Learn to Program with C_ Learn to Program using the Popular C Programming Language ( PDFDrive )

Chapter 2 ■ C – The Basics
The first statement assigns 12 to a; the second assigns 5 to b; the third assigns 14 to c; no
problem so far. However, when the computer attempts to execute the fourth statement, it runs
into a problem. There is no value for e, so the expression c + e cannot be evaluated. We say that e
is undefined—it has no value.
Before we can use any variable in an expression, it must have been assigned a value by some
previous statement. If not, we will get an “undefined variable” error and our program will halt.
The moral of the story: a valid program is not necessarily a correct program.
Exercise: What is printed by the following?
a = 13;
b = a + 12;
printf("%d %d\n", a, b);
c = a + b;
a = a + 11;
printf("a = %d b = %d c = %d\n", a, b, c);
2.9 printf
We have seen several examples of the printf statement. We have used it to print string constants,
integer values, and floating-point values. And we have printed values with and without field
widths. We have also seen how to use the escape sequence \n to force output onto a new line.
It is worth emphasizing that the characters in the format string are printed exactly as they
appear except that a format specification is replaced by its corresponding value. For example, if a
is 25 and b is 847, consider the statement
printf("%d%d\n", a, b);
This will print
25847
The numbers are stuck together and we cannot tell what is a and what is b! This is so because
the specification %d%d says to print the numbers next to each other. If we want them separated by
one space, say, we must put a space between %d and %d, like this:
printf("%d %d\n", a, b);
This will print
25 847
To get more spaces between the numbers, we simply put how many we want between %d and %d.
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