Program Looping

Program Control

• Usually statements within a program are executed one after the other in a top-down fashion, the first, second, and then the third and so on.
• some statements can redirect the flow of the program to what is not necessarily the next statement. This concept of redirection is called program control.

The for Statement

The general format:

for (expression1; expression2; expression3;)
     statement;

1. expression1 is usually an assignment statement that assigns a value to a variable.
2. expression2 is the condition, typically a relational expression.
3. expression3 is an increment
4. if expression2 evaluates to false, then the for statement terminates and execution passes to the next statement following the statement.
5. if expression2 evaluates to true then the statement is executed. Then the variable is incremented according to expression3.

Examples:

for ( i  =  0;  i  < 100;  i++){
j = 10 * i;
}

for ( i  = 100;  i  < 10;  i--){
j = 10 * i;
}

for ( i  = 0;  i  < 1000;  i  += 5){
j = 10 * i;
}

i = 1;
for ( ;  i  < 1000;  i  += 5){
j = 10 * i;
}

for ( i  = 0;  i  <  1000;  i + +){
	for ( j  =  0;  j  < 10;  j++)
	k = j* i;
}

The while Statement

The while statement is useful for having the program loop through one statement or a block of statements as long as a specified condition is true.

The general format:

while (expression)
     statement; (or a block that begins with \{ and ends with \})

1. the expression is evaluated
2. if it evaluates to false, the while statement terminates and the execution passes to the first statement following statement.
3. if the expression evaluates to true, then the statement(s) is executed and the execution returns to step 1.

Example:

main()
{
     int count;
     while (count <= 9)
     {
          printf("count=%d", count);
          ++count;
     }
}

• the while statement is same as the for statement without initialization and decrement parts.

  for (  ; expression ; )
  while ( expression )

• most C programmers prefer for over while if initialization is required because one can find initial value, test condition and increment, all at one location.

The do ... while loop

General format:

do
     statement; 
while (expression);

• Unlike for and while, it tests the condition at the end of the loop rather than at the beginning.
• Thus the loop executes at least once.
• although more straight forward, used less frequently

The if and if-else Statements

The general format is

if (expression) statement1; or a block that begins with { and ends with }

The expression evaluates to either TRUE or FALSE.

Examples:

if (a > b)
 z = a;

if (a > b)
 z = a;
 z = b;

/* Demonstrates the use of if statements */  
#include <stdio.h>  
int main()  
{  
    int x, y;  
    /* Input the two values to be tested */  
    printf("\nInput an integer value for x: ");  
    scanf("%d", &x);  
    printf("\nInput an integer value for y: ");  
    scanf("%d", &y);  
    /* Test values and print result */  
    if (x == y)  
      printf("x is equal to y");  
    if (x > y)  
      printf("x is greater than y");  
    if (x < y)  
      printf("x is smaller than y");  
    return 0;  
}

Two or more simple expressions can be combined using AND - && or OR - ||, e. g. if (grade >= 70 && grade <= 79)

Else - if

if (a > b)
 z = a;
else 
z = b;

Example:

main()
{
     int grade;
     char last_name[30];
     /* last name can be up to 30 chars */
     printf("enter your name followed by a space then your grade\n");
     scanf("%s %d", & last_name,&grade);
     if (grade >= 90)
          printf("\n%s, congratulations, you have an A !",last_name);
     else
     printf("\n%s, sorry, you don't have an A", last_name);
}

The break and continue statements

• Break can be placed only in the body of the for loop, while loop, do ... while loop or in a switch statement
• The break statement causes the program to immediately exit from the loop. All the statements in the loop that come after the break statement are skipped.

  for ( count = 0; count < 10; count++ )
  {
    if ( count == 5 )
       break;
  }

• If the break statement is encountered inside a nested loop, it causes the exit of the inner most loop only.
• continue statement, like the break statement can only be placed within the body of a loop.
• when a continue statement is encountered, the next iteration of the enlosing loop begins immediately.

The goto statement

• goto is an unconditional jump. When program encounters goto, execution immediately jumps to the location specified in the goto statement
• goto statement and the target label should be located in the same function, but they can be in different blocks
• the target can come before or after the goto statement
• goto can be used to transfer execution both into and out of loops
• however, it is strongly recommended not to use goto because
  (1) it is not needed. One can write a program without using a goto.
  (2) to avoid spaghetti code. Many debuggers (software that helps to trace the program) cannot interrogate a goto properly.

/* Demonstrates the goto statement */
#include <stdio.h>
void main()
{
  int n;
start: ;
  puts("Enter a number between 0 and 10: ");
  scanf("%d", &n);
  if (n < 0 ||n > 10 )
    goto start;
  else if (n == 0)
    goto location0;
      else if (n == 1)
        goto location1;
          else
            goto location2;
location0: ;
  puts("You entered 0.");
  goto end;
location1: ;
  puts("You entered 1.");
  goto end;
location2: ;
  puts("You entered something between 2 and 10.");
end: ;
}

The switch statement

• C's most flexible program control statement
• executes different statements based on an expression that can have more than two values

General Format:

switch (expression)
{
case template_1: statement(s);
case template_2: statement(s);
......
case template_n: statement(s);
default:  statement(s);
}

• If a match is found between the expression and one of the templates, execution is transferred to the statement that follows the case label
• If no match is found, goes to the default label. If there is no default, then the first statement after the switch block is executed.

Example:

#include <stdio.h>
void main()
{
  int reply;
  puts("Enter a number between 1 and 5:");
  scanf("%d", &reply);
  switch (reply)
  {
    case 1:
      puts("You entered 1.");
    case 2:
      puts("You entered 2.");
    case 3:
      puts("You entered 3.");
    case 4:
      puts("You entered 4.");
    case 5:
      puts("You entered 5.");
    default:
      puts("Out of range, try again.");
  }
}

About this document ...

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The translation was initiated by Vijay & on Thu Feb 5 10:38:53 EST 1998