Variable Practice (Answers)

Here are some exercises that will help you get a better grasp of using variables! (Try solving these by hand without a compiler!)

1. Write one line of code that declares an integer called cups_of_tea and assigns to it a value of 5.

Answer:

int cups_of_tea = 5;

2. What is wrong with the following code? How can you fix it?

#include <stdio.h>

int main(void)
{
  printf("The value of x is %d.\n", x);

  int x = 7;

  return 0;
}

Answer: x is declared after the printf() statement! This means that x does not yet exist at the time of the printf(). Even if it did somehow exist, it would not have a defined value.

To fix the issue, simply swap the lines with the printf() statement and the assignment to x. A revised version of the code might look like the following:

#include <stdio.h>

int main(void)
{
  int x = 7;

  printf("The value of x is %d.\n", x);

  return 0;
}

3. Which of the following are not valid variable names? If any are invalid, explain why.

Answer:

• __basic
  • Valid.
• apple#3
  • Invalid - The # symbol is not allowed in a variable.
• 2023_year
  • Invalid - A variable name cannot begin with a number.
• INT
  • Valid. (Since it is not the lowercase int, this is an allowed variable name.)
• orANGE
  • Valid.
• total students
  • Invalid - A variable name cannot contain a space.
• 982
  • Invalid - A variable name cannot begin with a number.
• float_
  • Valid.

4. Which of the following statements are not valid in C? If any are invalid, explain why.

Answer:

• int float = 34123.4123 * 4;
  • Invalid. float cannot be used as a variable name.
• char student_name = 'John Smith';
  • Invalid. The single quotation marks (') can only contain one character.
• int i = 0; i = i + 1;
  • Valid.
• 4 = gpa;
  • Invalid. The statement is in the wrong order. To assign the value 4 to gpa, the order should be flipped to gpa = 4.

5. What is wrong with the following code? How can you fix it?

#include <stdio.h>

int main(void)
{
  ounces_of_milk = 16.0;
  printf("You have %lf ounces of milk.\n", ounces_of_milk);

  return 0;
}

Answer: ounces_of_milk is assigned a value without ever being declared first! You can’t use a variable and assign it without first declaring it.

To fix this problem, you can place double (notice the %lf hints at which type to use) before ounces_of_milk to make one line that both declares and initializes the variable. A revised version of the code might look like the following:

#include <stdio.h>

int main(void)
{
  double ounces_of_milk = 16.0;
  printf("You have %lf ounces of milk.\n", ounces_of_milk);

  return 0;
}

Alternatively, you could add a new line of code before ounces_of_milk = 16.0; that declares it:

#include <stdio.h>

int main(void)
{
  double ounces_of_milk;
  ounces_of_milk = 16.0;
  printf("You have %lf ounces of milk.\n", ounces_of_milk);

  return 0;
}

Author’s note: The second solution works, but is unnecessarily verbose, declaring and assigning ounces_of_milk on two consecutive lines. Should there have been other code between these lines, this decision may make more sense, but the first solution is preferrable.

6. What is the final output of this program?

#include <stdio.h>

int main(void)
{
  int apples = 16;
  int oranges = 90;

  apples = oranges;
  oranges = apples;

  printf("You have %d apples and %d oranges.\n", apples, oranges);

  return 0;
}

Answer:

You have 90 apples and 90 oranges.