Friday, February 3, Lab Notes. Functions in C Arrays as parameters Pass by value vs pass by reference Programs 1 and 2

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1 Friday, February 3, 2017 Lab Notes Topics for today File Input/Output Functions in C Arrays as parameters Pass by value vs pass by reference Programs 1 and 2 Submitting program assignments You can turn in several version of an assignment up to the deadline but only the last one will be graded. Blackboard will let you make only one submission; you can turn in other versions through File input/output Notes on C Part 2 We can attach text files to programs and read from them and write to them in a simple way as in the following examples. Comp 162 Lab Notes Page 1 of 7 February 3, 2017

2 Example 6 Program prompts user for the name of a file. The file contains N followed by N integers. The program opens the file, reads and sums the N integers. #include <stdio.h> #define MAXSTRING 100 main() char filename[maxstring]; int sum=0, N, I, temp; FILE *ifp; printf("input a file name: "); scanf("%s", filename); ifp = fopen(filename,"r"); /* open file for reading */ if (ifp==null) printf( Sorry, cannot open %s\n, filename); return; fscanf(ifp, %d,&n); for (i=0; i<n; i++) fscanf(ifp, %d,&temp); sum += temp; fclose(ifp); printf( Sum of the %d numbers is %d\n,n,sum); Notes on the program ifp is a file variable to which we can attach a file. The fopen function call specifies the file name and the mode in which it is attached to the program. We can check to see if fopen succeeded. fscanf is the more general versions of scanf. The additional first parameter is the file variable. Comp 162 Lab Notes Page 2 of 7 February 3, 2017

3 Example 7 This program reads words from a file (the dictionary used by a spell checker). Each time it finds a word longer than the longest so far, it writes it to an output file. #include <stdio.h> #include <string.h> main() FILE *ifp, *ofp; char temp[100]; int longest=0; ifp = fopen("words","r"); /* open file for reading */ ofp = fopen("results99", "w"); /* open file for writing */ while (fscanf(ifp,"%s",temp)!= EOF) if (strlen(temp)>longest) longest = strlen(temp); fprintf(ofp,"%s\n",temp); fclose(ifp); fclose(ofp); Notes on the program ifp and ofp are file variables to which we can attach files. The fopen function call specifies the file name and the mode in which it is attached to the program fscanf and fprintf are the more general versions of scanf and printf. The additional first parameter is the file variable. We read from the file of words until we reach the end of file (EOF) Every time we encounter a word longer than the longest one we have seen so far we output it to the results file. The string.h library contains the function strlen Comp 162 Lab Notes Page 3 of 7 February 3, 2017

4 Here is a run of the program. hp9k2 22: gcc ctestc.c hp9k2 23: a.out hp9k2 24: cat results99 10th Aarhus abalone abbreviate aboveground abovementioned acknowledgeable anthropomorphism arteriolosclerosis electroencephalogram electroencephalograph electroencephalography 2. Functions in C All subprograms in C are called functions. They are all declared at the same level (i.e., there is no nesting of functions inside functions); we have seen that the main program is a function called main. Some compilers require that a function be declared before it is used but the function prototype mechanism (see below) is a convenient way not to have to worry about this. Recursion is allowed in C. Example 9 A simple function that print N instances of character C. We will see a version of this in Pep/9 later. void printchars (int N, char C) int i; for (i=0; i<n; i++) printf("%c",c); Comp 162 Lab Notes Page 4 of 7 February 3, 2017

5 Notes on Example 9 1. In a function declaration, the name of the function is preceded by the type of value returned. If the function does not return a value then the keyword void is used. 2. The formal parameter list shows names and types of formal parameters. 3. The scope of the local variable i is the function body. 4. The following are typical calls of printchars (assume i is a variable of type int and inchar is a variable of type char): printchars(5,'*'); printchars(i+4,inchar); 5. Parameters in C are passed by value and thus cannot be altered by the action of a function. Later we will see how pass by reference is accomplished. Example 10 Here is the definition of a function that returns a value. float min (float A, float B) if (A<B) return A; else return B; Notes on Example Note that the parameter list is not written: float A,B 2. The return statement exits out of a function. If the word return is followed by an expression then the value of that expression is the value returned by the function call. 3. C compilers are normally good at detecting paths through a function that fail to return a value of the appropriate type. Function prototypes A function prototype is a bare-bones declaration of a function. It contains enough information to enable the compiler to check whether or not a call to the function is valid. The function prototypes of our two examples are: void printchars (int,char); float min (float,float); Comp 162 Lab Notes Page 5 of 7 February 3, 2017

6 There are a couple of reasons why it is not a bad idea to list at the beginning of the program the function prototypes for all the functions in your program. First, you then do not need to worry about the order in which the actual function are declared and second it gives you a convenient place to put summary information about the functions, thus: void printchars(int,char); /* prints multiple instances of char */ float min(float,float); /* returns smaller of two arguments */ 3. Arrays as parameters of functions When an array is passed as a parameter to a function, only the address of the array is give; nothing about its size. Therefore we normally need additional parameters to indicate the number of elements to be processed. For example, the following function returns the sum of the first N elements of the array int sum (int AR[], int N) int i, temp=0; for (i=0, i<n; i++) temp+=ar[i]; return temp; If we have an array with daily rainfall totals We can find out how much rain fell in January int RAIN[365] Jan = sum(rain,31); Because of the lack of array bound checking, the following will also run Junk = sum(rain,20000) But give nonsense results. 4. Parameters: Pass by value and pass by reference Sometimes we wish a function to be able to modify its actual parameter. The following example illustrates different types of parameter passing in C. When it runs, the output is Comp 162 Lab Notes Page 6 of 7 February 3, 2017

7 #include <stdio.h> void change ( int *number, int increment ) // This function adds increment to Number *number += increment; int main() int a=14, b=6; printf("%d %d\n",a,b); change(&a,b); printf("%d %d\n",a,b); The second parameter of changed is passed by value. A copy of the value of the actual parameter (b in this case) is given to the call. The value in the second actual parameter cannot be altered by the actions of the function. The second actual parameter could be a constant or expression. The first parameter of changed is passed by reference. The address of the actual parameter (&a) is given to the call. This enables the value in that variable to be altered by the function. The second actual parameter must be the address of a variable. 5. Programs 1 and 2 Program 1 is not due until next week but you should aim to have it pretty much wrapped up today. Program 2, an assignment in C using arrays, will be posted next week. Comp 162 Lab Notes Page 7 of 7 February 3, 2017