Week 3 - Friday

 What did we talk about last time?  Bitwise practice  Precedence  Control flow

Unix is simple. It just takes a genius to understand its simplicity. Dennis Ritchie Unix is simple. It just takes a genius to understand its simplicity. Dennis Ritchie

 Loops can go on forever if you aren’t careful int n = 40; int i = 1; while( i <= 40 ) { printf("%d", i); //supposed to print all the numbers //less than 40, but i never increases } int n = 40; int i = 1; while( i <= 40 ) { printf("%d", i); //supposed to print all the numbers //less than 40, but i never increases }

 Infinite for loops are unusual, but possible:  This situation is more likely: for( ; ; ) printf("Hey!"); for( ; ; ) printf("Hey!"); int i; for(i = 0; i < 10; i++ ) { printf("%d", i); //lots of other code i--; //whoops, maybe changed from while? } int i; for(i = 0; i < 10; i++ ) { printf("%d", i); //lots of other code i--; //whoops, maybe changed from while? }

 Overflow and underflow will make some badly written loops eventually terminate int i; for( i = 1; i <= 40; i-- ) //whoops, should have been i++ printf("%d", i); int i; for( i = 1; i <= 40; i-- ) //whoops, should have been i++ printf("%d", i);

 Being off by one is a very common loop error int i; for( i = 1; i < 40; i++ ) //runs 39 times printf("%d", i); int i; for( i = 1; i < 40; i++ ) //runs 39 times printf("%d", i);

 If the condition isn’t true to begin with, the loop will just be skipped int i; for( i = 1; i >= 40; i++ ) //oops, should be <= printf("%d", i); int i; for( i = 1; i >= 40; i++ ) //oops, should be <= printf("%d", i);

 A misplaced semicolon can cause an empty loop body to be executed  Everything looks good, loop even terminates  But, only one number will be printed: 41  Misplaced semicolon usually makes a while loop infinite int i; for( i = 1; i <= 40; i++ ); //semicolon is wrong { printf("%d", i); } int i; for( i = 1; i <= 40; i++ ); //semicolon is wrong { printf("%d", i); }

 Systems programming concepts  Functions and prototypes

 Read LPI chapter 3 and K&R chapter 4  Project 1 due tonight by midnight!