String input and output in C - Step-by-Step Execution

Concept Flow - String input and output

Start Program

↓

Declare char array

↓

Input string using scanf

↓

Store input in array

↓

Print string using printf

↓

End Program

The program declares a character array, reads a string from the user, stores it, then prints it back.

Execution Sample

C

#include <stdio.h>
int main() {
  char name[20];
  scanf("%19s", name);
  printf("Hello, %s!\n", name);
  return 0;
}

This program reads a string input from the user and prints a greeting with that string.

Execution Table

Step	Action	Input/Value	Variable State	Output
1	Declare char array 'name' of size 20	-	name = uninitialized	-
2	Read string input with scanf	User types: Alice	name = "Alice"	-
3	Print greeting with printf	-	name = "Alice"	Hello, Alice!
4	Return 0 and end program	-	-	-

💡 Program ends after printing the greeting.

Variable Tracker

Variable	Start	After Input	Final
name	uninitialized	"Alice"	"Alice"

Key Moments - 3 Insights

Why do we use %19s in scanf instead of just %s?

Using %19s limits input to 19 characters plus the null terminator, preventing buffer overflow as shown in step 2 of the execution_table.

What happens if the user types a string longer than 19 characters?

scanf stops reading after 19 characters, so the extra characters remain in the input buffer, preventing overflow and keeping 'name' safe as seen in step 2.

Why does printf use %s to print the string?

The %s format tells printf to print characters from the array until it finds the null terminator, which is how strings are stored in C, demonstrated in step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'name' after step 2?

A uninitialized

B "Alice"

C "Hello"

D empty string

💡 Hint

Check the 'Variable State' column in row for step 2.

At which step does the program output the greeting?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Output' column in the execution_table.

If the user inputs a string longer than 19 characters, what prevents buffer overflow?

A Using %s in scanf

B Using printf to print the string

C Using %19s in scanf

D Declaring name as char array

💡 Hint

Refer to the key_moments explanation about scanf format specifier.

Concept Snapshot

String input and output in C:
- Declare char array to hold string
- Use scanf("%19s", array) to safely read input
- Use printf("%s", array) to print string
- Strings end with null character '\0'
- Limit input size to avoid overflow

Full Transcript

This program shows how to read a string from the user and print it in C. First, a character array named 'name' is declared to hold the input. Then, scanf with %19s reads up to 19 characters safely, storing them in 'name'. After input, printf prints a greeting including the string. The program ends after printing. Using %19s prevents buffer overflow by limiting input size. The string is printed until the null terminator is found. This simple flow helps beginners understand string input and output in C.