Control Structures

Control structures are divided into 2 types :

1. Excute certain commands only when certain condition(s) is satisfied.
   For e.g : if-else Family statements.
2. Execute certain commands repeatedly and use a certain logic to stop the iteration.
   For e.g : for loop, while loop etc.

if-else Family

if, if-else and if-elseif-else are a family of constructs where

• A condition is first checked, if satisfied then the operations are performed.
• If the condition is not satisfied, then, code exits the construct or, moves on to other options.

1. if Construct

Syntax :

if(condition){
  statements..
}

2. if-else Construct

Syntax :

if(condition){
  statements
} else{
  alternate statements
}

3. if-elseif-else Construct

Syntax :

if(condition){
  statements
} else if(condition){
  alternate statements
} else{
  alternate statements
}

Example :

Let’s write the R-code for the following problem :
- If \(x\) is greater than \(7\),then, operations within the curved braces will occur.
- Else the next condition,i.e., \(x>8\) would be checked and if this is not true
- Final else condition is checked and if that too is false, then, there will be no change is \(x\).

Let’s assume the initial value of \(x=6\). So, the code would be :

x = 6
if(x>7){
  x = x+1
}else if(x>8){
  x=x+2
}else{
  x = x+3
}

print(x)

## [1] 9

for Loop Construct

The structure of a for loop construct comprises :

• A sequence which could be a vector or, list.
• iter is an element of the sequence
• statements

Before understanding the for loop, we first need to understand the Sequence function.

Sequence Function

• A sequence function is one of the components of for loop.
• Syntax :

seq(from, to, length)

The sequence function creates an equi-spaced points between from and to - from : Starting Number - to : Ending Number - by : Increment/Decrement (width) - length : Number of elements required

Example :

seq(from = 1, to = 10, by = 2)

## [1] 1 3 5 7 9

seq(from = 1, to = 10, length = 4)

## [1]  1  4  7 10

for Loop

Syntax :

for(iter in sequence){
  statements
}

Nested for Loop

When one or, more for loop constructs are located within one another then, its known as “Nested for Loop”

Syntax :

for(iter_1 in sequence_1){
  for(iter_2 in sequence_2){
    statements
  }
}

Example :

n = 5
sum = 0

for(i in seq(1,n,1)){
  sum = sum + i
  print(c(i,sum))
}

## [1] 1 1
## [1] 2 3
## [1] 3 6
## [1]  4 10
## [1]  5 15

for Loop with if-break

A break statement once executed, program exits the loop even before the iterations are complete.

break command comes out of the innermost loop for nested loops.

Example : Let’s say we want to terminate the looping when sum is greater than \(15\) :

n = 100
sum = 0

for (i in seq(1,n,1)){
  sum = sum + i
  print(c(i, sum))
  
  if(sum >= 15){
    break
  }
}

## [1] 1 1
## [1] 2 3
## [1] 3 6
## [1]  4 10
## [1]  5 15

While Loop

A while loop is used whenever we want to execute statements until a specific condition is violated.

Example : Consider a sequence of natural numbers.
What is the value of the natural number up to which the calculated sum is less than the specified Fin_sum ?

1+2+3+…..+n = Fin_sum(15)

sum = 0
i = 0
Fin_sum = 15

while(sum < Fin_sum){
  i = i+1
  sum = sum+i
  print(c(i,sum))
}

## [1] 1 1
## [1] 2 3
## [1] 3 6
## [1]  4 10
## [1]  5 15