String input and output in C - Time & Space Complexity

Time Complexity: String input and output

O(n)

Understanding Time Complexity

When working with strings in C, it's important to know how the time to read or print a string grows as the string gets longer.

We want to understand how the program's work changes when the input string size changes.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

#include <stdio.h>

int main() {
    char str[1000];
    scanf("%999s", str);  // Read a string safely
    printf("%s", str); // Print the string
    return 0;
}
    

This code reads a string from the user and then prints it back to the screen.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Reading and printing each character of the string one by one.
• How many times: Once for each character in the string until the end (null character).

How Execution Grows With Input

As the string gets longer, the program reads and prints more characters, so the work grows with the string length.

Input Size (n)	Approx. Operations
10	About 10 character reads and 10 prints
100	About 100 character reads and 100 prints
1000	About 1000 character reads and 1000 prints

Pattern observation: The number of operations grows directly with the string length.

Final Time Complexity

Time Complexity: O(n)

This means the time to read and print the string grows in a straight line with the string length.

Common Mistake

[X] Wrong: "Reading or printing a string always takes the same time no matter how long it is."

[OK] Correct: The program must handle each character, so longer strings take more time.

Interview Connect

Understanding how string input and output time grows helps you explain how programs handle user data efficiently.

Self-Check

"What if we used fgets() instead of scanf() to read the string? How would the time complexity change?"

Practice

1. Which function is used to read a single word string from the user in C?

easy

A. fgets(str, size, stdin);

B. printf("%s", str);

C. gets(str);

D. scanf("%s", str);

Solution

1. Step 1: Understand string input functions

   scanf with "%s" reads a single word string until whitespace.

2. Step 2: Identify correct function for single word input

   printf is for output, gets is unsafe, fgets reads whole line including spaces.

3. Final Answer:

   scanf("%s", str); -> Option D

4. Quick Check:

   Single word input = scanf("%s", str); [OK]

Hint: Use scanf with %s for single word input [OK]

Common Mistakes:

• Using printf instead of scanf for input
• Using fgets expecting single word input
• Using gets which is unsafe

2. Which of the following is the correct syntax to print a string variable str in C?

easy

A. printf("%d", str);

B. printf("%s", str);

C. scanf("%s", str);

D. gets(str);

Solution

1. Step 1: Identify the print format specifier for strings

   "%s" is used in printf to print strings.

2. Step 2: Match correct function and format

   printf with "%d" is for integers, scanf is for input, gets reads input.

3. Final Answer:

   printf("%s", str); -> Option B

4. Quick Check:

   Print string = printf("%s", str); [OK]

Hint: Use printf with %s to print strings [OK]

Common Mistakes:

• Using %d to print strings
• Using scanf instead of printf for output
• Using gets for printing

3. What will be the output of the following C code?

#include <stdio.h>
int main() {
    char str[20];
    scanf("%s", str);
    printf("%s", str);
    return 0;
}

If the user inputs: Hello World

medium

A. Hello

B. Hello World

C. World

D. Compilation error

Solution

1. Step 1: Understand scanf with %s behavior

   scanf("%s", str) reads input until first whitespace, so it reads only "Hello".

2. Step 2: Output printed by printf

   printf("%s", str) prints the stored string, which is "Hello".

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   scanf with %s stops at space = Hello [OK]

Hint: scanf %s reads only until space, not full line [OK]

Common Mistakes:

• Expecting scanf %s to read full line with spaces
• Confusing scanf with fgets
• Assuming output includes 'World'

4. Identify the error in the following code snippet for reading a full line including spaces:

#include <stdio.h>
int main() {
    char str[50];
    scanf("%s", str);
    printf("%s", str);
    return 0;
}

medium

A. char array size is too large

B. printf("%s", str) cannot print strings

C. scanf("%s", str) reads only until first space, not full line

D. Missing return type for main

Solution

1. Step 1: Analyze scanf with %s for input

   scanf("%s", str) stops reading input at first whitespace, so it cannot read full line with spaces.

2. Step 2: Check other parts of code

   printf("%s", str) correctly prints string, array size is sufficient, main has return type int.

3. Final Answer:

   scanf("%s", str) reads only until first space, not full line -> Option C

4. Quick Check:

   scanf %s stops at space, so not full line input [OK]

Hint: Use fgets to read full line with spaces [OK]

Common Mistakes:

• Assuming scanf %s reads full line
• Thinking printf can't print strings
• Confusing array size issues

5. How can you correctly read a full line of text including spaces into a string str of size 100 in C?

hard

A. fgets(str, 100, stdin);

B. scanf("%s", str);

C. printf("%s", str);

D. gets(str);

Solution

1. Step 1: Understand functions for reading strings with spaces

   scanf("%s", str) reads only until space, gets is unsafe and deprecated.

2. Step 2: Identify safe function to read full line

   fgets(str, 100, stdin) reads up to 99 characters including spaces safely.

3. Final Answer:

   fgets(str, 100, stdin); -> Option A

4. Quick Check:

   Use fgets for full line input with spaces [OK]

Hint: Use fgets to read lines with spaces safely [OK]

Common Mistakes:

• Using scanf %s for full line input
• Using gets despite safety issues
• Confusing printf with input functions