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POHow to evaluate a string expression and return the value in C?
 

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  1. POHow to evaluate a string expression and return the value in C?
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    <p>I am trying to evaluate an expression taken as a character array and return the result of the expression.</p> <p>eg:</p> <pre><code>char *myeExpression []= "(1+2) * 3" </code></pre> <p>should return result 9.</p> <p>Here is my code:</p> <pre><code>struct node { double element; struct node *next; } *head; void push(int c); // function to push a node onto the stack int pop(); // function to pop the top node of stack void traceStack(); // function to //print the stack values int prece(char j) { if(j=='*'||j=='/') { j=3; } else { if(j=='+'||j=='-') { j=2; } else { j=1; } } return j; } int evaluate(char * a) { int i = 0, j = 0,k,l,a1,b1; // indexes to keep track of current position char *exp = (char *)malloc(sizeof(char)*100); double res = 0; char stack[5]; char tmp; head = NULL; // converting an infix to a postfix for(i=0;i&lt;10;i++) { a1=prece(a[i]); b1=prece(stack[k]); if(a1&lt;=b1) { exp[l]=a[i]; l++; } else { stack[k]=a[i]; k++; } } for(i=k;i&gt;0;i--) { exp[l]=stack[i]; l++; } //end i=0; j=0; k=0; while( (tmp=exp[i++]) != '\0') { // repeat till the last null terminator // if the char is operand, pust it into the stack if(tmp &gt;= '0' &amp;&amp; tmp &lt;= '9') { int no = tmp - '0'; push(no); continue; } if(tmp == '+') { int no1 = pop(); int no2 = pop(); push(no1 + no2); } else if (tmp == '-') { int no1 = pop(); int no2 = pop(); push(no1 - no2); } else if (tmp == '*') { int no1 = pop(); int no2 = pop(); push(no1 * no2); } else if (tmp == '/') { int no1 = pop(); int no2 = pop(); push(no1 / no2); } } return pop(); } void push(int c) { if(head == NULL) { head = malloc(sizeof(struct node)); head-&gt;element = c; head-&gt;next = NULL; } else { struct node *tNode; tNode = malloc(sizeof(struct node)); tNode-&gt;element = c; tNode-&gt;next = head; head = tNode; } } int pop() { struct node *tNode; tNode = head; head = head-&gt;next; return tNode-&gt;element; } </code></pre> <p>Infix Expression evaluation happens but not completely. Getting wrong result ie 3 in stead of 9.</p>
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    1. USSpooferman
    1. USSpooferman
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      1. POHow to evaluate a string expression and return the value in C?
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    1. COIf this is your complete code, and you are "lost", then you need to first think about what you are trying to do, think about the algorithm, and the steps and then move forward. You still have quite a bit to do.
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      1. POHow to evaluate a string expression and return the value in C?
      1. USasheeshr
    2. COSoory, had posted the wrong code.. please check now.
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      1. POHow to evaluate a string expression and return the value in C?
      1. USSpooferman
    3. COAsking people to spot errors in your code is not productive. You should use the debugger (or add print statements) to isolate the problem, and then construct a [minimal test-case](http://sscce.org).
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      1. POHow to evaluate a string expression and return the value in C?
      1. USOliver Charlesworth
 

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