Reverse input without array

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This page looks at how to read input and then print it out in reverse order without an explicit array.

Here is the code to read in input until an end of file and then print the lines in reverse order. Here is the C code.

#include <stdio.h>
#include <string.h>
#define LMAX 16
#define SMAX 256

int main() {
	// declarations
	char lineList1[LMAX+2][SMAX];
	int llen1;
	// input
	printf("Input: (forward order)\n");
	printf("\n");
	llen1 = 0;
	while (gets(lineList1[llen1+1])) {
		llen1++;
		printf("\t%d. \"%s\"\n",llen1,lineList1[llen1]);
		}
	// output
	printf("\n");
	printf("Output: (reverse order)\n");
	printf("\n");
	for (int lpos1=llen1; lpos1 >= 1; lpos1--) {
		printf("\t%d. \"%s\"\n",lpos1,lineList1[lpos1]);
		}
	return 0;
	}

Note: This code has a built-in limit of 16 on the maximum number of lines in the input.

Here are some examples of input and output for the above program code.

Here is an example input.

one
two
three
four

For the above example input, here is the expected output.

Input: (forward order)

	1. "one"
	2. "two"
	3. "three"
	4. "four"

Output: (reverse order)

	4. "four"
	3. "three"
	2. "two"
	1. "one"

Now let us try to write this program without an array. To do so, the input and output will not be directly separated and recursion will be used. Here is the C code.

#include <stdio.h>
#include <string.h>

void lineDo1(int pos1) {
	char line1[256];
	if (gets(line1)) {
		printf("\t%d. \"%s\" (in)\n",pos1,line1);
		lineDo1(pos1+1);
		printf("\t%d. \"%s\" (out)\n",pos1,line1);
		}
	else {
		printf("\n");
		printf("Output: (reverse order)\n");
		printf("\n");
		}
	}

int main() {
	// input
	printf("Input: (forward order)\n");
	printf("\n");
	lineDo1(0);
	return 0;
	}

Note: This code has no built-in limit on the maximum number of lines in the input.

Here are some examples of input and output for the above program code.

Here is an example input.

one
two
three
four

For the above example input, here is the expected output.

Input: (forward order)

	0. "one" (in)
	1. "two" (in)
	2. "three" (in)
	3. "four" (in)

Output: (reverse order)

	3. "four" (out)
	2. "three" (out)
	1. "two" (out)
	0. "one" (out)

A data flow solution is largely based on the functional idea that no variable is ever assigned a value more than once.

Notice that in the above recursive program no variable is ever assigned a value more than once.

Why did the above work without an array?

It worked because the runtime stack was used to store the string values.

Note the following:

Fewer variables
Less code
Clearer logic - for a computer scientist