
printf("The approximate value of pi is %f\n", pi);
}
13-28
Problem 2: Finding Prime Numbers
Print all prime numbers less than 100.
• A number is prime if its only divisors are 1 and itself.
• All non-prime numbers less than 100 will have a divisor between 2 and 10.
Start
Stop
Initialize
Print primes
13-29
Primes: 1st refinement
Start
Stop
Initialize
Print primes
Initialize num = 2
num < 100
Print num if prime
num = num + 1 F
T
13-30
Primes: 2nd refinement
Initialize num = 2
num < 100
Print num if prime
num = num + 1 F
T
Divide num by 2 through 10
no divisors?
Print num
F
T
13-31
Primes: 3rd refinement
Divide num by 2 through 10
no divisors?
Print num
F
T
Initialize divisor = 2
divisor <= 10
Clear flag if num%divisor > 0
divisor = divisor + 1
F
T
13-32
Primes: Using a Flag Variable
To keep track of whether number was divisible, we use a "flag" variable.
• Set prime = TRUE, assuming that this number is prime.
• If any divisor divides number evenly, set prime = FALSE.
Once it is set to FALSE, it stays FALSE.
• After all divisors are checked, number is prime if the flag variable is still TRUE.
Use macros to help readability.
#define TRUE 1
#define FALSE 0
13-33
Primes: Complete Code
#include <stdio.h>
#define TRUE 1
#define FALSE 0 main () {
int num, divisor, prime;
/* start with 2 and go up to 100 */
for (num = 2; num < 100; num ++ ) {
prime = TRUE; /* assume num is prime */
/* test whether divisible by 2 through 10 */
for (divisor = 2; divisor <= 10; divisor++)
if (((num % divisor) == 0) && (num != divisor)) prime = FALSE; /* not prime */
if (prime) /* if prime, print it */
printf("The number %d is prime\n", num);
} }
Optimization: Could put
a break here to avoid some work.
(Section 13.5.2)
13-34
Switch
switch (expression) { case const1:
action1; break;
case const2:
action2; break;
default:
action3;
}
evaluate expression
= const1?
= const2?
action1
action2
action3
T
T F
F
Alternative to long if-else chain.
If break is not used, then
case "falls through" to the next.
13-35
Switch Example
/* same as month example for if-else */
switch (month) { case 4:
case 6:
case 9:
case 11:
printf(“Month has 30 days.\n”);
break;
case 1:
case 3:
/* some cases omitted for brevity...*/
printf(“Month has 31 days.\n”);
break;
case 2:
printf(“Month has 28 or 29 days.\n”);
break;
default:
printf(“Don’t know that month.\n”);
}
13-36
More About Switch
Case expressions must be constant.
case i:
/* illegal if i is a variable */If no break, then next case is also executed.
switch (a) { case 1:
printf(“A”);
case 2:
printf(“B”);
default:
printf(“C”);
}
If a is 1, prints “ABC”.
If a is 2, prints “BC”.
Otherwise, prints “C”.
13-37
Problem 3: Searching for Substring
Have user type in a line of text (ending with linefeed) and print the number of occurrences of "the".
Reading characters one at a time
• Use the getchar() function -- returns a single character.
Don't need to store input string;
look for substring as characters are being typed.
• Similar to state machine:
based on characters seen, move toward success state or move back to start state.
• Switch statement is a good match to state machine.
13-38
Substring: State machine to flow chart
matched 't'
matched 'th'
matched 'the'
't'
'h'
'e' 't'
't'
't'
no match
other other other other
increment count
read char
match = 0
match = 1
match = 2
if 't', match=1
if 'h', match=2 if 't', match=1 else match=0
if 'e', count++
and match = 0 if 't', match=1 else match=0 T
T
T F
F
F
13-39
Substring: Code (Part 1)
#include <stdio.h>
main() {
char key; /* input character from user */
int match = 0; /* keep track of characters matched */
int count = 0; /* number of substring matches */
/* Read character until newline is typed */
while ((key = getchar()) != '\n') {
/* Action depends on number of matches so far */
switch (match) {
case 0: /* starting - no matches yet */
if (key == 't') match = 1;
break;
13-40
Substring: Code (Part 2)
case 1: /* 't' has been matched */
if (key == 'h') match = 2;
else if (key == 't') match = 1;
else
match = 0;
break;
13-41
Substring: Code (Part 3)
case 2: /* 'th' has been matched */
if (key == 'e') {
count++; /* increment count */
match = 0; /* go to starting point */
}
else if (key == 't') { match = 1;
else
match = 0;
break;
} }
printf("Number of matches = %d\n", count);
}
13-42
Break and Continue
break;
• used only in switch statement or iteration statement
• passes control out of the “smallest” (loop or switch) statement containing it to the statement immediately following
• usually used to exit a loop before terminating condition occurs (or to exit switch statement when case is done)
continue;
• used only in iteration statement
• terminates the execution of the loop body for this iteration
• loop expression is evaluated to see whether another iteration should be performed
• if for loop, also executes the re-initializer
13-43
