Find and Fix Errors
Find and Fix Errors
A compile error prevents a valid executable from being built; a logic error can remain in code that compiles.
Comparing an actual report with a hand-calculated result exposes wrong arithmetic or conditions.
Before this lesson, review Read Statements and Comments when needed.
What you’ll learn
- Use errors and debugging to find and Fix Errors.
- Explain how the complete example produces 75.
- Make the practice change and explain why the result changes or stays the same.
Follow a complete C++ example
Comparing an actual report with a hand-calculated result exposes wrong arithmetic or conditions.
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 total = 35 + 40;
std::cout << total << '\n';
return 0;
}
Expected output
75Connect the code to its result
C++17 · source used in this explanation
#include <iostream>
int main() {
int total = 35 + 40;
std::cout << total << '\n';
return 0;
}
Expected output
75A compile error prevents a valid executable from being built; a logic error can remain in code that compiles. Comparing an actual report with a hand-calculated result exposes wrong arithmetic or conditions.
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 75. In this example, distinguish what the program calculates or changes from what it actually displays.
Important mistakes and boundaries
Passing compilation is not proof of a correct answer. Read the first useful diagnostic and test boundary inputs after a fix.
Change the school example
C++17 · edit this complete example
#include <iostream>
int main() {
int total = 35 + 40;
std::cout << total << '\n';
return 0;
}
Your task: Change 40 to 45 and predict the corrected total: 80.. 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: Change 40 to 45 and predict the corrected total: 80.
A compile error prevents a valid executable from being built; a logic error can remain in code that compiles. Comparing an actual report with a hand-calculated result exposes wrong arithmetic or conditions.
Passing compilation is not proof of a correct answer. Read the first useful diagnostic and test boundary inputs after a fix.
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 total = 35 + 45;
std::cout << total << '\n';
return 0;
}
Expected output
80Check your understanding
C++17 · original program for this question
#include <iostream>
int main() {
int total = 35 + 40;
std::cout << total << '\n';
return 0;
}
What to remember
A compile error prevents a valid executable from being built; a logic error can remain in code that compiles. Passing compilation is not proof of a correct answer. Read the first useful diagnostic and test boundary inputs after a fix.
Useful connections: Read Statements and Comments, 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.