Function

Intermediate Programming Course Day 02_1

Keypoints

• Compiling and running a simple C++ program
• Basic C/C++ programming workflow
• Inside the compiler
• How to call a function
• How to declare a function
• Head file and source file

Hello world

// hello_world.c
#include <stdio.h>

// Print "Hello, world!" followed by newline and exit
int main(void){
    printf("Helllo, world\n");
    return 0;
}

// Compilation command line 
$ gcc hello_world.c -std=c99 -pedantic -Wall -Wextra // or g++ -o generated_file_name target_file_name,   ex: g++ res hello_world.c
$ ./a.out
// Output 
Hello, world!

• #include is a preprocessor directive, similar to import in Python
• **** means it's defined in stdio.h(standard input output package); **.h** is "header file"
• main is a function, every program has exactly one main
• int is the return type. Because return 0, so its return type is int. If without return, using void as return type.
• (void) says the main function takes no inputs/parameters. If with parameter inputs, ex: void main(int num1, int num2).
• printf prints a string to “standard out” (terminal)
• \n denotes the newline character
• return 0 means “program completed without error”
• gcc is the compiler; hello_world.c is the cpp file name;
• Switches in compilation command: -std=c99 is to use the c99 standard from 1999 year; -pedantic -Wall -Wextra is to get full feedback, i.e. error and warning (potential error)
• ./ is the execution sign; . is current directory
• a.out is the compilation default file name

Basic C/C++ programming workflow

• Edit file (using EMACS or VIM)
• Compile using GNUplot C compiler (gcc) to compile, link, and create excut
  • If compile-time errors reported, edit .c file and re-compile
• Run the excutable file
  • If run-time errors reported/detected, go back to edit step

Inside the compiler

• Step 1：preprocessor
  • Bring together all the code that belongs together
  • Process the directives that start with #, such as #include
    • also #define (to ensure library exists)
• Step 2：compiler
  • Turn human-readable source code into object code
  • Might yield warnings and errors if mistakes are “visible” to compiler
• Step 3：linker
  • Bring together all the relevant object code into a single excutable file
  • Might yield warnings and errors if relevant code is missing, naming conflicts, etc

    many .c files --compiler--> many .o files --linker--> one excutable file 
    

How to call a function

int add(int num1, int num2){
    int a = num1 + num2;
    return a;
}

int main(){
    int a = 10;
    int b = 20;
    int c = add(a+b);
}

NOTE: We initialize and assign value to another variable a in function add. But the a value in function add doesn’t make any difference to a value in main function due to different thread, i.e the value change of parameters doesn’t change the value of argument.

How to declare a function

It’s to notify compiler there is a function before defining it. With so, we can define the function seperately/later.

int max(int a, int b);

int main(){
    int a = 10;
    int b = 20;
    return max(a, b)
}

int max(int a, int b){
    return a > b ? a:b;
}

• We initialize and assign value to another variable a in function add. But the a value in function add doesn’t make any difference to a value in main function due to different thread, i.e the value change of parameters doesn’t change the value of argument.
• a > b ? a:b: If the argument before ? is True, then return the value before :, otherwise return the latter one.

Head file and source file

• Set up head file ending with .h
• Set up source file ending with .cpp
• Declare function in head file
• Define function in source file

// head file swap.h
# include <iostream>
using namespace std;

void swap(int a, int b);            // declare the function

• using namespace std means that we can use names for objects and variables from the standard library without writing “std::cout”

// source file
# include "swap.h"

void swap(int a, int b){
    int temp = a;
    a = b;
    b = temp;
    
    cout << "a = "<< a << endl;
    cout << "b = "<< b << endl;
}

• include the head file using “” not <>. <> is for package.

// main file
# include "swap.h"
# include <iostream>
using namespace std;

int main() {
    int a = 10;
    int b = 20;
    swap(a, b);
    system("pause");
    return 0;
}