1 Introduction to C

if (condition)
   block

The condition is given by a Boolean or a logic expression. When it is true the block of statements will be executed; otherwise the block will be skipped.

Example:

float a = 3.41415;
if (a > 0.0) {
  printf("number %f is positive",a);
}

Note that the block scope braces {} can be eliminated if there is only one statement in the block. The above program example can be written as:

float a = 3.41415;
if (a > 0.0) 
  printf("number %f is positive",  a);

Extra spaces, new lines, tabs can also be added for readability, and eliminated for compressing. The above program can also be written as:

float a=3.41415;if(a>=0.0)printf("the number %f is positive",a);

if (condition) 
  block1 
else 
  block2

Example:

if (mark >= 50.0)
  printf("pass");
else
  printf("fail");

C provides a conditional ternary operator (?:) for a simple if-else expression.

The syntax of the conditional operator is: condition? expression1 : expression2

It means that if condition is true, use expression1; else use expression2.

Examples:

int a = 1>2 ? 10 : 20; // This assigns 20 to a.
int b = 1<2 ? 10 : 20; // This assigns 10 to b. 

if (condition1) 
  block1; 
else if (condition2) 
  block2;
else 
  block3;

It allows to have multiple else-if statements, and to remove the else statement.

Example:

if (mark >= 90.0)  
  printf("A+");
else if (marks >= 80.0)
  printf("A");
else if (mark >= 70.0)
  printf("B");
else if (mark >= 60.0)
  printf("C");
 else if (mark >= 50.0)
  printf("D");
 else
   printf("F");

C provides a switch statement to select blocks based on integer or char values.

Example:

switch( grade ) {
  case 'A' : 
    printf( "Excellent" );
    break;         
  case 'B':
    printf( "Good" );             
    break;
  case 'C' :  
    printf( "OK" );           
    break;
   default: 
    printf( "What is your grade?" ); 
}

For a given value of grade, it checks the first case, namely evaluates grade == 'A'. If the evaluation is true, it prints “Excellent”. If it is not true, it checks the next case, and so on. The break; statement means to skip the rest statements of the switch block. Without the break statement, it will move to the next case. For example, given char grade = 'B';, it prints Good, and then leaves the switch statement.

Syntax:

for (a list of initial statements; a list of conditional statements; a list of next-iteration statements)  
  repetition-block

The for loop statement controls the program flow by the following steps.

Step 1. Start from executing the list of initial statements. 
Step 2. Execute the list of conditional statements.   
        If one of them gives true, i.e., none-zero, then goto step 3  
        else goto step 5.
Step 3. Execute the repetition-block.
Step 4. Execute the list of next-iteration statements, goto step 2.
Step 5. Leave the for loop, i.e., move to the next statement after the loop block.   

Example:

int count;
for (count = 1; count < 3; count++)
  printf("%d ", count);  

The output of the above code segment is: 1 2

It runs step-by-step as follows.

count = 1
count < 3 gives 1
print count value 1   

count = count+1 = 1+1 = 2
count < 3 gives 1
print count value 2

count = count+1 = 2+1 = 3 
count < 3 gives 0
leave the for loop. 

Syntax:

while (list of conditional statements)  
  repetition-block

The while loop controls the program flow by the following steps.

Step 1. Execute the list of conditional statements,   
        if one of them values true, then goto step 2 
        else goto step 3.
Step 2. Execute the repetition-block, goto step 1.
Step 3. Leave the while loop. 

Example:

int count = 1;
while (count < 3) {
  printf("%d ", count);
  count++;
}

The output of the above program is: 1 2

It runs step-by-step as follows.

count < 3 gives 1
print count value 1
count = count+1 = 1+1 = 2

count < 3 gives 1
print count value 2
count = count+1 = 2+1 = 3 

count < 3 gives 0
leave the while loop. 

Syntax:

do {
  repetition-block
} while (list of conditional statements);

Example:

int count = 1;
do {
  printf("%d ", count);
  count++;
} while (count < 10);

The output of the above program is: 1 2

It runs step-by-step as follows.

print count value 1
count = count+1 = 1+1 = 2

count < 3 gives 1
print count value 2
count = count+1 = 2+1 = 3 

count < 3 gives 0
leave the do-while loop. 

The break statement break; can be used in a repetition-block for jumping out of a loop. When the compiler encounters a break statement, it adds flow control to transfer to the statement that follows the loop statement.

Example:

int count; 
for(count = 1; ; count++) { 
   if (count >= 3) break;
   printf("%d ", count); 
}

The output of the program is: 1 2

It leaves the loop statement when count >=3. Without statement if (count >= 3) break;, the above program runs into an infinite loop.

The statement continue; can be used in a repetition-block to skip the rest of statements of the repetition-block. If it is placed within a for-loop, the continue statement causes a jump to the next-iteration statements. If it is in a while-loop or do-while loop, it jumps to the loop conditional statements.

Example:

/* output the integers between 10 and 20, not divisible by 3*/
#include <stdio.h>
int main(void) {
  int n;
  for (n = 10; n <= 20;  n++)
  {
    if (n%3==0)
      continue;
    printf("%d ", n);
  }
  return 0;
}

The above program will print the integers between 10 and 20, which are not divided by 3. That is, it skips the printing n dividable by 3.

The goto statement transfers flow control to a labeled position within a function scope.

A label is an identifier that specifies the place where the branch is to be made. It can be any valid variable name that is followed by a colon (:). Labels can be placed anywhere in the program either before or after the goto statement within its block. Whenever the goto statement is encountered, the control is immediately transferred to the statements following the label.

Example:

#include <stdio.h>
int main () {
   int i = 0;
   LOOP:
   printf("%d ", i);
   i++;
   if (i < 10) 
     goto LOOP;
   return 0;
}