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LEC07+Notes

# LEC07+Notes - 14:440:127 Introduction to Computers for...

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14:440:127– Introduction to Computers for Engineers Notes for Lecture 7 Rutgers University, Fall 2010 Instructor- Brenda V. Cortez 1 Loop Examples Lets re- iterate the loop concept with some examples. 1.1 Example - Sum Primes Lets say we wanted to sum all 1, 2, and 3 digit prime numbers . To accomplish this, we could loop through all 1, 2, and 3 digit integers, testing if each is a prime number (using the isprime function). If and only if a particular value is prime, then well add it to our running total. Note that if a particular number is not prime, we dont do anything other than advancing to the following number. For-loop format --------------- for k = VECTOR % replace capitalized parts STATEMENTS % replace capitalized parts end total = 0; for k = 1:999 if(isprime(k)) total = total + k; end end disp(total) One interesting difference between MatLab and other programming languages is that it uses a vector to indicate what values a loop variable should take. Thus, if you simply write that x equals an arbitrary vector rather than a statement like x = 1:100, your program will work fine. Here, we rewrite the previous example for summing all 1, 2, and 3 digit prime numbers by first cre- ating a vector of all the prime numbers from 1 to 999, and simply looping through those values: total = 0; for k = primes(999) total = total + k; end disp(total)

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1.2 Example - Finding The Maximum in a Vector In the previous few examples, weve seen cases where weve kept a running total or running count as weve gone through our loop. However, sometimes, youll instead want to keep a running maximum, or something along those lines. For instance, lets say we wanted to find the largest value in some vector V. Lets first define our process. At all times, well keep track of the “maximum so far,” which well save in the variable maxvalue. Well loop through each element of the vector, and for each of these elements, compare it to our maxvalue so far. If the number were currently looking at is bigger than maxvalue, then that should replace maxvalue with the current element of V since thats the new largest number weve seen so far. If its not bigger than maxvalue, then dont do anything. Thus, at the end of our loop, the variable maxvalue will contain the overall maximum value, since that will be the largest value weve seen so far, and well have seen every element of the vector. Theres one complication: when we try and compare the first element of the matrix to maxvalue, the variable maxvalue wont have a value yet and well thus get an error message. To x this, lets initially set maxvalue to be -inf (negative infinity) since every value is bigger than negative infinity. Similarly, if we were trying to find the minimum of a vector, wed want to set our initial value to +inf, since every value is smaller than positive infinity.
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LEC07+Notes - 14:440:127 Introduction to Computers for...

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