Many of us know the traditional uses of scanf. Well, here are some of the lesser known facts
How to read only a part of the input that we need? For example, consider some input stream that contains only characters followed by an integer or a float. And we need to scan only that integer or float.
That is ,
Input: “this is the value 100”,
Output: value read is 100
Input : “this is the value 21.2”,
Output : value read is 21.2
/* C program to demonstrate that we can ignore some string in scanf() */#include <stdio.h> int main() { int a; scanf("This is the value %d", &a); printf("Input value read : a = %d", a); return 0; } // Input : This is the value 100 // Output : Input value read : a = 100 |
Now, assume we don’t know what the preceding characters are but we surely know that the last value is an integer. How can we scan the last value as an integer?
Below solution works only if input string has no spaces.
/* Sample C program to demonstrate use of *s */#include<stdio.h> int main() { int a; scanf("%*s %d", &a); printf("Input value read : a=%d",a); return 0; } // Input: "blablabla 25" // Output: Value read : 25 |
Explanation: The %*s in scanf is used to ignore some input as required. In this case, it ignores the input until the next space or new line. Similarly if you write %*d it will ignore integers until the next space or new line.
The above fact may not seem as an useful trick at the first glance. Inorder to understand its usage, let us first see fscanf().
fscanf() : Tired of all the clumpsy syntax to read from files? well, fscanf comes to the rescue.
int fscanf(FILE *ptr, const char *format, ...)
fscanf reads from a file pointed by the FILE pointer (ptr), instead of reading from the input stream.
Consider the following text file abc.txt
NAME AGE CITY abc 12 hyderbad bef 25 delhi cce 65 bangalore
Now, we want to read only the city field of the above text file, ignoring all the other fields. A combination of fscanf and the trick mentioned above does this with ease
/*c program demonstrating fscanf and its usage*/#include<stdio.h> int main() { FILE* ptr = fopen("abc.txt","r"); if (ptr==NULL) { printf("no such file."); return 0; } /* Assuming that abc.txt has content in below format NAME AGE CITY abc 12 hyderbad bef 25 delhi cce 65 bangalore */ char* buf[100]; while (fscanf(ptr,"%*s %*s %s ",buf)==1) printf("%s
", buf); return 0; } |
Output:
CITY hyderbad delhi bangalore
Exercise: Count the number of words, characters and lines in a file using fscanf!
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
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