C Loops

If the body of the loop contains multiple statements it has to be wrapped in brackets
Statements that are wrapped in brackets are called compound statements

do-while Loop

In a do-while loop the body is evaluated at least once every time
Always use with brackets even if the body is not a compound statement

for Loop

The 1st expression in a for loop is the initialization expression
The 2nd expression in a for loop is the controlling expression
The 3rd expression in a for loop is evaluated after the body of the loop is executed
Increment and decrement behaves the same when used as the 3rd expression

// For loop
for (i = 10; i > 0; i--)
	printf("T minus %d and counting\n", i);
 
// (Equivalent) While loop
i = 10;
while (i > 0) {
	printf("T minus %d and counting\n", i);
	i--;
}

Multiple expressions can be declared in place of the 1st and 3rd expression
In the 1st expression we can initialize multiple variables provided they have the same type

int sum = 0
for(int i = 0, j = n; i <= n && j >= 0; i++, j --) {
	sum += i;
	printf("%d\n", j);
}
 
printf("Sum : %d\n", sum);

In C99, the ability to declare variable the variable in the 1st expression was added

for (int i = 0; i < n; i++) {
	// statements
}

Comma Operator

Depending on the usage comma can be a separator or a operator

When used as a operator the 1st expression is evaluated and its value is discarded
The 2nd expression is evaluated and its value becomes the value of the expression
The 1st expression should cause a side-effect for it to have a purpose

Comma operator has the lowest precedence of the operators, it is left associative

Comma operator - Wikipedia

int a=1, b=2, c=3;       // Seperator           
int i = (a, b);          // Operator
{ int i = a, b; }        // Seperator
i = a += 2, a + b;       // Operator
return 1, 2, 3;          // Operator
return a=4, b=5, c=6;    // Seperator

break Statement

Break statement allows us to break out of one level of nested
When break is used in a nested loop the control is only broken from the inner most loop
Break causes the control to jump to a point right after the loop body

for (d = 2; d < n; d++) {
	if (n % d == 0)
		break;
}
/* break jumps to here */
		
if (d < n)
	printf("%d is divisible by %d\n", n, d);
else
	printf("%d is prime\n", n);

continue Statement

Continue causes the control to jump to a point right before the end of the loop body
Cannot be used with a switch statement

n = 0;
sum = 0;
 
while (n < 10) {
	scanf("%d", &i);
	
	if (i == 0)
		continue;
	
	sum += i;
	n++;
	/* continue jumps to here */
}

goto Statement

Transfer controls to any labelled statement that is inside the same function
C99: goto cannot be used to skip over the declaration of a variable-length array (Might access element of the array that is not declared)

// Check if number is prime
for (d = 2; d < n; d++) {
	if (n % d == 0) {
		goto done;
	}
}
 
done:
if (d < n)
	printf("%d is divisible by %d\n", n, d);
else
	printf("%d is prime\n", n);

goto statement is useful for breaking out of a nested loop

// Print even numbers
for (i = 0; i <= 10; i++) {
	if (i % 2 != 0) {
		goto done;
	}
	printf("%d ", i);
done:;
}

WARNING

A break statement followed by a return can never be executed
Needs to take into considering when using switch statement at the end of program

#include <stdio.h>

int main(void) { int cmd; float balance = 0.0f, credit, debit;

printf(”*** ACME checkbook-balancing program ***\n”); printf(“Commands: 1=credit, 2=debit, ”); printf(“4=exit\n\n”);

for (;;) { printf(“Enter command: ”); scanf(“%d”, &cmd);

   switch (cmd) {
       case 1:
           printf("Enter amount of credit: ");
           scanf("%f", &credit);
           balance += credit;
           break;

       case 2:
           printf("Enter amount of debit: ");
           scanf("%f", &debit);
           balance -= debit;
           break;

       case 3:
           printf("Current balance: $%.2f\n", balance);
           break;

       case 4:
           return 0;

       default:
           printf("Commands: 0=clear, 1=credit, 2=debit, ");
           printf("3=balance, 4=exit\n\n");
           break;
   }

} }

NULL Statement

A null statement (consisting of just a semicolon) performs no operations
It is useful when the syntax of the language calls for a statement but no expression evaluation

/* Skip whitespace after first word */
while ((character = getchar()) != ' ');

Adding a semicolon after the parenthesis of a if, for or while statement creates a null statement which causes the loop to be exited prematurely

// The printf statement is not inside the if statement
if (d == 0);
	printf("Error: Division by zero\n");
 
// Infinite loop as loop body will not be executed
i = 10;
while (i > 0); 
{
	printf("T minus %d and counting\n", i);
	--i;
}
 
// Loop body only executed once (after loop terminates)
for (i = 10; i > 0; i--);
	printf("T minus %d and counting\n", i);