1. Programmation impérative - Prof. Béat Hirsbrunner
Chap. 3 Control Flow
3.1 Statements and Blocks
3.2 if, if … else …
3.3 if … else if … else …
3.4 switch
3.5 Loops: while and for
3.6 Loops: do
3.7 break and continue
3.8 goto and labels
System-oriented Programming, B. Hirsbrunner, diuf.unifr.ch/pai/sp, Lecture 3 – 8 March 2016 KR – Chap. 3: ~30’; Tutorials: AST for control flow structures: 10'; UNIT Testing: ~25’ + gdb: ~25' +; Exercises: ~90’

2. Programmation impérative - Prof. Béat Hirsbrunner
3.1 Statements and Blocks
Statement
An expression such as x=0 or i++ becomes a statement when it is followed by a semicolon, as in :
x=0;
i++;
Compound statement or block
Braces { and } are used to group declarations and statements.
A block is syntactically equivalent to a single statement.
Miscellaneous
There is no semicolon after the right brace that ends a block.

3. Programmation impérative - Prof. Béat Hirsbrunner
3.2 if [else], else if
Syntax
if (expression)
statement
Shortcuts
if (expression)
is equivalent to
if (expression != 0)
if (expression)
statement1
else
statement2
Nested if sequence
Because the else part is optional, there is an ambiguity when the else is omitted from a nested if sequence:
if (n > 0)
if (a > b)
z = a;
else
z = b;
This is resolved by associating the else with the closest previous else-less if
if (expression1)
statement1
else if (expression2)
statement2
else if (expression3)
statement3
else
statement4

4. Programmation impérative - Prof. Béat Hirsbrunner
3.4 Switch
switch (expression) {
case const-expr1: statements1
case const-expr2: statements2
case const-expr3: statements3
default: statements4 }
Example
while ((c = getchar()) != EOF) {
switch (c) {
case ‘0’: printf(“zero\n”); break;
case ‘1’: printf(“one\n”); break;
default : printf(“other\n”); break; }

5. Programmation impérative - Prof. Béat Hirsbrunner
3.5 Loops – while, for
Syntax
while (expression)
statement
for (e1_opt; e2_opt; e3_opt)
Equivalence
for (expr1; expr2; expr3)
statement
is equivalent to (except if statement contains a continue statement):
expr1;
while (expr2) {
expr3; }
The postfix _opt indicates that the expression is optional
Infinite loop (sic!)
for (;;) ;
Typical use
while ((c = getchar()) == ‘ ‘ || c == ‘\n’ || c == ‘\t’) …
for (i = 0; i < n; i++)

6. Programmation impérative - Prof. Béat Hirsbrunner
3.6 Loops – do
Syntax do
statement
while (expression);
Comma “,” operator
for (i = 0, j = strlen(s) - 1; i < j; i++, j--)
c = s[i], s[i] = s[j], s[j] = c;
Remark. A pair of expressions separated by a comma is :
evaluated left to right, and
the type and value of the result are the ones of the last evaluated expression,
e.g. in the above example the ones of the expression s[j] = c

7. 3.7 break and continue, 3.8 goto and labels
Programmation impérative - Prof. Béat Hirsbrunner
été 2002
3.7 break and continue, goto and labels
The break statement provides an early exit from for, while, do, and switch.
The continue statement causes the next iteration of the enclosed for, while, or do loop.
Note: break and continue operate on the
closest enclosing loop or switch
C provides the infinitely-abusable goto statement, and labels to branch to.
A label has the same form as a variable name, and is followed by a colon. It can be attached to any statement in the same function as the goto. The scope of a label is the entire function.

8. Programmation impérative - Prof. Béat Hirsbrunner
Goto’s Typical Use
int foo () {
...
for ( ... ) {
if (disaster)
goto error; }
return ...
error:
/* clean up the mess */

9. Programmation impérative - Prof. Béat Hirsbrunner
Goto’s Typical Use
for (i = 0; i < n; i++)
for (j = 0; j < m; j++)
if (a[i] == b[j])
goto found;
/* didn't find any common element */
...
found:
/* got one: a[i] == b[j] */

10. Programmation impérative - Prof. Béat Hirsbrunner
Goto is not necessary
Code involving a goto can always be written without one, though perhaps at the price of some repeated tests or an extra variable. For example, the array search becomes:
found = 0;
for (i = 0; i < n && !found; i++)
for (j = 0; j < m && !found; j++)
if (a[i] == b[j])
found = 1;
if (found)
/* got one: a[i-1] == b[j-1] */
...
else
/* didn't find any common element */