The Program

Problem: What is the sum of the first ten positive integers?

The program from Overview appears below. The lines are numbered on the left so we can refer to them. The numbers are NOT part of the program. 

1.    #include<stdio.h>
2.    int main() {
3.      int num = 1, 
4.      sum = 0;
5.      while (num<10) 
6.      {
7.        sum += num;
8.        num += 1;
9.      }
10.     printf("%d\n", sum);
11.   }

This program is one function definition; the definition of function main. Function definitions consist of a header, in our case 
   int main()
and the body of the function. The body is a compound statement. Compound statements start with the open brace ({) and continue until the matching closing brace (}) is found. For the body, the opening brace is the first character visible after the header (line 2) and the matching closing brace is in line 11.

Compound statements contain 0 or more statements between the braces. Computers execute compound statements by executing each of the statements contained inside. The statements are executed in the order that they found inside the compound statement. What compound statements are good for will become apparent below. Note the body of main contains a compound statement inside of it. This is the statement that starts with a open brace on line 6 and ends with a closing brace on line 9.

Our program contains three types of "simple statements": variable definition, assignment, and function call. It also contains 

 
   while (num<10)
in line 5.

Variable Definitions. A variable is a named storage location in computer memory. As such it has a content and an address. In addition, the program gives it a type and a value. The value is the content of the storage location. The program can change the value in a variable.

The statement in lines 3 and 4 defines two variables to be used by the program: 

3.   int num = 1,
4.       sum = 0;
The variable names are num and sum. Variable num is initialized to 1, and variable sum is initialized to 0. Variable names are just made up. They must start with a letter or underbar (_) and the rest of the name can be more letters, digits and underbars. You should not use keywords of C/C++ as variable names (list). You should use names that make sense in the context of what the program is trying to do.

The keyword, int, that starts the statement determines the type of the variables. "int" says that the content of the variables will be interpreted as integers. Click on type keywords to see more types.

The C and C++ languages require that all variables be defined before they can be used. Here are a few more variable definitions. 

int num=1;
int sum=0;
float pi=3.14;
float rate = 3.5e-2;
The first two lines are an alternate way of doing what lines 3 and 4 do in our program. The third and fourth lines set up variables pi and rate to store non-integers. The value 
3.5e-2
is scientific notation: 
        -2 
3.5 x 10
That is, 0.035.

Assignment Statements. The format for assignment statements is: 

variable assignOp expression
Lines 7 and 8 are the two assignment statements in our program 
7.           sum += num;
8.           num += 1;
There are many assignment operators. See Assignment Operators The assignment operator in the statements above is "+=". No spaces between the "+" and the "=". This operator changes the value stored in the variable on its left by adding to that variable the value on its right. So if variable sum has the value 36 stored in it and variable num has the value 9 stored in it, then line 7 will replace the 36 with 45 (36+9). Afterwards line 8 executes and replaces the 9 in num with a 10.

The while prefix. Take a statement, say the compound statement containing the two assignments: 

6.      {
7.        sum += num;
8.        num += 1;
9.      }
Prefix that statement with while(num<10): 
5.      while (num<10) 
6.      {
7.        sum += num;
8.        num += 1;
9.      }
This makes a NEW statement. We need to talk about how it executes. The first thing that happens is the condition 
num<10
inside the parentheses is evaluated. If it's true (num is less than 10), the statement after the parentheses is executed, and then the condition is re-evaluated. So long as the condition is true we continue executing the statement after the parentheses and then re-checking the condition. If the condition tests false, we skip the following statement and go to the next statement in the program. In our program that is line 10.

IMPORTANT NOTE: by having both assignments contained inside the compound statement and then putting the while in front of the compound statement we repeat the combination below several times 

add num to sum
make num one larger
Each repetition adds a new number into sum since the second statement makes num one larger.

Other comparisons are possible as conditions: More Comparisons.

Function Calls. Line 10 

10.      printf("%d\n", sum);
uses or calls the printf function. Some functions do some work for the calling function (in this case, main). Other functions do some work and return a value to the calling function. Function printf prints stuff and sets up where the next printf will print.

Lots of functions require that values be plugged into the parentheses that follow the function name. These values are called the arguments of the functions. The arguments are separated by commas. Our call to function printf has two arguments plugged into the function: 

"%d\n"
sum
The first argument is a string constant. The string is the sequence of characters between the quotes. Mostly printf just prints the string. Like the call 
    printf("Hello out there!");
prints 
    Hello out there!
However both %d and \n are special. The \n is the new line character. It is a single character. Function printf prints it by moving down one line and all the way back to the left. So the call 
    printf("cat\ndog");
prints 
    cat
    dog
The %d tells printf to look at the next argument which should be an int and print it in place of the %d using decimal digits. Thus the call 
    int num = 35;
    printf("%d\n", num);   
    printf("Bob");
prints 
    35
    Bob
The new line character is printed. The next print starts all the way at the left of the new line. Finally in the call 
    printf("age=%d weight=%s\n", 35, "too much");
the first string (the format string) tells printf to expect two more arguments, the first should be an int and the second should be a string (%s). What prints is 
    age=35 weight=too much
and the next printf, if there is one will print on the line below.

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