Programming Structures: ConditionalRepeat Statements Programming Constructs if .. else while loop for loop C code to Pseudo-C C allows goto as means of transferring control Closer to machine-level programming style Generally considered
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Presentation Transcript
01
Programming Structures:
Conditional/Repeat Statements Programming Constructs
if .. else
while loop
for loop<br>
02
C code to Pseudo-C C allows “goto” as means of transferring control
Closer to machine-level programming style
Generally considered bad coding style
But, essential for assembly programming
More explicit control
Directly mapped to assembly instructions<br>
03
if (t)
then-statement;
else
else-statement; Summary 1 while (Test)
Body loop:
if (!Test) goto done;
Body
goto loop
done: if - else while if (!t) goto false;
then-statement;
goto done;
false:
else-statement;
done: Pseudo-C<br>
04
Summary 2 Init;
loop: if (!Test) goto done;
Body
Update ;
goto loop;
done: for (Init; Test; Update )
Body Init;
while (Test ) {
Body
Update ;
} for loop
equivalent to:<br>
05
if (t)
then-statement;
else
else-statement; //t = test_expr;
if (!t) goto else;
then-statement;
goto done;
else:
else-statement;
done: int goto_max(int x, int y){
int rval;
if (x <= y)goto else;
rval = x;
goto done
else:
rval = y;
done:
return rval;
} int max(int x, int y){
if (x > y)
return x;
else
return y;
} General if-else translation<br>
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int goto_max(int x, int y){
int rval = y;
if (x <= y)goto done;
rval = x;
done:
return rval;
} int max(int x, int y){
int rval = y;
if (x > y)
rval = x;
} General if-else translation int max(int x, int y){
if (x > y)
return x;
else
return y;
}<br>
07
C Code int fact_while(int x){
int result = 1;
while (x > 1) {
result *= x;
x = x-1;
};
return result;
} Pseudo-C Version int fact_while_goto(int x){
int result = 1;
loop:
if (!(x > 1)) goto done;
result *= x;
x = x-1;
goto loop;
done:
return result;
} “While” Loop while (Test)
Body loop:
if (!Test) goto done;
Body
goto loop
done:<br>
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“For” “While” for (Init; Test; Update )
Body Init;
while (Test ) {
Body
Update ;
} Init;
loop: if (!Test) goto done;
Body
Update ;
goto loop;
done: While Version For Version<br>
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int result;
for (result = 1;
p != 0;
p = p>>1) {
if (p & 0x1)
result *= x;
x = x*x;
} result = 1;
loop: if (p == 0) goto done;
if (p & 0x1)
result *= x;
x = x*x;
p = p >> 1;
goto loop;
done: for (Init; Test; Update )
Body Init;
loop: if (!Test) goto done;
Body
Update ;
goto loop;
done:<br>
10
C Code int fact_do(int x)
{
int result = 1;
do {
result *= x;
x = x-1;
} while (x > 1);
return result;
} Pseudo-C Version int fact_goto(int x)
{
int result = 1;
loop:
result *= x;
x = x-1;
if (x > 1)
goto loop;
return result;
} “Do” Loop Use backward branch to continue looping
Only take branch when “while” condition holds do
Body
while (Test); loop:
Body
if (Test)
goto loop<br>
