Name and Update Values
Name and Update Values
A variable has a type, a name and a stored value.
Initialize a student mark before reading it, then make an explicit correction when the value changes.
Before this lesson, review Read and Display Input when needed.
What you’ll learn
- Use variables to name and Update Values.
- Explain how the complete example produces 72.
- Make the practice change and explain why the result changes or stays the same.
Follow a complete C++ example
Initialize a student mark before reading it, then make an explicit correction when the value changes.
Read the declarations above main before tracing its statements. Each declared name supplies the value, type or operation used later. The std:: prefix selects a name from the standard library, and cout << sends a value to the output stream.
C++17 · main.cpp · complete program
#include <iostream>
int main() {
int marks = 70;
marks += 2;
std::cout << marks << '\n';
return 0;
}
Expected output
72Connect the code to its result
C++17 · source used in this explanation
#include <iostream>
int main() {
int marks = 70;
marks += 2;
std::cout << marks << '\n';
return 0;
}
Expected output
72A variable has a type, a name and a stored value. Initialize a student mark before reading it, then make an explicit correction when the value changes.
Trace the result: Read the complete program below from its starting values to the output operations. Each std::cout statement displays the value supplied at that point; \n begins the next line. The resulting lines, in order, are 72. In this example, distinguish what the program calculates or changes from what it actually displays.
Important mistakes and boundaries
Reading an uninitialized ordinary local int is invalid. Assignment copies a value; it does not create a live connection to an expression.
Change the school example
C++17 · edit this complete example
#include <iostream>
int main() {
int marks = 70;
marks += 2;
std::cout << marks << '\n';
return 0;
}
Your task: Start marks at 60 instead. Before running your edited program, predict the new output and explain which stored value, condition or object changes. Keep the rest of the example intact.
Use the complete source repeated here; this task does not depend on remembering code from another section.
Worked answer and reasoning
Worked variation: Start marks at 60 instead. The final value is 62.
A variable has a type, a name and a stored value. Initialize a student mark before reading it, then make an explicit correction when the value changes.
Reading an uninitialized ordinary local int is invalid. Assignment copies a value; it does not create a live connection to an expression.
For a baseline run, save this source as main.cpp and use clang++ -std=c++17 -Wall -Wextra -pedantic main.cpp -o school, or equivalent g++ flags. Recompile after editing so that you run the updated executable.
The source and output below belong to the edited program. Compare them with the original example to see the consequence of this single change.
C++17 · worked variation · complete program
#include <iostream>
int main() {
int marks = 60;
marks += 2;
std::cout << marks << '\n';
return 0;
}
Expected output
62Check your understanding
C++17 · original program for this question
#include <iostream>
int main() {
int marks = 70;
marks += 2;
std::cout << marks << '\n';
return 0;
}
What to remember
A variable has a type, a name and a stored value. Reading an uninitialized ordinary local int is invalid. Assignment copies a value; it does not create a live connection to an expression.
Useful connections: Read and Display Input, Give a Task a Name.
Language and design guidance: C++ Core Guidelines and C++ working draft. The draft describes an evolving language; these examples deliberately target C++17.
Reaching the recap and completing the quizzes are separate milestones.