======Exercise 11: While-Loop And Boolean Expressions====== You've had your first taste of how C does loops, but the boolean expression i < argc might have not been clear to you. Let me explain something about it before we see how a while-loop works. In C, there's not really a "boolean" type, and instead any integer that's 0 is "false" and otherwise it's "true". In the last exercise the expression i < argc actually resulted in 1 or 0, not an explicit True or False like in Python. This is another example of C being closer to how a computer works, because to a computer truth values are just integers. Now you'll take and implement the same program from the last exercise but use a while-loop instead. This will let you compare the two so you can see how one is related to another. #include <stdio.h> int main(int argc, char *argv[]) { // go through each string in argv int i = 0; while(i < argc) { printf("arg %d: %s\n", i, argv[i]); i++; } // let's make our own array of strings char *states[] = { "California", "Oregon", "Washington", "Texas" }; int num_states = 4; i = 0; // watch for this while(i < num_states) { printf("state %d: %s\n", i, states[i]); i++; } return 0; } You can see from this that a while-loop is simpler: while(TEST) { CODE; } It simply runs the CODE as long as TEST is true (1). This means that to replicate how the for-loop works we need to do our own initializing and incrementing of i. ======What You Should See====== The output is basically the same, so I just did it a little different so you can see another way it runs. $ make ex11 cc -Wall -g ex11.c -o ex11 $ ./ex11 arg 0: ./ex11 state 0: California state 1: Oregon state 2: Washington state 3: Texas $ $ ./ex11 test it arg 0: ./ex11 arg 1: test arg 2: it state 0: California state 1: Oregon state 2: Washington state 3: Texas $ ======How To Break It====== In your own code you should favor for-loop constructs over while-loop because a for-loop is harder to break. Here's a few common ways: * Forget to initialize the first int i; so have it loop wrong. * Forget to initialize the second loop's i so that it retains the value from the end of the first loop. Now your second loop might or might not run. * Forget to do a i++ increment at the end of the loop and you get a "forever loop", one of the dreaded problems of the first decade or two of programming. ======Extra Credit====== * Make these loops count backward by using i-- to start at argc and count down to 0. You may have to do some math to make the array indexes work right. * Use a while loop to copy the values from argv into states. * Make this copy loop never fail such that if there's too many argv elements it won't put them all into states. * Research if you've really copied these strings. The answer may surprise and confuse you though. Copyright (C) 2010 Zed. A. Shaw Credits