Insert at Middle Specific Position in DSA C - Execution Trace

Concept Flow - Insert at Middle Specific Position

Start

↓

Check if list is empty

↓

Create new node

↓

Insert new node

↓

Update links

↓

End

Start by checking if the list is empty. If yes, create the new node as head. Otherwise, traverse to the node before the desired position, insert the new node, and update links.

Execution Sample

DSA C

void insertAtPosition(Node** head, int data, int pos) {
  Node* newNode = malloc(sizeof(Node));
  newNode->data = data;
  newNode->next = NULL;
  if (pos == 1) {
    newNode->next = *head;
    *head = newNode;
    return;
  }
  Node* temp = *head;
  for (int i = 1; i < pos - 1 && temp != NULL; i++) {
    temp = temp->next;
  }
  if (temp == NULL) return;
  newNode->next = temp->next;
  temp->next = newNode;
}

This code inserts a new node with given data at the specified position in a singly linked list.

Execution Table

Step	pos	temp position	Action	List state after step
1	3	head (pos 1)	Create new node with data=99	1 -> 2 -> 3 -> 4 -> null
2	3	pos 1	Traverse to node before position 3 (pos 2)	1 -> 2 -> 3 -> 4 -> null
3	3	pos 2	Insert new node after pos 2	1 -> 2 -> 99 -> 3 -> 4 -> null
4	3	pos 2	Update links	1 -> 2 -> 99 -> 3 -> 4 -> null
5	3	pos 2	End	1 -> 2 -> 99 -> 3 -> 4 -> null

💡 Insertion complete at position 3, list updated successfully.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
pos	3	3	3	3	3
temp	head (pos 1)	head (pos 1)	pos 2	pos 2	pos 2
newNode->data	N/A	99	99	99	99
newNode->next	N/A	NULL	NULL	pos 3	pos 3

Key Moments - 3 Insights

Why do we traverse to position-1 instead of the exact position?

Because we need to update the 'next' pointer of the node before the insertion point to point to the new node. See execution_table step 2 where temp stops at position 2 before inserting at position 3.

What happens if the position is 1?

The new node becomes the new head. Its 'next' points to the old head. This is handled separately in the code before traversal, not shown in this trace.

What if the position is beyond the list length?

The traversal will reach NULL before position-1, so insertion is skipped. This prevents invalid memory access.

Visual Quiz - 3 Questions

Look at the execution_table, what is the list state after step 3?

A 99 -> 1 -> 2 -> 3 -> 4 -> null

B 1 -> 2 -> 3 -> 4 -> null

C 1 -> 2 -> 99 -> 3 -> 4 -> null

D 1 -> 99 -> 2 -> 3 -> 4 -> null

💡 Hint

Check the 'List state after step' column in execution_table row 3.

At which step does the temp pointer move from position 1 to position 2?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'temp position' column in execution_table rows 1 and 2.

If pos was 1, what would be the new head's data after insertion?

A Data of old head

B 99

C NULL

D Data of node at position 2

💡 Hint

Recall key_moments answer about insertion at position 1 making new node the head.

Concept Snapshot

Insert at Middle Specific Position in singly linked list:
- Create new node with data
- If position is 1, new node points to head, becomes new head
- Else, traverse to node before position
- Insert new node by adjusting next pointers
- Handles invalid positions by skipping insertion

Full Transcript

This visual trace shows how to insert a new node at a specific middle position in a singly linked list. We start by creating the new node with the given data. If the position is 1, the new node becomes the head. Otherwise, we traverse the list to the node just before the desired position. Then we insert the new node by updating the next pointers to link it in. The execution table tracks each step, showing the position, traversal, and list state. The variable tracker shows how variables like temp and newNode change. Key moments clarify why we stop traversal at position-1 and how edge cases like position 1 or invalid positions are handled. The quiz tests understanding of list state after insertion and pointer movement. This method ensures safe and correct insertion at any valid middle position.