Basic formatting in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of formatted floating-point number

What is the output of this C++ code snippet?

C++

#include <iostream>
#include <iomanip>

int main() {
    double pi = 3.1415926535;
    std::cout << std::fixed << std::setprecision(3) << pi << std::endl;
    return 0;
}

A 3.1416

B 3.14159

C 3.141

D 3.142

💡 Hint

Look at how many digits are shown after the decimal point with setprecision.

std::fixed with std::setprecision(3) formats the number to show exactly 3 digits after the decimal point, rounding the value. 3.1415926535 rounded to 3 decimals is 3.142.

❓ Predict Output

intermediate

Output of formatted integer with width and fill

What is the output of this C++ code?

C++

#include <iostream>
#include <iomanip>

int main() {
    int num = 42;
    std::cout << std::setw(5) << std::setfill('0') << num << std::endl;
    return 0;
}

A 00042

B 0042

C 43

D 42

💡 Hint

setw sets the total width, setfill sets the padding character.

setw(5) sets the total width to 5 characters. setfill('0') fills empty spaces with zeros on the left by default. So 42 becomes 00042.

❓ Predict Output

advanced

Output of mixed formatting flags

What is the output of this C++ code snippet?

C++

#include <iostream>
#include <iomanip>

int main() {
    int num = 255;
    std::cout << std::hex << std::uppercase << std::setw(6) << std::setfill('*') << num << std::endl;
    return 0;
}

A ****FF

B ***FF

C ***0FF

D FF0***

💡 Hint

hex converts to base 16, uppercase makes letters capital, setw and setfill control width and fill.

255 in hex is FF. setw(6) means total width 6, setfill('*') fills with '*'. The output is ****FF because the number is right aligned and the fill fills the left side.

❓ Predict Output

advanced

Output of left and right alignment with width

What is the output of this C++ code?

C++

#include <iostream>
#include <iomanip>

int main() {
    std::cout << std::left << std::setw(6) << 123 << "X" << std::endl;
    std::cout << std::right << std::setw(6) << 123 << "X" << std::endl;
    return 0;
}

A

   123X
123   X

B

123   X
   123X

C

123X
123X

D

123   X
123   X

💡 Hint

left aligns text, right aligns text, setw sets width.

First line uses std::left with setw(6), so 123 is printed left aligned in 6 spaces, then X. Second line uses std::right with setw(6), so 123 is right aligned in 6 spaces, then X.

🧠 Conceptual

expert

Effect of std::boolalpha on boolean output

Which option shows the output of this C++ code?

C++

#include <iostream>

int main() {
    bool flag = true;
    std::cout << std::boolalpha << flag << std::endl;
    std::cout << std::noboolalpha << flag << std::endl;
    return 0;
}

A

1
1

B

1
true

C

true
1

D

true
true

💡 Hint

std::boolalpha prints true/false, std::noboolalpha prints 1/0.

std::boolalpha causes bool values to print as 'true' or 'false'. std::noboolalpha resets this, so bool prints as 1 or 0. So first line prints 'true', second line prints '1'.

Practice

1. What does the setw manipulator do in C++ output formatting?

easy

A. Sets the number of decimal places for floating numbers

B. Sets the width of the next output field

C. Changes the text color in the console

D. Aligns text to the center automatically

Solution

1. Step 1: Understand the purpose of setw

   The setw manipulator sets the width of the next output field in characters.

2. Step 2: Compare with other manipulators

   setprecision controls decimal places, not width. Color and alignment are not handled by setw.

3. Final Answer:

   Sets the width of the next output field -> Option B

4. Quick Check:

   setw = width setting [OK]

Hint: Remember: setw sets width, setprecision sets decimals [OK]

Common Mistakes:

• Confusing setw with setprecision
• Thinking setw changes text color
• Assuming setw aligns text automatically

2. Which of the following is the correct way to include setw in a C++ program?

easy

A. #include <string>

B. #include <format>

C. #include <iomanip>

D. #include <iostream>

Solution

1. Step 1: Identify the header for setw

   The setw manipulator is defined in the iomanip header.

2. Step 2: Check other headers

   iostream is for input/output streams but does not define setw. format and string are unrelated here.

3. Final Answer:

   #include <iomanip> -> Option C

4. Quick Check:

   setw needs iomanip header [OK]

Hint: Use <iomanip> for manipulators like setw [OK]

Common Mistakes:

• Including only <iostream> and expecting setw to work
• Confusing <format> with <iomanip>
• Forgetting to include any header for setw

3. What is the output of this code?

#include <iostream>
#include <iomanip>
using namespace std;

int main() {
  cout << setw(5) << 42 << endl;
  cout << setw(5) << 1234 << endl;
  return 0;
}

medium

A. 42
1234

B. 42
1234

C. 4321
24

D. 42
1234

Solution

1. Step 1: Understand setw effect on output

   setw(5) sets the width to 5 characters, padding with spaces on the left by default.

2. Step 2: Apply setw to numbers

   Number 42 is printed right-aligned in 5 spaces: 3 spaces + '42'. Number 1234 is 4 digits, so 1 space + '1234'.

3. Final Answer:

   42 1234 -> Option A

4. Quick Check:

   setw pads left with spaces [OK]

Hint: setw pads on left by default for numbers [OK]

Common Mistakes:

• Assuming setw pads on the right
• Ignoring spaces added by setw
• Thinking setw truncates numbers

4. Identify the error in this code snippet:

#include <iostream>
using namespace std;

int main() {
  cout << setw(10) << 3.14159 << endl;
  return 0;
}

medium

A. No error, code runs fine

B. Using cout without std:: prefix

C. Incorrect use of endl

D. Missing <iomanip> header for setw

Solution

1. Step 1: Check header includes

   The code uses setw but does not include <iomanip>, which is required.

2. Step 2: Verify other parts

   cout is used with using namespace std;, so no prefix needed. endl usage is correct.

3. Final Answer:

   Missing <iomanip> header for setw -> Option D

4. Quick Check:

   setw needs iomanip header [OK]

Hint: Always include <iomanip> when using setw [OK]

Common Mistakes:

• Forgetting to include <iomanip>
• Thinking cout needs std:: with using namespace std;
• Assuming endl usage causes error

5. How would you format the output of a floating-point number to always show exactly 3 decimal places and a total width of 8 characters, right-aligned?

hard

A. cout << fixed << setprecision(3) << setw(8) << number;

B. cout << setprecision(3) << setw(8) << number;

C. cout << setw(8) << fixed << number << setprecision(3);

D. cout << fixed << setw(3) << setprecision(8) << number;

Solution

1. Step 1: Set fixed decimal places

   Use fixed and setprecision(3) to show 3 decimals exactly.

2. Step 2: Set total width and alignment

   Use setw(8) to set total width to 8 characters, right-aligned by default.

3. Final Answer:

   cout << fixed << setprecision(3) << setw(8) << number; -> Option A

4. Quick Check:

   fixed + setprecision + setw(8) = correct format [OK]

Hint: Use fixed before setprecision, then setw for width [OK]

Common Mistakes:

• Omitting fixed causes variable decimals
• Placing setw before fixed and setprecision incorrectly
• Using wrong values for setw or setprecision