C Language

Selection Statements

if () Statements

One of the simplest ways to control program flow is by using if selection statements. Whether a block of code is to be executed or not to be executed can be decided by this statement.

The syntax for if selection statement in C could be as follows:

if(cond) 
{
  statement(s);  /*to be executed, on condition being true*/
}

For example,

if (a > 1) {
    puts("a is larger than 1");
}

Where a > 1 is a condition that has to evaluate to true in order to execute the statements inside the if block. In this example “a is larger than 1” is only printed if a > 1 is true.

if selection statements can omit the wrapping braces { and } if there is only one statement within the block. The above example can be rewritten to

if (a > 1)
    puts("a is larger than 1");

However for executing multiple statements within block the braces have to used.

The condition for if can include multiple expressions. if will only perform the action if the end result of expression is true.

For example

if ((a > 1) && (b > 1)) {
    puts("a is larger than 1");
    a++;
}

will only execute the printf and a++ if both a and b are greater than 1.

if () … else statements and syntax

While if performs an action only when its condition evaluate to true, if / else allows you to specify the different actions when the condition true and when the condition is false.

Example:

if (a > 1)
    puts("a is larger than 1");
else 
    puts("a is not larger than 1");

Just like the if statement, when the block within if or else is consisting of only one statement, then the braces can be omitted (but doing so is not recommended as it can easily introduce problems involuntarily). However if there’s more than one statement within the if or else block, then the braces have to be used on that particular block.

if (a > 1) 
{
    puts("a is larger than 1");
    a--;
}
else 
{
    puts("a is not larger than 1");
    a++;
}

switch () Statements

switch statements are useful when you want to have your program do many different things according to the value of a particular test variable.

An example usage of switch statement is like this:

int a = 1;

switch (a) {
case 1:
    puts("a is 1");
    break;
case 2:
    puts("a is 2");
    break;
default:
    puts("a is neither 1 nor 2");
    break;
}

This example is equivalent to

int a = 1;

if (a == 1) {
    puts("a is 1");
} else if (a == 2) {
    puts("a is 2");
} else {
    puts("a is neither 1 nor 2");
}

If the value of a is 1 when the switch statement is used, a is 1 will be printed. If the value of a is 2 then, a is 2 will be printed. Otherwise, a is neither 1 nor 2 will be printed.

case n: is used to describe where the execution flow will jump in when the value passed to switch statement is n. n must be compile-time constant and the same n can exist at most once in one switch statement.

default: is used to describe that when the value didn’t match any of the choices for case n:. It is a good practice to include a default case in every switch statement to catch unexpected behavior.

A break; statement is required to jump out of the switch block.

Note: If you accidentally forget to add a break after the end of a case, the compiler will assume that you intend to “fall through” and all the subsequent case statements, if any, will be executed (unless a break statement is found in any of the subsequent cases), regardless of whether the subsequent case statement(s) match or not. This particular property is used to implement Duff’s Device. This behavior is often considered a flaw in the C language specification.

Below is an example that shows effects of the absence of break;:

int a = 1;

switch (a) {
case 1:
case 2:
    puts("a is 1 or 2");
case 3:
    puts("a is 1, 2 or 3");
    break;
default:
    puts("a is neither 1, 2 nor 3");
    break;
}

When the value of a is 1 or 2, a is 1 or 2 and a is 1, 2 or 3 will both be printed. When a is 3, only a is 1, 2 or 3 will be printed. Otherwise, a is neither 1, 2 nor 3 will be printed.

Note that the default case is not necessary, especially when the set of values you get in the switch is finished and known at compile time.

The best example is using a switch on an enum.

enum msg_type { ACK, PING, ERROR };
void f(enum msg_type t)
{
  switch (t) {
  case ACK:
    // do nothing
    break;
  case PING:
    // do something
    break;
  case ERROR:
    // do something else
    break;
  }
}

There are multiple advantages of doing this:

  • most compilers will report a warning if you don’t handle a value (this would not be reported if a default case were present)
  • for the same reason, if you add a new value to the enum, you will be notified of all the places where you forgot to handle the new value (with a default case, you would need to manually explore your code searching for such cases)
  • The reader does not need to figure out “what is hidden by the default:”, whether there other enum values or whether it is a protection for “just in case”. And if there are other enum values, did the coder intentionally use the default case for them or is there a bug that was introduced when he added the value?
  • handling each enum value makes the code self explanatory as you can’t hide behind a wild card, you must explicitly handle each of them.

Nevertheless, you can’t prevent someone to write evil code like:

enum msg_type t = (enum msg_type)666; // I'm evil

Thus you may add an extra check before your switch to detect it, if you really need it.

void f(enum msg_type t)
{
   if (!is_msg_type_valid(t)) {
      // Handle this unlikely error
   }

   switch(t) { 
    // Same code than before
   }
}

if()…else Ladder Chaining two or more if () … else statements

While the if ()... else statement allows to define only one (default) behaviour which occurs when the condition within the if () is not met, chaining two or more if () ... else statements allow to define a couple more behaviours before going to the last else branch acting as a “default”, if any.

Example:

int a = ... /* initialise to some value. */

if (a >= 1) 
{
    printf("a is greater than or equals 1.\n");
} 
else if (a == 0) //we already know that a is smaller than 1
{
    printf("a equals 0.\n");
}
else /* a is smaller than 1 and not equals 0, hence: */
{ 
    printf("a is negative.\n");
}

Nested if()…else VS if()..else Ladder

Nested if()...else statements take more execution time (they are slower) in comparison to an if()...else ladder because the nested if()...else statements check all the inner conditional statements once the outer conditional if() statement is satisfied, whereas the if()..else ladder will stop condition testing once any of the if() or the else if() conditional statements are true.

An if()...else ladder:

#include <stdio.h>

int main(int argc, char *argv[])
{
  int a, b, c;
  printf("\nEnter Three numbers = ");
  scanf("%d%d%d", &a, &b, &c);
  if ((a < b) && (a < c))
  {
    printf("\na = %d is the smallest.", a);
  }
  else if ((b < a) && (b < c))
  {
    printf("\nb = %d is the smallest.", b);
  }
  else if ((c < a) && (c < b))
  {
    printf("\nc = %d is the smallest.", c);
  }
  else
  {
    printf("\nImprove your coding logic");
  }
  return 0;
}

Is, in the general case, considered to be better than the equivalent nested if()...else:

#include <stdio.h>

int main(int argc, char *argv[])
{
  int a, b, c;
  printf("\nEnter Three numbers = ");
  scanf("%d%d%d", &a, &b, &c);
  if (a < b)
  {
    if (a < c)
      {
        printf("\na = %d is the smallest.", a);
      }
    else
      {
        printf("\nc = %d is the smallest.", c);
      }
  }
  else
  {
    if(b < c)
    {
      printf("\nb = %d is the smallest.", b);
    }
    else
    {
      printf("\nc = %d is the smallest.", c);
    }
  }
  return 0;  
}
This modified text is an extract of the original Stack Overflow Documentation created by the contributors and released under CC BY-SA 3.0 This website is not affiliated with Stack Overflow