a Exercise
Write program that accepts an input file containing integers representing daily high temperatures.
Example input file: 42 45 37 49 38 50 46 48 48 30 45 42 45 40 48
Your program should print the difference between each adjacent pair of temperatures, such as the following:
Temperature Temperature Temperature Temperature Temperature Temperature Temperature Temperature Temperature Temperature Temperature Temperature Temperature Temperature changed changed changed changed changed changed changed changed changed changed changed changed changed changed by by by by by by by by by by by by by by 3 deg F -8 deg F 12 deg F -11 deg F 12 deg F -4 deg F 2 deg F 0 deg F -18 deg F 15 deg F -3 deg F 3 deg F -5 deg F 8 deg F
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Solution
#include <stdio.h> int main() { FILE *pFile = fopen("weather.txt", "r"); if (pFile == NULL) { printf("Could not open file. Exiting...\n"); exit(1); } int temp1, temp2; fscanf(pFile, "%d", &temp1); while (fscanf(pFile, "%d", &temp2) != EOF) { printf("Temperature changed by %d deg F\n", temp2 - temp1); temp1 = temp2; } }
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Line-based processing
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fgets man page
SYNOPSIS #include <stdio.h> char *fgets(char *s, int n, FILE *stream); DESCRIPTION The fgets() function reads at most one less than the number of characters specified by n from the given stream and stores them in the string s. Reading stops when a newline character is found, at end-of-file or error. The newline, if any, is retained. If any characters are read and there is no error, a '\0' character is appended to end the string. RETURN VALUES Upon successful completion, fgets() returns a pointer to the string. If end-of-file occurs before any characters are read, it returns NULL and the buffer contents remain unchanged.
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Who's next in line?
Reading a file line-by-line, general syntax:
FILE *pFile = fopen("<file name>", "r"); // do error-checking on file char buffer[1000]; while (fgets(buffer, 1000, pFile) != NULL) { <process the line in the buffer>; }
NOTE: fgets stores the newline character ('\n') in the buffer.
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Reading lines
23 3.14 John Smith 45.2 "Hello world" 19 19\n
23\t3.14 John Smith\t"Hello world"\n\t\t45.2 ^ fgets(buffer, 1000, pFile); 23\t3.14 John Smith\t"Hello world"\n\t\t45.2 ^
19\n
fgets(buffer, 1000, pFile); 23\t3.14 John Smith\t"Hello world"\n\t\t45.2
19\n ^
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Exercise
Write a program that reads a text file and "quotes" it by putting a > in front of each line. Input:
Hey, How are you? computers. Sincerely, Your student This class is too much work. I hate
Output:
> > > > > > > Hey, How are you? computers. Sincerely, Your student This class is too much work. I hate
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Solution
#include <stdio.h> int main() { FILE *pFile = fopen("message.txt", "r"); // do error-checking on file char buffer[1000]; while (fgets(buffer, 1000, pFile) != NULL) { printf("> %s", buffer); } }
Notice that since fgets copies the newline character to the buffer, we don't need a newline character at the end of the printf format string.
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Example
Example file contents:
123 Susan 12.5 8.1 7.6 3.2 456 Brad 4.0 11.6 6.5 2.7 12 789 Jennifer 8.0 8.0 8.0 8.0 7.5
Consider the task of computing the total hours worked for each person represented in the above file.
Susan (ID#123) worked 31.4 hours (7.85 hours/day) Brad (ID#456) worked 36.8 hours (7.36 hours/day) Jennifer (ID#789) worked 39.5 hours (7.9 hours/day)
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Line-based or token-based?
Neither line-based nor token-based processing works. The better is solution a hybrid approach
Break the input into lines. Break each line into tokens.
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Processing lines
FILE *pFile = fopen("<file name>"); // do error-checking on file while (fgets(buffer, 1000, pFile) != NULL) { <process this line>; }
You can also use sscanf to interpret lines. It works like scanf and fscanf but takes it's input from a string.
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Exercise
Write a program that computes the total hours worked and average hours per day for a particular person represented in the following file:
123 Susan 12.5 8.1 7.6 3.2 456 Brad 4.0 11.6 6.5 2.7 12 789 Jennifer 8.0 8.0 8.0 8.0 7.5 7.0
Sample runs: (run #1)
Enter a name: Brad Brad (ID#456) worked 36.8 hours (7.36 hours/day)
(run #2)
Enter a name: Harvey Harvey was not found
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Searching for a line
When going through the file, how do we know which line to process? If we are looking for a particular line, often we look for the token(s) of interest on each line.
If we find the right value, we process the line. e.g. If the second token on the line is "Brad", process it.
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Solution
// This program searches an input file of employees' hours worked // for a particular employee and outputs that employee's hours data. #include <stdio.h> #include <string.h> #include <stdlib.h> void getSearchName(char *buffer); void getEmployeeData(char *searchName, char *line); void processLine(char *line); int main() { char name[1000]; getSearchName(name); char line[1000]; getEmployeeData(name, line); if (strlen(line) > 0) { processLine(line); } else { printf("%s was not found\n", name); } }
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Solution
void getEmployeeData(char *searchName, char *line) { line[0] = '\0'; // initialize to 0 length FILE *pFile = fopen("hours.txt", "r"); if (pFile == NULL) { printf("Could not open file. Exiting ..."); exit(1); } char buffer[1000]; while (fgets(buffer, 1000, pFile) != NULL) { int num; char name[1000]; sscanf(buffer, "%d %s", &num, name); // e.g. 456 Brad if (strcmp(name, searchName) == 0) { strcpy(line, buffer); } } } void getSearchName(char *buffer) { printf("Enter a name: "); scanf("%s", buffer); // e.g. "BRAD" }
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Solution
// totals the hours worked by one person and outputs their info void processLine(char *line) { char *pId = strtok(line, " "); // "456" char *pName = strtok(NULL, " "); // "Brad" double sum = 0.0; int count = 0; char *pToken = strtok(NULL, " "); while (pToken != NULL) { sum += atof(pToken); count++; pToken = strtok(NULL, " "); } double average = sum / count; printf("%s (ID#%s) worked %lf hours (%lf hours/day)", pName, pId, sum, average); }
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Exercise
Write a program that reads in a file containing HTML text, but with the tags missing their < and > brackets.
Whenever you see any all-uppercase token in the file, surround it with < and > before you print it to the console. You must retain the original order of the tokens on each line.
You will find the functions isupper and isalpha in ctype.h useful.
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Exercise: Example input
Input file: HTML HEAD TITLE My web page /TITLE /HEAD BODY P There are pics of my cat here, as well as my B cool /B blog, which contains I awesome /I stuff about my trip to Vegas. /BODY /HTML Output to console: <HTML> <HEAD> <TITLE> My web page </TITLE> </HEAD> <BODY> <P> There are pics of my cat here, as well as my <B> cool </B> blog, which contains <I> awesome </I> stuff about my trip to Vegas. </BODY> </HTML>
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