Branching and Looping

If statements and while loops in C both rely on the idea of Boolean expressions, as they do in Pascal. In C, however, there is no Boolean type: You use plain integers instead. The integer value 0 in C is false, while any other integer value is true.

Here is a simple translation from Pascal to C. First, the Pascal code:

if (x=y) and (j>k) then     
    z:=1  
else    
    q:=10;

The C translation looks very similar, but there are some important differences, which we will discuss next.

if ((x==y) && (j>k))    
    z=1;  
else    
    q=10;

Notice that = in Pascal became == in C. This is a very important difference, because C will accept a single = when you compile, but will behave differently when you run the program. The and in Pascal becomes && in C. Also note that z=1; in C has a semicolon, that C drops the then, and that the Boolean expression must be completely surrounded by parentheses.

The following chart shows the translation of all boolean operators from Pascal to C: The == sign is a problem because every now and then you may forget and type just =. Because integers replace Booleans, the following is legal in C:

void main()  
{    
    int a; 
    
    printf("Enter a number:");    
    scanf("%d", &a);    
    if (a)    
    {      
        blah blah blah    
    }  
}

if a is anything other than 0, the code that blah blah blah represents gets executed. Suppose you take the following Pascal statement:

if a=b then

and incorrectly translate it to C as:

if (a=b)  /* it SHOULD be "if (a==b)" */ 

In C, this statement means “Assign b to a, and then test a for its Boolean value.” So if a becomes 0, the if statement is false; otherwise, it is true. The value of a changes as well. This is not the intended behavior (although this feature is useful when used correctly), so be careful with your = and == conversions.

While statements are just as eay to translate. For example, the following Pascal code:

while a < b do  
begin    
    blah blah blah  
end;

in C becomes:

while (a < b)  
{    
    blah blah blah  
} 

C also provides a “do-while” structure to replace Pascal’s “repeat-until,” as shown below:

do  
{    
    blah blah blah  
}  
while (a < b); 

The for loop in C is somewhat different from a Pascal for loop, because the C version is simply a shorthand way of expressing a while statement. For example, suppose you have the following code in C:

x=1;  
while (x<10)  
{    
    blah blah blah    
    x++; /* x++ is the same as saying x=x+1. It's an increment. */  
}

You can convert this into a for loop as follows:

for(x=1; x<10; x++)  
{    
    blah blah blah  
}

Note that the while loop contains an initialization step (x=1 ), a test step (x<10), and an increment step (x++ ). The for loop lets you put all three parts onto one line, but you can put anything into those three parts. For example, suppose you have the following loop:

a=1;  
b=6;  
while (a < b)  
{    
    a++;    
    printf("%d\n",a);  
}

You can place this into a for statement as well:

for (a=1,b=6; a < b; a++,printf("%d\n",a)); 

It is confusing, but it is possible. The comma operator lets you separate several different statements in the initialization and increment sections of the for loop (but not in the test section). Many C programmers like to pack a lot of information into a single line of C code. I think it makes the code harder to understand, so I break it up.

C Errors to avoid

1. Putting = when you mean == in an if or while statement.

2. Accidentally putting a ; at the end of a for loop or if statement, so that the statement has no effect. For example,

   for (x=1; x<10; x++);    
   printf("%d\n",x); 

   only prints out one value because of the semicolon after the for statement.