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:
Pascal C = == < < > > <= <= >= >= <> != and && or || not !
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.
for (x=1; x<10; x++);
printf("%d\n",x);
only prints out one value because of the semicolon after the for statement.